Water

Water exists in all the three physical states : as solid (ice), as liquid (water) and as gas (water vapour). It occurs in both free as well as combined states.

Occurrence of water in free state:

• Solid state — In the form of ice, snow, frost.
• Liquid state —
  • On the earth's surface as river water, lake water, sea water, spring water.
  • Below the earth's surface in well water and moisture accumulation in the soil.
  • Above the earth's crust as dew.
• Gaseous state [vapour] — as water vapour, clouds, mist, fog.

Occurrence of water in combined state :

• Water occurs in the combined form in all living matter i.e., plants and animals.
• Water is present in hydrated salts e.g., MgCl₂.6H₂O and in certain minerals.
• Earth's surface - Covers nearly 75% of earth's surface.
• Human body - Nearly, 70% of the body weight.
• Food products - Green vegetables [80-90%], Milk [80-85%], Dry cereals [3-5%]

Henry Cavendish synthesized water from its elements [2 vols. of hydrogen and 1 vol. of oxygen] by igniting the elements in their respective ratio, thereby leading to the conclusion that water is not an element but a compound of hydrogen and oxygen combined in the ratio 2:1.

(a) Mumbai and Chennai are on the shores of the sea. As water has high specific heat capacity, the presence of a large amount of water is able to modify the climate of the nearby land areas, making them warmer in winter and cooler in summer. Land and sea breeze also take place because of this great moderating property. Hence, the temperature in Mumbai and Chennai does not fall as low as it does in Delhi.

(b) Properties of water responsible for controlling the temperature of our body:

1. High specific heat capacity — Water has a high specific heat capacity enabling it to absorb heat from the body and release it to the environment without causing significant changes in its own temperature.
2. High latent heat of vaporization — As water has a high latent heat of vaporization, it requires a large amount of heat energy to change from liquid to gas. This property allows sweat to evaporate from the skin, taking heat from the body and cooling it down.

Water dissolves many substances, forming aqueous solutions (water solutions). Not only solids but gases and other liquids can also dissolve in water to a large extent. For the same reason, water is called a universal solvent.

The sudden release of latent heat of condensation causes the violence associated with torrential rain.

Due to its high specific heat capacity, the presence of a large amount of water is able to modify the climate of the nearby land areas, making them warmer in winter and cooler in summer. Land and sea breezes also take place because of this great moderating property of water.

At 4°C, water has its maximum density, 1g/cm³ or 1000 kg/m³, and minimum volume. Its density decreases as the temperature increases or decreases from this point. The consequences of this are:

1. Water expands on freezing, i.e., 92 volumes of water become 100 volumes of ice. Therefore, with relative density of ice being 0.92, it floats on water.
2. It enables marine life to exist in the colder regions of the world because even when the water freezes on the surface, it remains in liquid state below the ice layer as the density of water is greater than the density of ice and ice is a bad conductor of heat.
3. It can cause pipes to burst in winter season because when water freezes it expands slightly.

The boiling point of water increases and the melting point of water decreases due to the presence of dissolved impurities in it.

The property of anomalous expansion of water enables marine life to exist in the colder regions of the world because even when the water freezes on the surface, it remains in liquid state below the ice layer as the density of water is greater than the density of ice and ice is a bad conductor of heat.

Water is composed of Hydrogen and Oxygen in the ratio of 2:1. Its properties are distinct from those of its component elements in the following ways:

At 4°C, water has its maximum density, 1g/cm³ or 1000 kg/m³, and minimum volume. Its density decreases as the temperature increases or decreases from this point. This enables marine life to exist in the colder regions of the world because even when the water freezes on the surface, it remains in liquid state below the ice layer as the density of water is greater than the density of ice and ice is a bad conductor of heat.

Observation — On looking at the watch glass against light, a number of concentric rings of solid matter are seen. These are the dissolved solids left behind after evaporation of water.

Conclusion — Tap water contains dissolved solids.

Importance of dissolved salts in water are as follows:

1. Salts and minerals are essential for the growth and development of plants.
2. They add taste to water.
3. They supply the essential minerals needed by our bodies.

Solubility of CO₂, and O₂ in water is important for the following reasons:

1. Marine life like fish use the oxygen of the air dissolved in water for respiration and thus aquatic life is sustained. 1 dm³ (1 litre) of water contains 40 cm³ of dissolved oxygen.
2. Aquatic plants make use of dissolved carbon dioxide for photosynthesis, i.e., to prepare their food.
   6CO₂ + 12H₂O $\xrightarrow[\text{sunlight}]{\text{Chlorophyll}}$ C₆H₁₂O₆ + 6O₂ + 6H₂O
3. Carbon dioxide dissolved in water reacts with limestone to form calcium bicarbonate.
   CaCO₃ + CO₂ + H₂O ⟶ Ca(HCO₃)₂
   Marine organisms such as snails, oysters, etc., extract calcium carbonate from calcium bicarbonate to build their shells.

Ordinary air contains 78% nitrogen, 21% oxygen and 0.01% carbon dioxide. But oxygen is more soluble in water as compared to nitrogen. Hence, the composition of air dissolved is different from ordinary air.

The composition of air dissolved in water is 33% oxygen, 66% nitrogen and 1% carbon dioxide.

(B) Latent heat of vaporization — It is the heat energy required to change water into its vapour at its boiling point without any change in temperature.

(C) Latent heat of condensation — It is the heat energy released when a gas converts to liquid.

(D) Latent heat of solidification — It is the heat energy released when a liquid converts to solid.

(a) Pure water is tasteless. The taste in water is due to the gases and solids dissolved in it i.e., impurities present in it. As boiled and distilled water are pure containing no impurities hence they taste flat i.e., are tasteless.

(b) The latent heat of fusion of ice is 336 J g^-1 so it absorbs 336 J of heat and changes to water at 0°C and then water at 0°C absorbs heat and temperature is raised. Therefore, ice at 0°C absorbs extra heat in comparison to water at 0°C (due to latent heat of fusion). Hence, ice at 0°C has greater cooling effect than water at 0°C.

(c) Steam has a higher heat content on account of high specific latent heat of condensation that is 2268 J g^-1. Hence, steam at 100°C carries more heat than water. Therefore, burns caused by steam are more severe than burns caused by boiling water.

(d) Rivers and lakes do not freeze easily because the specific latent heat of fusion of ice is sufficiently high (= 336 J g^-1). The water in lakes and rivers will have to liberate a large quantity of heat to the surrounding before freezing. The layer of ice formed over the water surface, being a poor conductor of heat, will also prevent the loss of heat from the water of lake, hence the water does not freeze all at once.

(e) Air dissolved in water contains a higher proportion of oxygen because oxygen is more soluble in water compared to nitrogen. The composition of air dissolved in water is 33% oxygen compared to 21% in ordinary air.

