Water

Iron [III] chloride (FeCl₃)

(a) Hydrated copper sulphate [CuSO₄.5H₂O] contain water of crystallization.

(b) Potassium nitrate [KNO₃] does not contain water of crystallization.

(i) NaCl — Solubility increases slightly with increase in temperature.
(ii) CaSO₄ — Decreases with increase in temperature.

(i) Table salt is not deliquescent but contains a small amount of calcium and magnesium chloride impurities which are deliquescent. Hence, the impurities absorb moisture from the atmosphere, thereby turning table salt sticky and wet especially during humid conditions.

(ii) Washing Soda is a efflorescent substance and so when it is exposed to the atmosphere, it loses its moisture [water of crystallization] partly and changes into amorphous state. Hence, a white powder forms on the surface of washing soda crystals which are left exposed to the air.

Anhydrous CaCl₂ is deliquescent in nature, absorbs moisture and hence used as drying agent or desiccating agent.

Washing soda [Na₂CO₃.10H₂O] is efflorescent in nature.

(i) Washing Soda is an efflorescent substance and so when it is exposed to the atmosphere, it loses its moisture [water of crystallization] partly and changes into amorphous state. Hence, a white powder forms on the surface of washing soda crystals which are left exposed to the air.

(ii) When copper sulphate loses water of crystallization on being heated, it becomes white powder of CuSO₄ and is called anhydrous copper sulphate.

(iii) Fused calcium chloride or conc. sulphuric acid is deliquescent in nature, absorbs moisture and hence used as drying agent or desiccating agent.

The solubility of a gas at constant pressure may be increased by decreasing the temperature.

The fixed number of water molecules which enter into a loose chemical combination with the substance when the substance is crystallized from it's hot saturated solution is called water of crystallization.

KNO₃ is a substance that is crystalline but does not contain water of crystallization.

The salt loses it's geometric shape and becomes a white powder that is amorphous in nature.

(i) The phenomenon by which a substance absorbs moisture from the atmosphere when exposed to it, without changing its state [i.e., a solid substances remains a solid after absorption of moisture] is known as Hygroscopy.

(ii) Crystalline hydrated salts which on exposure to the atmosphere lose their moisture [water of crystallization] partly or completely to the atmosphere and change into the amorphous state are called efflorescent substances and the phenomenon is known as Efflorescence.

Solubility of KNO₃ increases with rise in temperature and that of calcium sulphate decreases with increase in temperature.

When we add more of solute to a given solution and on stirring, the solute dissolves, it is unsaturated solution. If the solute settles down and does not dissolve, then it is a saturated solution.

Solubility of a given volume of gas can be increased by increasing the pressure on the surface of water.

(a) Distilled water

(b) Petrol

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.

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.

Water has the ability to dissolve a large number of substances e.g., salt, sugar etc. It also dissolves a large number of gases e.g., hydrogen chloride, ammonia etc. Hence water dissolves almost every common substance and is therefore called a 'universal solvent'.

(i) A substance which dissolves or disappears in the solvent to form a solution is called a solute.

(ii) Liquid [generally water] or medium of dissolution - which allows the solute to dissolve in it so as to form a solution is called a solvent.

(iii) A homogeneous mixture of a solute in a solvent [i.e., a uniform mixture of two or more substances] whose composition may be gradually changed by changing the relative amount of components is called solution.

Characteristics properties of a true solution are :

• Nature — It is clear, transparent and homogeneous in nature.
• Particle size — Particle size is molecular and the particles -
  • can pass through the pores of filter paper.
  • cannot be seen under a microscope.
  • Do not settle down.
• Separation — Solute can be recovered from the solvent by physical and not by chemical means.

A true solution is therefore a mixture and not a compound.

1. By heating the saturated solution slowly - The saturated solution can dissolve more of the solute at a higher temperature and hence becomes unsaturated.
2. By adding more solvent to the saturated solution - Increased amount of solvent can dissolve more of the solute at that temperature.

The solubility of a solute in a solvent at a particular temperature is the maximum amount of the solute in grams that will saturate 100 g. of the solvent at that temperature.