(f) Substances that are apparently insoluble in water actually dissolve in it in traces. Even, when we put water in a glass vessel, an extremely small amount of glass dissolves in it. It is for this reason that when distilled water is kept in a sealed bottle for a long time, it leaves etchings on the inside surface of the glass.

(g) As rainwater does not contain dissolved solids hence, it does not leave behind concentric rings when boiled.

Given,
Mass of solute = 136 g
Mass of solvent = 500 g

At the temperature 293 K

Solubility = $\frac{\text{Mass of solute}}{\text{Mass of solvent}}$ x 100

= $\frac{136}{500}$ x 100

= 27.2 g

∴ The solubility of given salt at 293 K is 27.2 g

Given,
Mass of solute = 15 g
Mass of solvent = 285 g
Mass of solution = Mass of solute + Mass of solvent
              = 15 + 285
              = 300

Mass percent = $\frac{\text{Mass of solute}}{\text{Mass of solution}}$ x 100

= $\frac{15}{300}$ x 100

= 5%

∴ Concentration of sodium chloride in 285 g of water is 5%

Given,
Volume of solute = 4 litres
Volume of solution = 90 litres

Volume percent = $\frac{\text{Volume of solute}}{\text{Volume of solution}}$ x 100

= $\frac{4}{90}$ x 100

= 4.44 %

∴ Volume percent of acetone in 90 litres of aqueous solution is 4.44 %

(a) From the table, the solubility of KNO₃ at 313 K is 62 g per 100 g of water.

62 g of KNO₃ per 100 g of water

Mass of solute per 50 g of water is

= $\frac{62}{100}$ x 50

= 31 g

∴ Mass of solute per 50 g of water is 31 g.

(b) The solubility of KCl at 353 K is 54 g per 100 g of water.

The solubility of KCl at 293 K (room temperature) is 35 g per 100 g of water.

When the solution cools, the solubility decreases.

The excess KCl will precipitate out of the solution as crystals.

(c) Considering given salts are dissolved in 100g of water

Solubility of KNO₃ at 293 K

Mass of solute = 32 g
Mass of solvent = 100 g

Solubility = $\frac{\text{Mass of solute}}{\text{Mass of solvent}}$ x 100

= $\frac{32}{100}$ x 100

= 32 g

∴ Solubility of KNO₃ at 293 K is 32 g.

Solubility of NaCl at 293 K

Mass of solute = 36 g
Mass of solvent = 100 g

Solubility = $\frac{\text{Mass of solute}}{\text{Mass of solvent}}$ x 100

= $\frac{36}{100}$ x 100

= 36 g

∴ Solubility of NaCl at 293 K is 36 g.

Solubility of KCl at 293 K

Mass of solute = 35 g
Mass of solvent = 100 g

Solubility = $\frac{\text{Mass of solute}}{\text{Mass of solvent}}$ x 100

= $\frac{35}{100}$ x 100

= 35 g

∴ Solubility of KCl at 293 K is 35 g.

Solubility of NH₄Cl at 293 K

Mass of solute = 37 g
Mass of solvent = 100 g

Solubility = $\frac{\text{Mass of solute}}{\text{Mass of solvent}}$ x 100

= $\frac{37}{100}$ x 100

= 37 g

∴ Solubility of NH₄Cl at 293 K is 37 g.

(d) From the table, when we calculate the solubility of KNO₃ at 283 K,

Mass of solute = 21 g
Mass of solvent = 100 g

Solubility = $\frac{\text{Mass of solute}}{\text{Mass of solvent}}$ x 100

= $\frac{21}{100}$ x 100

= 21 g

Whereas, the solubility of NaCl, KCl and NH₄Cl will be 36 g, 35 g and 24 g respectively.

Hence, the least solubility will be of KNO₃ at 283 K

(e) The solubility of most salts in water usually increases with rise in temperature. Example, Potassium nitrate.
There are some salts which show anomalous solubility. Their solubility first increases, and then decreases, with rise in temperature.
Example: Na₂SO₄.10H₂O (Glauber's salt).
Solubility curve of Na₂SO₄.10H₂O rises till it reaches 32.8°C, and then it falls slightly. This is because Na₂SO₄.10H₂O is hydrous below 32.8°C and loses its water and becomes anhydrous above 32.8°C.

Given,
Mass of empty dish (M) = 50 g
Mass of dish and solution (M₁ ) = 65 g
Mass of dish and residue (M₂ ) = 54.3 g

Mass of saturated solution = M₁ - M = 65 - 50 = 15 g
Mass of solute = M₂ - M = 54.3 - 50 = 4.3 g

Mass of solvent = mass of saturated solution - mass of solute
              = (M₁ - M) - (M₂ - M)
              = 15 - 4.3
              = 10.7 g

Solubility = $\frac{\text{Mass of solute}}{\text{Mass of solvent}}$ x 100

= $\frac{4.3}{10.7}$ x 100

= 40.18 g

∴ Solubility of KNO₃ at 20°C is 40.18 g.

Given,
Mass of solvent = 50 g

Solubility of NaNO₃ at 50°C = 114 g

Solubility of NaNO₃ at 30°C = 86 g

Solubility = $\frac{\text{Mass of solute}}{\text{Mass of solvent}}$ x 100

Rearranging the above equation

Mass of solute = $\frac{\text{Solubility x Mass of solvent}}{100}$

∴ Mass of solute at 50°C = $\frac{114}{100}$ x 50

= 57 g

∴ Mass of solute at 30°C = $\frac{86}{100}$ x 50

= 43 g

∴ Amount of NaNO₃ that separate = 57 g - 43 g = 14 g

Given,
Mass of solute = 36 g
Mass of solvent = 100 g
Mass of solution = Mass of solute + Mass of solvent
= 36 + 100
= 136 g

Mass percent = $\frac{\text{Mass of solute}}{\text{Mass of solution}}$ x 100

= $\frac{36}{136}$ x 100

= 26.47%

∴ Concentration of sodium chloride in 100 g of water at 293 K is 26.47%

Air is a homogeneous mixture of a number of gases like nitrogen, oxygen, carbon dioxide, water vapour and inert gases, etc. These gases in air are physically mixed, not chemically bonded to each other so, air cannot be a compound. It cannot be an element as element contains only one type of atom. Hence, we can say that air is a mixture and not an element or compound.

Hydrogen chloride gas is a pure substance, but when it is dissolved in water then it is a homogeneous mixture.

(a) The antiseptic liquid was not effective because antiseptics are mainly used to kill or prevent the growth of microorganisms, not to stop bleeding. They do not have properties that help in clotting blood.

(b) Blood consists of haemoglobin, which are charged colloidal particles. Alum dissolves in water to provide ions. As they come in contact with blood, they neutralize the charge on the colloidal particles. The blood becomes thick and bleeding stops.

(c) Rani’s husband has a better understanding in this situation.
He knew that if alum is rubbed on the cut, blood stops flowing as alum neutralizes the charge on the colloidal particles of haemoglobin.

The solution is a colloidal solution as it shows Tyndall effect.

(a) A mixture of blue copper sulphate and potassium permanganate dissolved in water.
Both are coloured and dissolve to form a uniform (homogeneous) solution.

(b) A mixture of oil and water.
They do not mix and form separate layers, making it a heterogeneous mixture.

The colloids are milk, smoke, butter, face cream, and jelly.

Explanation:

Milk : It is an emulsion.

Smoke : It is a solid aerosol.

Butter : It is a gel.

Face cream : It is an emulsion.

Jelly : It is a gel.

Copper sulphate solution, sugar solution, lemonade, soda water are true solutions and do not show scattering of light. Muddy water is a suspension.

(a) Fog
Dispersed phase : Liquid (tiny water droplets)
Dispersion medium : Gas (air)

(b) Cheese
Dispersed phase : Liquid (water droplets)
Dispersion medium : Solid (protein/fat matrix)

(c) Coloured gem stones
Dispersed phase : Solid (metal oxides or impurities giving colour)
Dispersion medium : Solid (the gemstone itself, like silica)

Starch solution exhibits the Tyndall effect.

Explanation:
Tyndall effect is shown by colloidal solutions, not by true solutions.
Sodium chloride solution and Tincture of iodine are true solutions. Air is a true mixture of gases. Therefore, they do not show Tyndall effect.

Three differences between true solutions and suspensions are :

The type of colloid in each of the following are :

Colloids are different from true solutions in following ways :

Comparison between suspensions and colloidal solutions are :

Colloids scatter a beam of light passing through them. This phenomenon is known as the Tyndall effect.

Milk and starch solution will show Tyndall effect.

Explanation:
Milk and starch solution are colloids. Both have large particles to scatter light. Thus, they show Tyndall effect.
Salt and copper sulphate solutions are true solutions. They have small particles and do not scatter light. Thus, they do not show Tyndall effect.

(a) Solution — A solution is a homogeneous mixture of two or more components whose composition may be gradually changed by changing the relative amounts of the components.

(b) Solute — The substance that dissolves in the solvent to form a solution is known as solute.

(c) Solvent — The medium of dissolution that allows one or more components to dissolve in it to form a solution is known as solvent.

The solubility of a solution usually decreases with a fall in its temperature. Hence, if the temperature of a saturated solution is lowered, a part of the dissolved solute separates out in the form of crystals.

The rate of dissolution or rate of solubility of a solid in a solvent depends on the following factors:

1. Size of solute particles — The smaller the size of the solute particles, the greater is its total surface area exposed to the solvent. Therefore, greater is the solubility of that solute.
2. Stirring — This brings more of the solvent in contact with the solute and thus increases the rate of formation of solution.
3. Temperature — The solubility of a gas in a liquid always decreases with rise in temperature. But the solubility of most solids in water usually increases with rise in temperature.

(a) Preparation of saturated solution of copper sulphate crystals:
Take 100 g of distilled water in a beaker. Add one gram of copper sulphate crystals and stir with a glass rod till the crystals dissolve. Add one more gram of copper sulphate crystals and stir it. It too will dissolve. Continue adding a gram of copper sulphate crystals and stir vigorously after each addition.
A stage is reached when no more copper sulphate dissolves. At this stage, we have a saturated solution of copper sulphate at room temperature.

(b) Add some more copper sulphate crystals to the saturated solution. The crystals do not dissolve no matter how long it is left there or how vigorously it is stirred.

KNO₃ shows a considerable increase in solubility with rise in temperature.

Solubility of calcium sulphate decreases (after attaining a certain temperature) with further rise in temperature.

Solubility of NaCl at 40°C is 36.5 g means that 36.5 g of NaCl dissolves in 100 g of water at 40°C.

Add a small amount of solute in the solution and stir it while keeping the temperature constant.

1. If more solute does not dissolve in the given solution, then it will be a saturated solution.
2. If the solute gets dissolved, then it is an unsaturated solution.
3. If on slightly disturbing the solution by shaking, stirring, scratching the wall of container or adding a solute crystal to the solution, the additional amount of the solute precipitates out, then the solution is a supersaturated solution.

When a saturated solution of nitre (KNO₃) prepared at 60°C is allowed to cool at room temperature, the solubility of KNO₃ decreases. The solution can no longer hold the same amount of dissolved KNO₃, so the excess KNO₃ separates out in the form of crystals.

(a) (i) Henry’s law — At any given temperature, the mass of a gas dissolved by a fixed volume of liquid is directly proportional to the pressure on the surface of the liquid.

(ii) Crystallisation — It is a process by which crystals of a substance are obtained by cooling a a hot saturated solution.

(iii) Seeding — It is a process of inducing crystallisation by adding a crystal of a pure substance into its saturated solution.

(b) Three methods of crystallisation are:

1. By cooling a a hot saturated solution gently
2. By cooling a fused mass
3. By sublimation

Drops of colourless liquid condense on the cooler parts of the test tube, leaving behind a residue that is anhydrous (without water) and amorphous (non-crystalline), i.e., with no definite shape or structure.

CuSO₄.5H₂O ⇌ CuSO₄ + 5H₂O

(a) hydrated substance —

1. Sodium carbonate decahydrate [Washing soda crystals] : Na₂CO₃.10H₂O
2. Copper(II) sulphate pentahydrate [Blue vitriol]: CuSO₄.5H₂O

(b) anhydrous substance —

1. Anhydrous sodium chloride [table salt] : NaCl
2. Nitre (KNO₃)

(c) liquid drying agent —

1. Sulphuric acid: H₂SO₄
2. Phosphoric acid: H₃PO₄

(d) a basic drying agent —

1. Quick lime: CaO
2. Potassium carbonate: K₂CO₃

An increase in pressure on the surface of water increases the solubility of a gas in water.

For example: the solubility of carbon dioxide in water under normal atmospheric pressure is rather low, but when the water surface is subjected to higher pressure, a lot more carbon dioxide gas dissolves in it, as is seen in the case of soda water. On opening the soda water bottle, the dissolved gas rapidly bubbles out since pressure on the surface of the water suddenly decreases.

(a) Non-aqueous solution

(b) Deliquescence

(c) Hydrated substance

(d) Hygroscopy

(e) Efflorescence

(f) Dehydrating agent

(a) Water is an effective coolant. By allowing water to flow in pipes around the heated parts of a machine, heat energy from such parts is removed. Water in pipes can extract more heat from the surroundings without much rise in its temperature because of its high specific heat capacity. This is why radiators in cars and generators use water for cooling.