Determination of solubility of 'X' in water:

Step 1: Preparation of saturated solution of 'X'

• Take a boiling tube half filled with about 100 ml of distilled water.
• Add crystals of 'X' to distilled water and stir slowly.
• Continue stirring till the crystals dissolve. Repeat the process till no more salt can dissolve.
• Pour the saturated solution — in a clean dry test-tube.

Step 2:

(ii) Calculations To Determine The Solubility Of Solute :

• Weigh a clean and dry evaporating dish = M g
• Add the above saturated solution to it and reweigh = M₁ g
• Heat the solution to dryness and reweigh the dish with residue = M₂g
• ∴ Weight of solute = (M₂ – M) g
• Note the temperature of saturated solution = t° C

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

Solubility = $\frac{[\text{M}_2 – \text{M]} \text{ g}}{\text{[M}_1 - \text{M}_2] \text{ g} }$ x 100 at t° C

(a) KClO₃, NaNO₃, NH₄Cl — solubility increases with rise in temperature.

(b) CaSO₄, Ca(OH)₂ above 70° C — solubility decreases with rise in temperature.

(c) NaCl, KCl, Ca(OH)₂ below 70° C — solubility is fairly independent or increases slightly with rise in temperature.

When the solubility of a solute - in a given solvent are plotted against their respective temperatures - a graph showing the effect of temperature on solubility of the substance is obtained. This graph is called the solubility graph or curve.

Two applications of the solubility curve :

1. Medical : Enables a pharmacist to determine the amount of drugs that must be dissolved together in a given quantity of solvent at different temperatures to give a prescribed drug preparation.
2. Chemists and Research Workers : Enables them to determine the most suitable solvent to be used at various temperatures for extraction of essential chemicals from their natural sources.

Two benefits of the solubility curve :

1. To determine solubility of a given solute at a particular temperature.
2. To compare solubilities of different solutes in a solvent at a given temperature.

(i) Pressure — An increase in pressure on the surface of water causes increase in solubility of gases in water.

(ii) Temperature — An increase in temperature of water causes decrease in solubility of gases in water.

(i) As soluble gases in water contribute to the taste of water and on boiling, the temperature of the water increases, thereby the solubility of the dissolved gases decreases and the dissolved gases are expelled out. Hence, the water tastes flat.

(ii) Soda contains carbon dioxide dissolved in water under pressure.
Solubility of carbon dioxide under normal atmospheric pressure is very low, but when subjected to high pressure, it dissolves to a great extent as in case of soda water in which carbon dioxide is dissolved in water under pressure and the bottle is corked.
On opening the bottle, the gas rapidly bubbles out with a 'fizz', since the pressure on the surface of the water suddenly decreases and so does the solubility of carbon dioxide gas in water.

(a) When a hot saturated solution is cooled the excess solid is generally thrown out in the form of particles having a definite geometric shape. These particles are called crystals.

(b) The process by which crystals are separated or deposited from a hot saturated solution of a substance on cooling gently is called crystallization.

(c) A well-formed crystal from the cooled filtrate of saturated solution used as seed for the formation of large sized crystal is called seed crystal.

E.g., A seed crystal of potassium nitrate (KNO₃) is used to prepare a large size crystal of potassium nitrate.

The fixed number of water molecules which enter into a loose chemical combination with the substance when the substance is crystallized from it's hot saturated solution is called water of crystallization.

Changes that occur when hydrated copper sulphate is heated are :

1. White powder of anhydrous copper sulphate is formed from blue crystals of hydrated copper sulphate.
2. No geometric shape is seen.
3. Anhydrous copper sulphate is amorphous in nature.

Anhydrous calcium chloride is kept in a desiccator because it is hygroscopic (it absorbs moisture from the air readily). Hence, it acts as a good drying agent. Potassium chloride is not used because it is not hygroscopic and cannot absorb sufficient moisture from the air.

Anhydrous calcium chloride is hygroscopic in nature, absorbs moisture from atmosphere and becomes wet whereas iron [III] chloride is deliquescent in nature, it absorbs moisture from the atmosphere and changes into liquid state. Hence, the two substances differ.