(b) Solutions are homogeneous in nature. The solute particles are dispersed uniformly throughout the solvent. The size of particles of a true solution is very small less than 10^-9 m. Due to homogeneity and small size, they do not scatter light significantly. Hence, a solution is always clear and transparent.

(c) Rivers and lakes do not freeze suddenly because the specific latent heat of fusion of ice is sufficiently high (= 336 J g^-1). The water in lakes and rivers will have to liberate a large quantity of heat to the surrounding before freezing. The layer of ice formed over the water surface, being a poor conductor of heat, will also prevent the loss of heat from the water of lake, hence the water does not freeze all at once.

(d) The size of particles of a true solution is less than 10^-9 m. In a solution, the solute particles and the solvent molecules cannot be distinguished even under a microscope. Due to such small size of particles, the solute cannot be separated from a solution by filtration.

(e) Fused CaCl₂ is deliquescent in nature, absorbs moisture and hence is used as a drying agent or desiccating agent, similarly, conc. sulphuric acid is hygroscopic in nature and can remove moisture from other substances; Hence, they are used as drying agents.

(f) The solubility of carbon dioxide in water under normal atmospheric pressure is rather low, but when the water surface is subjected to higher pressure, a lot more carbon dioxide gas dissolves in it, because, an increase in pressure on the surface of water increases the solubility of the gas in water. On opening the soda water bottle, the dissolved gas rapidly bubbles out since pressure on the surface of the water suddenly decreases. Hence, effervescence is seen on opening a bottle of soda water.

(g) Table salt [sodium chloride] contains impurities like magnesium chloride and calcium chloride, which are deliquescent. Hence, table salt absorbs moisture in rainy season and turns sticky.

No, solubility of a crystalline solid does not increase uniformly with temperature in all cases.

(a) Solubility increases rapidly with temperature — Potassium nitrate

(b) Solubility increases gradually with temperature — Potassium chloride

(c) Solubility increases slightly with temperature — Sodium chloride

(d) Solubility initially increases then decreases with rise in temperature — Calcium sulphate

Drying or desiccating agents are substances that can readily absorb moisture from other substances without chemically reacting with them.

Ex: Conc. Sulphuric acid [H₂SO₄], Phosphorus pentoxide [P₂O₅], Quicklime [CaO], Silica gel.

(a) Increase in mass: Iron, conc. sulphuric acid, Table salt

Reason — Increase in mass is due to the absorbed water in case of sulphuric acid, Table salt, whereas, gain in mass of iron is due to the increased weight of oxygen which has combined with the iron to form iron oxide or rust.

(b) Decrease in mass: Sodium carbonate crystals

Reason — Decrease in mass is because sodium carbonate loses its water of crystallisation on exposure to dry air.

(c) No change in mass: Sodium chloride

Reason — Pure sodium chloride is neither deliquescent nor efflorescent i.e., it does not absorb moisture from atmospheric air nor does it lose it, hence there is no change in mass.

Hydrated salts can be made anhydrous by:

1. direct heating
2. heating in dry or hot air
3. heating under vacuum
4. by using dehydrating/desiccating agents such as warm concentrated sulphuric acid.

Water

Reason — Water has a high dielectric constant, as a result it reduces the electrostatic force of attraction between the positive and negative ions and dissolves even inorganic compounds, which are usually electrovalent. Hence, sodium sulphate is dissolves in water.
Ether is a non-polar solvent, while Sodium sulphate is an ionic compound. Non-polar solvents cannot dissolve ionic salts, so no solubility. Alcohol (like ethanol) is partially polar, but not enough to effectively dissolve ionic salts. So sodium sulphate is only very slightly soluble or considered insoluble. Benzene is a non-polar solvent. It cannot break ionic bonds or stabilize ions, so no solubility.

It is polar and has a high dielectric constant.

Reason — Water is a polar covalent compound having a high dielectric constant. This makes water a universal solvent as it helps dissolve even inorganic compounds by reducing the electrostatic force of attraction between the ions.
Water is inorganic, not organic. So this is not the reason. Being liquid helps in dissolving substances, but it is not the main reason for its strong dissolving power. Boiling point has no direct role in making water a universal solvent.

Adding sodium carbonate

Reason — Permanent hardness of water can be removed by addition of washing soda [sodium carbonate]:

Permanent Hard Water:

$\underset{\text{Washing soda}}{\text{Na}_2\text{CO}_3} + \underset{\text{Calcium sulphate}}{\text{CaSO}_4} \longrightarrow \underset{\text{Calcium carbonate}}{\text{CaCO}_3↓} + \text{Na}_2\text{SO}_4$

Calcium sulphate itself causes permanent hardness, so adding it will worsen the problem. Potassium chloride does not remove hardness. It has no effect on calcium or magnesium salts causing hardness. Boiling removes only temporary hardness (due to bicarbonates), not permanent hardness.

Alloys

Reason — A homogeneous solution of a solid into another solid is called a solid solution and common metal alloys are solid solutions because an alloy is also a metal made by mixing two types of metals together.
Allotropy refers to different forms of the same element (e.g., carbon as diamond and graphite), not solutions. Isotopes are atoms of the same element with different masses, not mixtures. Isotones relate to atomic structure, not solutions.

Increase slightly

Reason — In an endothermic process, the solubility of a solute increases with an increase in temperature.

For example: solubility of sodium chloride increases with rise in temperature.

Only some salts increase rapidly. NaCl does not become less soluble with temperature. There is a slight change, so it is not completely constant.

Calcium hydroxide

Reason — In an exothermic process, the solubility increases on lowering the temperature.

For example: solubility of calcium sulphate and calcium hydroxide in water decreases on increasing the temperature.

Potassium chloride shows a slight increase in solubility with rise in temperature. Calcium hydroxide has very low solubility, but it does not show a clear decreasing trend with temperature in standard school context. Potassium nitrate shows a rapid increase in solubility with rise in temperature.

Potassium permanganate

Reason — The crystalline shape of a substance is not necessarily the result of the presence of water of crystallisation. In fact, there are number of crystalline solids that crystallize from water without holding any water of crystallisation. For example: Potassium permanganate

Plaster of Paris contains water of crystallisation (it is a hydrated salt). Potassium alum contains 12 molecules of water of crystallisation. Magnesium sulphate heptahydrate contains 7 molecules of water of crystallisation.

It contains impurities which are deliquescent

Reason — Table salt [sodium chloride] contains impurities like magnesium chloride and calcium chloride, which are deliquescent. Hence, table salt absorbs moisture in rainy season and turns sticky.
Pure sodium chloride is not deliquescent. NaCl does not absorb moisture directly; hygroscopic substances only absorb water without dissolving. Drying agents absorb moisture; table salt is not used for this purpose.

Conc sulphuric acid

Reason — Being hygroscopic sulphuric acid absorbs moisture but not enough to form a solution. Hence, it is a drying agent.

Sulphuric acid is a dehydrating agent as it can remove chemically combined water molecules from blue vitriol.

CuSO₄.5H₂O + H₂SO₄ ⇌ CuSO₄ + 5H₂O

Oxalic acid is an organic acid and does not act as a strong drying or dehydrating agent. Nitric acid is a strong acid and oxidising agent, but it is not mainly used as a drying or dehydrating agent. Sodium carbonate decahydrate is a basic salt used for cleaning, not for drying or dehydration.

FeCl₃

Reason — FeCl₃ absorbs moisture when exposed to the atmosphere and ultimately dissolves in the absorbed water. Hence, it is a deliquescent salt.
Copper(II) sulphate is not deliquescent in its normal crystalline form (it is hygroscopic only in hydrated form). Potassium chloride is stable in air and does not absorb moisture. Zinc sulphate is hygroscopic in hydrated form but not strongly deliquescent.

Anhydrous

Reason — A salt which does not contain water of crystallisation is called an anhydrous salt. Since it has no water molecules attached to its crystals, it does not give water vapour on heating.
Hydrated salts contain water of crystallisation and give water vapour on heating. Hence, the correct answer is Anhydrous.

Magnesium bicarbonate

Reason — Hardness in water is due to the presence of bicarbonates, chlorides or sulphates of calcium or magnesium.
Sodium sulphate does not cause hardness because sodium salts are soluble and do not affect soap action. Sodium chloride is a common salt and does not cause hardness. Calcium nitrate is soluble but does not significantly contribute to hardness in the usual sense taught at this level.

Boiling

Reason — By boiling carbon dioxide is driven off and the soluble hydrogen carbonates are converted into insoluble carbonates and could be removed by filtration or decantation.

Calcium carbonate and magnesium carbonate are precipitated leaving the water soft.

Ca(HCO₃)₂ $\xrightarrow{\text{Boil}\space}$ CaCO₃ ↓ + H₂O + CO₂ ↑

Mg(HCO₃)₂ $\xrightarrow{\text{Boil}\space}$ MgCO₃ ↓ + H₂O + CO₂ ↑

Sodium chloride does not remove hardness. Calcium carbonate is insoluble and does not remove hardness. Temporary hardness does not disappear on standing; it requires heating.

Only R

Reason — Water of crystallisation is the fixed number of water molecules chemically bound to a salt in its crystalline form. Salts like blue vitriol (CuSO₄.5H₂O) and gypsum (CaSO₄.2H₂O) are examples of hydrated salts. But baking soda (NaHCO₃) doesn't have any water molecules in its formula. Therefore, it does not contain any water of crystallisation.

Both Q and R

Reason — Washing soda (Na₂CO₃.10H₂O) and baking soda (NaHCO₃) are made from sodium chloride using the Solvay process.

NaCl + NH₃ + CO₂ + H₂O ⟶ NaHCO₃ + NH₄Cl

NaHCO₃ is filtered and heated to get sodium carbonate: ​

2NaHCO₃ $\xrightarrow{\Delta}$ Na₂CO₃ + CO₂ + H₂O

Sodium carbonate (Na₂CO₃) is then hydrated to form washing soda.

Only Q

Reason — A is Glauber's salt and B is sodium sulphate because the solubility curve of Na₂SO₄.10H₂O (Glauber's salt) rises till it reaches 32.8°C, and then it falls slightly. This is because Na₂SO₄.10H₂O is hydrous below 32.8°C and anhydrous above it.
Whereas solubility curve of sodium chloride and gypsum are almost flat, showing minimal change in solubility as temperature rises.

(a) Sugar is solute, water is solvent

(b) Water is a polar covalent compound having a dielectric constant which makes it a universal solvent.

(c) Solubility of Glauber's salt [Na₂SO₄.10H₂O] shows anomalous behaviour.

(d) Potassium nitrate

Water is composed of two atoms of hydrogen and one atom of oxygen in the ratio of 2:1.

(i) Hydrogen carbonates of calcium and magnesium make water temporary hard.

(ii) Sulphates and chlorides of magnesium and calcium make water permanently hard.

Hydrated substances can be made anhydrous by:

1. Direct heating
2. Heating in dry or hot air
3. Heating under vacuum
4. By using dehydrating/desiccating agents such as warm concentrated sulphuric acid.

Solubility of O₂, and CO₂ in water is important for the following reasons:

1. Marine life like fish use the oxygen of the air dissolved in water for respiration and thus aquatic life is sustained. 1 dm³ (1 litre) of water contains 40 cm³ of dissolved oxygen.
2. Aquatic plants make use of dissolved carbon dioxide for photosynthesis, i.e., to prepare their food.
   6CO₂ + 12H₂O $\xrightarrow[\text{sunlight}]{\text{Chlorophyll}}$ C₆H₁₂O₆ + 6O₂ + 6H₂O
3. Carbon dioxide dissolved in water reacts with limestone to form calcium bicarbonate.
   CaCO₃ + CO₂ + H₂O ⟶ Ca(HCO₃)₂
   Marine organisms such as snails, oysters, etc., extract calcium carbonate from calcium bicarbonate to build their shells.

As solubility of sodium nitrate decreases with decrease in temperature, hence, a part of the dissolved solute separates out in the form of crystals when temperature falls.

(a) Soft water — Water is said to be soft water if it readily forms lather with soap.

(b) Hard water — Water is said to be hard if it does not readily form lather with soap.

(c) Temporary Hard water — Water that contains only hydrogen carbonates of Calcium and Magnesium is called temporary hard water.

(d) Permanent hard water — Water that contains sulphates and chlorides of Magnesium and Calcium is called Permanent hard water.

(a) The presence of hydrogen carbonates of calcium and magnesium makes water temporarily hard.

(b) The presence of sulphates and chlorides of magnesium and calcium makes water permanently hard.

A saturated solution can be changed to an unsaturated solution by:

1. heating.
2. adding more solvent.

Soap is chemically a sodium salt of stearic acid (an organic acid with the formula C₁₇H₃₅COOH) and has the formula C₁₇H₃₅COONa (sodium stearate, can be represented by NaSt).

Soap is used for washing & cleaning purposes.

Detergents are more soluble in water than soap and are unaffected by hardness of water as their calcium and magnesium salts are soluble in water so they do not form scum and cleaning action is easily done.