Hard water — Water is said to be hard when it does not lather readily - with ordinary soap and hence wastes soap.

Soft Water — Water that lathers readily with ordinary soap and — hence does not waste soap.

Substances that cause hardness in water are Calcium and magnesium salts. e.g., Calcium and magnesium - bicarbonates, sulphates and chlorides.

Cause of temporary hardness — presence of calcium and magnesium bicarbonates - Ca(HCO₃)₂, Mg(HCO₃)₂

Cause of permanent hardness — presence of calcium and magnesium chlorides and sulphates in water — CaCl₂, MgCl₂, CaSO₄, MgSO₄

Hard water is unfit for laundries, unsafe for drinking, not suitable for preparing solutions and for industrial uses since it forms a crusty boiler scale or fur in boilers.

Temporary hardness in water is due to the presence of calcium or magnesium bicarbonates.

On boiling temporary hard water, calcium and magnesium bicarbonates in water decompose to give insoluble carbonates, carbon dioxide and water.

The insoluble carbonates are filtered out, CO₂ gas escapes and water is rendered soft.

$\underset{\text{Calcium bicarbonate}}{\text{Ca(HCO}_3)_2}\xrightarrow{\text{Boil}} \underset{\text{Calcium carbonate}}{\text{CaCO}_3↓} + \text{H}_2\text{O} + \text{CO}_2 [\text{g}]$

$\underset{\text{Magnesium bicarbonate}}{\text{Mg(HCO}_3)_2}\xrightarrow{\text{Boil}} \underset{\text{Magnesium carbonate}}{\text{MgCO}_3↓} + \text{H}_2\text{O} + \text{CO}_2 [\text{g}]$

Below are the balanced equations for removal of temporary & permanent hardness in water by addition of washing soda:

Permanent Hard Water:

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

Temporary Hard Water:

$\underset{\text{Washing soda}}{\text{Na}_2\text{CO}_3} + \underset{\text{Calcium bicarbonate}}{\text{Ca(HCO}_3)_2}\longrightarrow \underset{\text{Calcium carbonate}}{\text{CaCO}_3↓} + 2\text{NaHCO}_3$

A piece of ordinary soap is rubbed inside the sample of water. If the sample of water lathers, it is soft else it is hard.

Boil each sample of water separately. The boiled water is filtered. The clear residual sample left behind is rubbed with soap. If the sample of water does not lather after filtration, then the sample is permanent hard water.

(a) Sand and gravel act as filtering materials and remove suspended impurities from water during filtration.

(b) Alum acts as a coagulant and reacts with the alkalinity in water; binds with the suspended mud or clay particles, making water more acceptable for filtration.

Glaubers salt

Reason — Glauber's salt shows unusual solubility behavior. Its solubility in water first increases with temperature and then decreases after a certain temperature.
Anhydrous copper sulphate solubility increases continuously with rise in temperature. It does not decrease afterward. Potassium chloride solubility generally increases steadily with temperature. Calcium sulphate solubility is very low and generally decreases slightly with increase in temperature; it does not first increase and then decrease.

Na₂SO₄.10H₂O

Reason — Glauber's salt, Na₂SO₄.10H₂O, is a white hydrated crystalline salt. On exposure to air, it loses water of crystallization and changes into an amorphous powder.
FeSO₄.7H₂O is green vitriol. CuSO₄.5H₂O is blue vitriol. Ca(NO₃)₂.4H₂O is deliquescent.

Calcium chloride

Reason — Anhydrous CaCl₂ is hygroscopic as it absorbs moisture, while fused CaCl₂ is deliquescent as it dissolves in the absorbed moisture. Therefore, calcium chloride is the correct answer.

Calcium bicarbonate

Reason — Calcium bicarbonate causes temporary hardness in water by bicarbonates of calcium and magnesium.
Calcium chloride causes permanent hardness. Calcium sulphate causes permanent hardness. Calcium carbonate is insoluble in water; does not normally cause hardness.

Sodium carbonate (decahydrate)

Reason — Sodium carbonate (decahydrate) is efflorescent; it loses water of crystallization on exposure to air and is used to remove both temporary and permanent hardness of water by precipitating calcium and magnesium ions as insoluble carbonates.