Chemically pure water is not necessarily potable water because potable water must be free from harmful microorganisms, parasites, toxic chemicals and pollutants. It should also be clear, odourless and safe for regular drinking. Hence, chemically pure water is not called potable unless it is safe for human consumption.

Water that looks clean may still contain invisible, harmful bacteria, dissolved impurities, so clarity and lack of smell do not guarantee safety for drinking.

We should not allow household drainage water to flow into nearby water bodies because it contains organic wastes, disease-causing germs and harmful substances. These pollutants contaminate water, make it unsafe for drinking and increase the risk of water-borne diseases such as diarrhoea, typhoid, cholera and dysentery. The polluted water can also harm aquatic life. Therefore, household drainage water should be properly treated before disposal.

Three water-borne diseases that can be prevented by drinking potable water are :

1. Cholera
2. Typhoid
3. Dysentery

Residents who drink untreated river water may face serious health risks because river water can be contaminated by sewage, animal wastes, industrial wastes, pesticides and chemical fertilisers. Such polluted water may contain harmful microorganisms that cause water-borne diseases such as diarrhoea, typhoid, cholera, dysentery, hepatitis A and polio. It may also contain toxic chemicals, pesticides and heavy metals, which can cause long-term health problems such as cancer and organ damage, especially affecting the liver and kidneys.

Substances which contain water molecules along with salts like Sodium carbonate decahydrate [Washing soda crystals — Na₂CO₃.10H₂O] and Copper(II) sulphate pentahydrate [Blue vitriol — CuSO₄.5H₂O] are hydrated substances. This water gives the crystals their shape. In some cases it also gives them their colour (copper sulphate crystals are blue in colour).

Importance of dissolved impurities in water are as follows:

1. Salts and minerals are essential for growth and development of plants.
2. They add taste to water.
3. They supply the essential minerals needed by our body.

(i) Advantages of soft water

1. With soft water soaps and cleansing agents are consumed less, hence, money is saved.
2. Soft water does not leave deposits of minerals on pipes which makes plumbing works easy.
3. Clothes washed with soft water lasts long and stay bright.

(ii) Advantages of hard water

1. The presence of salts in hard water makes it tasty. It is used in preparation of beverages and wine.
2. Calcium and magnesium salts present in small amounts in hard water are essential for the growth of our bones and teeth.
3. Hard water checks poisoning of water by lead pipes. When these pipes are used for carrying water, some lead salts dissolve in water to make it poisonous. Calcium sulphate present in hard water forms insoluble lead sulphate in the form of a layer inside the lead pipe and this checks the lead poisoning.

In some limestone caves, sometimes conical pillar-like objects hanging from the roof of the caves and some rising from the floors are seen. These conical pillars which grow upward from the floor of the caves are known as stalagmites, and the structures which grow downwards from the roof are called stalactites.

These are formed by water dripping from the cracks in the rocks containing calcium hydrogen carbonate. Calcium hydrogen carbonate converts to calcium carbonate when pressure is released. Gradually, calcium carbonate deposits both on roof and floor to form stalagmites and stalactites.

(a) By boiling carbon dioxide is driven off and the soluble hydrogen carbonates are converted into insoluble carbonates and could be removed by filtration or decantation.

Calcium carbonate and magnesium carbonate are precipitated leaving the water soft.

Ca(HCO₃)₂ $\xrightarrow{\text{Boil}\space}$ CaCO₃ ↓ + H₂O + CO₂ ↑

Mg(HCO₃)₂ $\xrightarrow{\text{Boil}\space}$ MgCO₃ ↓ + H₂O + CO₂ ↑

(b) Lime stone is first thoroughly mixed with water in a tank and then fed into another tank containing the hard water. Revolving paddles thoroughly mix the two solutions.

Most of calcium carbonate settles down. If there is any solid left over, it is removed by a filter. This process goes by the name 'Clark's process'.

Ca(HCO₃)₂ + Ca(OH)₂ ⟶ 2CaCO₃ ↓ + 2H₂O

Mg(HCO₃)₂ + Ca(OH)₂ ⟶ MgCO₃ ↓ + CaCO₃ ↓ + 2H₂O

1. Furring of tea kettles is caused by sediment formed from boiling hard water. This fur is carbonates of calcium and magnesium.
2. Hard water is unfit for washing purposes because it is difficult to form lather with soap. Scum may form in a reaction with soap, wasting the soap.
3. Hard water is not suitable for producing steam. Solids in hard water incapable of changing into steam get deposited on the inner walls of the tubes. Hence, the tubes become narrower and eventually less steam is produced.

(i) The dissolved substances present in hard water do not convert into steam and get deposited on the inner walls of the tube. Hence, the tubes become narrower and eventually less steam is produced.

When bore of the tube becomes very narrow, the pressure of steam increases so much that at times the boiler itself bursts. Hence, hard water is unfit for use in boilers.

(ii) If the water is hard, calcium and magnesium ions of the water combine with the negative ions of the soap to form a slimy precipitate of insoluble calcium and magnesium usually called soap curd (scum).

Formation of soap curd will go on as long as calcium and magnesium ions are present. Till then, no soap lather will be formed and cleaning of clothes or body will not be possible. Moreover, these precipitates are difficult to wash from fabrics and sometimes form rusty spots if iron salts are present in water.

(a) Permanent hardness due to magnesium salts can be partly removed by slaked lime because magnesium ions form insoluble magnesium hydroxide.

MgSO₄ + Ca(OH)₂ ⟶ Mg(OH)₂ ↓ + CaSO₄

However, this alone does not completely soften water because calcium sulphate remains in solution. Washing soda is required for complete removal of permanent hardness.

(b) Washing soda is added to hard water, which results in settling down of insoluble carbonates which can be removed by filtration.

$\underset{\text{Washing soda}}{\text{Na}_2\text{CO}_3} + \underset{\text{Calcium sulphate}}{\text{CaSO}_4} \longrightarrow \underset{\text{Calcium carbonate}}{\text{CaCO}_3↓} + \text{Na}_2\text{SO}_4$

$\underset{\text{Washing soda}}{\text{Na}_2\text{CO}_3} + \underset{\text{Calcium chloride}}{\text{CaCl}_2} \longrightarrow \underset{\text{Calcium carbonate}}{\text{CaCO}_3↓} + 2\text{NaCl}$

Permutit is an artificial zeolite. Chemically, it is hydrated sodium aluminium orthosilicate, having the formula Na₂Al₂Si₂O₈.XH₂O. For the sake of convenience, let us give it the formula Na₂P.

A tall cylinder is loosely filled with lumps of permutit. When hard water containing calcium and magnesium ions percolates through these lumps, exchange of ions takes place. The sodium permutit is slowly changed into calcium and magnesium permutit, and with the removal of calcium and magnesium ions, the water becomes soft.

When no longer active, the permutit is regenerated by running a concentrated solution of brine over it and removing the calcium chloride formed by repeated washing.

CaP + 2NaCl ⟶ Na₂P + CaCl₂

If the water is hard, calcium and magnesium ions of the water combine with the negative ions of the soap to form a slimy precipitate of insoluble calcium and magnesium usually called soap curd (scum).

$\underset{\text{[soap]}}{2\text{NaSt}} + {\text{Ca(HCO}_3)_2} \longrightarrow \underset{\text{[soap curd]}}{\text{CaSt}_2} ↓ + {2\text{NaHCO}_3}$

${2\text{NaSt}} + {\text{Mg(HCO}_3)_2} \longrightarrow {\text{MgSt}_2} ↓ + {2\text{NaHCO}_3}$

In permanent hard water, the formation of soap-curd will go on as long as there are calcium and magnesium ions present.

${2\text{NaSt}} + {\text{CaCl}_2} {\longrightarrow} {\text{CaSt}_2} ↓ + {2\text{NaCl}}$

${2\text{NaSt}} + {\text{MgCl}_2} {\longrightarrow} {\text{MgSt}_2} ↓ + {2\text{NaCl}}$

${2\text{NaSt}} + {\text{CaSO}_4} {\longrightarrow} {\text{CaSt}_2} ↓ + {\text{Na}_2{\text{SO}_4}}$

${2\text{NaSt}} + {\text{MgSO}_4} {\longrightarrow} {\text{MgSt}_2} ↓ + {\text{Na}_2{\text{SO}_4}}$

As long as the formation of soap-curd continues, no soap lather will be formed and the cleaning of cloth or body will not be possible.

(a) Lead is toxic. When water flows through lead pipes, especially soft or acidic water, it can dissolve lead ions into the water. Ingesting lead causes serious health issues, especially in children (like brain and kidney damage).

(b) Chalk is mainly calcium carbonate (CaCO₃). Rainwater absorbs carbon dioxide (CO₂) from the air and forms carbonic acid (H₂CO₃), a weak acid:

CO₂ + H₂O ⟶ H₂CO₃

This reacts with calcium carbonate:

H₂CO₃ + CaCO₃ ⟶ Ca(HCO₃)₂

The product, calcium bicarbonate, is soluble in water, so the chalk slowly dissolves.

(c) The dissolved substances present in hard water do not convert into steam and get deposited on the inner walls of the tube. Hence, the tubes become narrower and eventually less steam is produced.
When bore of the tube becomes very narrow, the pressure of steam increases so much that at times the boiler itself bursts. Hence, hard water is unfit for use in boilers.

(d) Ferric chloride (FeCl₃) is deliquescent, it absorbs moisture from the air and dissolves in the water it absorbs, eventually forming a saturated solution. This property makes it unstable in open air.

(e) Concentrated sulphuric acid H₂SO₄ is highly hygroscopic, meaning it absorbs water vapour from the air. If left open, it will absorb moisture and become diluted. To maintain concentration and prevent hazards, it should always be tightly stoppered.

(a) The definite quantity of water combined with a salt is called Water of Crystallisation.

(b) Salts that lose their water molecules on exposure to air are called Efflorescent salts.
Washing soda and magnesium sulphate heptahydrate are the two examples.

(c) Table salt forms a solution in the rainy season because it contains impurities like Magnesium chloride, which is deliquescent. It absorbs moisture from the air and dissolves in it, forming a solution.

(d) Pure sodium chloride can be obtained by evaporation or crystallisation.

(a) The labelled curves are shown below:

(b) Solubility of a solid in a solvent depends on the following factors:

1. Size of solute particles — The smaller the size of the solute particles, the greater is its total surface area exposed to the solvent. Therefore, greater is the solubility of that solute.
2. Stirring — This brings more of the solvent in contact with the solute and thus increases the rate of formation of solution.
3. Temperature — The solubility of a gas in a liquid always decreases with rise in temperature. But the solubility of most solids in water usually increases with rise in temperature.

In case of NaCl, solubility increases only a little with increase in temperature.

In case of KNO₃, solubility increases considerably with increase in temperature.

In case of calcium sulphate, solubility decreases (after attaining a certain temperature) with further rise in temperature.

(c) (i) In an endothermic process, the solubility of a solute increases with an increase in temperature.

For example: solubility of KNO₃ increases with rise in temperature and solubility of NaCl increases only a little with increase in temperature.

(ii) In an exothermic process, the solubility increases on lowering the temperature.

For example: solubility of calcium sulphate in water decreases on increasing the temperature.

A is true but R is false.

Explanation — Water dissolves many substances, forming aqueous solutions. Not only solids but gases and other liquids can also dissolve in water to a large extent. For this reason, water is called a universal solvent. Hence the assertion (A) is true.

It is true that noble metals and glass do not dissolve in water, but it is not true that water dissolves all other substances. For instance, many non-polar substances, such as oils, do not dissolve in water. Hence reason (R) is false.

Therefore, A is true but R is false is the correct option.

Both A and R are true and R is the correct explanation of A.

Explanation — A is true as when a saturated solution is heated to a higher temperature, then it becomes unsaturated.
R is true as more solute can be dissolved in this solution now.

Hence, both the assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

A is true but R is false.

Explanation — Washing soda (hydrated sodium carbonate), when exposed to dry air, loses water of crystallisation. The result is the formation of white anhydrous sodium carbonate (Na₂CO₃) on the surface, which appears as a white powder. Hence the assertion (A) is true. Washing soda undergoes efflorescence, meaning it loses moisture (its water of crystallisation) to the air. So washing soda is efflorescent, not hygroscopic. Hence the reason (R) is false.

Therefore, A is true but R is false is the correct option.

Both A and R are true and R is the correct explanation of A.

Explanation — R is true as hard water contains bicarbonates of calcium and magnesium.
A is true as when heated in boilers, these decompose into their respective carbonates, which are insoluble and get deposited as a hard, crusty layer on the inner walls of the boiler, known as boiler scale.

Hence, both the assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

Both A and R are true and R is the correct explanation of A.

Explanation — In humid conditions, the atmospheric vapour pressure increases. Therefore, the difference between the vapour pressure of the hydrated salt and the atmosphere is less, and water does not easily escape from the crystals. Thus, efflorescence is minimized. Hence the assertion (A) is true.
Efflorescence occurs when vapour pressure in the hydrated crystals is higher than atmospheric vapour pressure. Hence reason (R) is true.
Reason (R) explains why efflorescence occur and how vapour pressure of crystal varies according to atmospheric humidity. Hence, reason (R) is correct explanation for assertion (A).