Magnesium chloride, Magnesium sulphate, Calcium chloride

Reason — Boiling removes temporary hardness only, which is caused by bicarbonates of calcium and magnesium. It cannot remove permanent hardness caused by chlorides and sulphates of calcium and magnesium. Therefore, among the given options, magnesium chloride, magnesium sulphate and calcium chloride are all correct.

When Washing soda is exposed to the atmosphere:

(i) Observations — It loses its moisture [ i.e., water of crystallization] and becomes amorphous.

(ii) Phenomenon — Efflorescence

(iii) Reason for Efflorescence — Efflorescence occurs when the vapour pressure of the hydrated crystals exceeds the vapour pressure of the atmospheric humidity. Due to this, the substance loses its water of crystallization to the atmosphere.

When Iron (III) Chloride is exposed to the atmosphere:

(i) Observations — It absorbs moisture from the atmosphere and changes to liquid state.

(ii) Phenomenon — Deliquescence

(iii) Reason for Deliquescence — Deliquescence occurs when the vapour pressure of the deliquescent crystal solution is very low compared to the vapour pressure of the atmospheric humidity.

Exposure of common salt to the atmosphere:

Pure common salt (NaCl) is neither efflorescent nor deliquescent as it is anhydrous and does not contain water of crystallization.
However, common salt usually contains small amounts of calcium or magnesium chloride impurities which are deliquescent. Hence, impure common salt may absorb moisture from air and turn sticky and wet, especially in humid conditions.

1. Water of crystallization is responsible for the geometric shape of blue vitriol crystals.
2. At any given temperature, the mass of gas dissolved in a fixed volume of liquid is directly proportional to the pressure on the surface of the liquid.
3. A chilled soda water on opening produces more effervescence compared to one kept at room temperature.
4. Particles of potassium chloride dissolved in water to form a true solution cannot be removed, both by filtration and chemical means.

1. Sodium sulphate (Na₂SO₄) is formed as the soluble salt.
2. The deliquescent impurity in common salt that makes it sticky in humid air is magnesium chloride.
3. The black charred mass formed when a liquid dehydrating agent is added to a carbohydrate is carbon (charcoal).

1. If pressure on the surface of water increases it's boiling point increases and freezing point decreases.
2. A saturated solution can be converted to an unsaturated solution by increasing the amount of the solvent.
3. Dissolved air in water contains a higher percentage of oxygen than ordinary air.
4. At low temperatures the solubility of a gas in water is more compared to that at ordinary temperatures.
5. Efflorescence occurs when the vapour pressure of the hydrated crystals is more than the vapour pressure of the atmospheric humidity.

1. An anhydrous crystal — lead chloride.
2. A substance which causes hardness in water. — CaCl₂.
3. A deliquescent salt of a divalent metal — CaCl₂
4. An anhydrate of a heptahydrate salt. — FeSO₄
5. A drying agent, deliquescent in nature used in a dessicator. — fused CaCl₂

When a saturated solution of a given mixture of solutes is cooled, those fractions with very low solubilities will be the first to crystallize out from the solution.

An increase in pressure on the surface of water causes increase in solubility of gas in water and an increase in temperature of water causes decrease in solubility of gas in water.

When hydrated copper sulphate crystals are heated, they lose water of crystallization and change from blue hydrated crystals to a white powder of anhydrous copper sulphate. Since water of crystallization is removed and the composition and properties of the substance change, heating hydrated copper sulphate crystals is deemed a chemical change.

Efflorescence occurs when the vapour pressure of the hydrated crystals exceeds the vapour pressure of the atmospheric humidity. Hence, efflorescence is minimum during humid conditions.

As hard water contains bicarbonates of calcium and magnesium and on boiling CaCO₃ or MgCO₃ is formed in large amounts which get deposited as crusty 'boiler scale'.

1. Washing soda (Na₂CO₃.10H₂O)
2. Iron (III) chloride (FeCl₃)
3. Concentrated sulphuric acid (conc. H₂SO₄)
4. CaSO₄
5. Washing soda (Na₂CO₃)