Both A and R are true and R is the correct explanation of A.

Explanation — When a saturated solution is heated to a higher temperature, the solubility of most solids increases, meaning the solution can now dissolve more solid solute. So, it becomes unsaturated. Hence the assertion (A) is true. R is true as the solution become unsaturated upon heating, more solute can be dissolved in this solution now.

Hence the reason (R) is true and it is correct explanation for assertion (A).

Both A and R are true and R is the correct explanation of A.

Explanation — A is true as carbon disulphide is a non-aqueous liquid solvent. Phosphorus when dissolved in carbon disulphide forms a non aqueous solution.
R is true as water is not the only solvent. Alcohol, petrol, ether, benzene, carbon disulphide, liquid ammonia, etc. are some non-aqueous liquid solvents in common use. The solutions made in these liquids are known as non-aqueous solutions.

Hence, both the assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

A is true but R is false.

Explanation — Concentrated sulphuric acid absorbs moisture (water vapour) from the atmosphere when it is exposed to air but it will not form solution. Such substances are called hygroscopic substances. Hence the assertion (A) is true.
Conc. H₂SO₄ is hygroscopic substances not deliquescent substance because deliquescent substance absorbs moisture from the air until it dissolves completely in the absorbed water, forming a solution. Hence the reason (R) is false.

Therefore, A is true but R is false is the correct option.

A is false but R is true.

Explanation — *Hardness of temporary hard water which contains only hydrogen carbonates of calcium and magnesium can be removed just by boiling. Water containing sulphates and chlorides, of magnesium and calcium is called permanent hard water. This hardness cannot be removed by boiling. Hence the assertion (A) is false.
Water of some springs, wells and rivers contain dissolved mineral matter like hydrogen carbonates, sulphates or chlorides of calcium and magnesium, turning the water hard. Hence the reason (R) is true.

Therefore, A is false but R is true is the correct option.

(a) One formula unit of hydrated copper sulphate contains 5 water molecules of crystallisation.

(b) On heating, hydrated copper sulphate loses its water of crystallisation and forms anhydrous copper sulphate (white powder).

(c) The white anhydrous copper sulphate becomes blue again when water is added.

CuSO₄ + 5H₂O ⟶ CuSO₄.5H₂O

(d) The chemical formula of gypsum is :

CaSO₄.2H₂O

When gypsum is heated at 100°C, it loses water partially and forms plaster of Paris.

(a) Calcium oxide is preferred because it removes moisture without reacting with ammonia gas. It acts as a suitable drying agent for ammonia. Other drying agents like concentrated acids react with ammonia, so they cannot be used.

(b) Concentrated sulphuric acid is not used to dry ammonia gas because it is acidic and reacts with basic ammonia gas to form ammonium sulphate. Thus, ammonia gets chemically absorbed instead of only being dried.
2NH₃ + H₂SO₄ ⟶ (NH₄)₂SO₄

(c) Concentrated sulphuric acid is a good dehydrating agent because it can remove chemically combined water molecules from compounds. For example, it removes water of crystallisation from blue vitriol, CuSO₄.5H₂O, forming anhydrous CuSO₄.
CuSO₄.5H₂O ⟶ CuSO₄ + 5H₂O

(d) A drying agent removes moisture from substances without chemically reacting with them, whereas a dehydrating agent removes chemically combined water molecules from compounds, usually causing a chemical change.

(a) Y > X > Z

Reason - Solubility is increasing with increase in temperature in case of X and Y but solubility is decreasing with increase in temperature in case of Z.
Y is highest and Z is the lowest with X in middle.

Therefore, Y > X > Z is the correct option.

(b) 10 g solute in 250 g solvent

Solubility = $\frac{10}{250}$ ${\times100}$

= 4 g

(c) (i) and (iv)

Reason - Heating the solution increases the solubility.
Adding more of the solvent so that more solute can dissolve.

Cooling the solution decreases the solubility. A saturated solution cannot dissolve more solute at that temperature. Any extra solute added will simply remain undissolved at the bottom.

(d) The maximum amount of solute which can be dissolved in 100 g of the solvent to form a saturated solution at a given temperature.

(a) Water has following properties that make it an exceptional solvent :

• Water is an exceptional solvent because its molecules are polar in nature. The oxygen atom has a partial negative charge and the hydrogen atoms have partial positive charges. Due to this polarity, water molecules can attract different types of particles and help them dissolve.
• Water also has a high dielectric constant. Therefore, it reduces the electrostatic force of attraction between positive and negative ions and dissolves many inorganic compounds. Hence, water can dissolve a wide range of solids, gases and liquids, forming aqueous solutions.

(b) Water’s property of being a universal solvent also makes it easily polluted. It can dissolve or carry harmful substances such as sewage, industrial wastes, pesticides, fertilisers and toxic chemicals. As a result, water may become unsafe for drinking and can cause diseases such as diarrhoea, typhoid, cholera and dysentery. Long-term consumption of chemically polluted water may also cause toxicity, cancer or organ damage.

(a) Milk is a colloidal solution. The particles of a colloid are large enough to scatter light. Therefore, when torch light passes through milk, the light gets scattered in different directions, making the path of light visible and the milk appear illuminated. This phenomenon is called the Tyndall effect.

(b) The same result is not observed with salt solution because salt solution is a true solution. Its solute particles are extremely small and do not scatter light. Hence, light passes through it without making its path visible.

(c) Two more solutions which would show the same effect are:

1. Starch solution
2. Soap solution

Some physical and chemical processes used to make river water potable are:

1. Coagulation and settling
   Raw untreated water is mixed with chemicals called coagulants, such as alum. Alum forms tiny sticky particles called flocs. Mud, bacteria and other suspended impurities stick to these flocs and settle at the bottom.

2. Filtration
   The water is passed through filters made of layers of sand, gravel and coal. These layers remove the remaining suspended impurities from water.

3. Chlorination
   Chlorine is added to water to kill harmful microorganisms and make the water germ-free.

4. Defluoridation
   Excess fluoride is removed from drinking water to bring fluoride to safe levels and make the water fit for consumption.

Coagulation and settling are most effective at removing solid impurities from water. Raw untreated water is first mixed with chemicals called coagulants.
When alum is added to water, it forms tiny sticky globs called flocs. Bacteria, mud and other impurities stick to these flocs. The water is then passed into a settling basin, where the flocs gradually become heavier and settle at the bottom.

Percentage of population with access to safe affordable drinking water.

Reason - The sustainable development goal related to drinking water aims to ensure universal and equitable access to safe and affordable drinking water. The number of dams built only measures infrastructure, average water use per person measures consumption, and bottled water sold is commercial data. These do not directly indicate access to safe drinking water.