Water

rain water

Reason — Rainwater is the purest form of natural water. It is formed naturally by evaporation followed by condensation of water vapour in the atmosphere.
Seawater contains a large amount of dissolved salts, minerals and impurities. River water contains mud, microorganisms, dissolved salts, and pollutants. Spring water contains dissolved minerals and salts from rocks.

1:8

Reason — One atom of Hydrogen has mass of 1 amu, while one atom of Oxygen has mass of 16 amu. One molecule of water contains 2 atoms of Hydrogen and 1 atom of Oxygen, so their mass ratio:
1 x 2 : 16 x 1 = 1 : 8

dissolved common salt

Reason — Seawater contains a large amount of dissolved common salt.

condensation

Reason — Condensation is the process by which water vapour in the atmosphere cools and changes back into liquid form, leading to the formation of clouds, dew, or even precipitation like rain, snow, etc. This is how water returns to Earth's surface from the atmosphere.
Combustion is burning of a substance in oxygen, producing heat and light. Respiration is the process of releasing energy from food in living organisms. Photosynthesis is the process by which plants make food using carbon dioxide, water, and sunlight.

underground water

Reason — Spring water comes from underground water that has been naturally filtered and then rises to the surface through a spring.

The four main sources of water are:

1. Ocean and sea water
2. River and lake water
3. Spring and well water
4. Rainwater

The water cycle is important in nature because:

1. It helps in regulating the climate on the earth.
2. It makes water available in various forms on the earth.

Water is formed from hydrogen and oxygen gases. The chemical composition of hydrogen and oxygen gases is 1:8 ratio by mass in water. The molecular formula of water is H₂O.

Water has a high specific heat, i.e., it needs a large amount of heat to become hot. Thus, water can absorb a lot of heat without much rise in its temperature. Hence, it is used as an excellent cooling agent.

During severe winters, when the temperature starts falling below 4°C, water in pipelines expands (due to property of anomalous expansion of water) and it exerts large pressure on the pipes causing them to burst.

The boiling point of water decreases with a decrease in pressure. Therefore, water boils at a lower temperature (i.e., less than 100°C) in hilly areas where the atmospheric pressure is lower as compared to the plains. Due to this, it takes a longer time to cook food making cooking difficult in hills.

The density of ice is less than the density of water, so ice floats on water.

The presence of impurities in water lowers its freezing point. Sea water contains salt which lowers the freezing point of water. So sea water does not freeze at 0°C.

Water shows anomalous expansion. As the temperature falls below 4°C, water at the surface becomes lighter and finally freezes at 0°C to form ice, which floats. This ice layer is a poor conductor of heat, so it acts like an insulating blanket and prevents the water below from freezing. Therefore, the lower layers remain liquid and aquatic plants and animals can survive in very cold regions.

Water needs a large amount of heat to become hot and takes a long time to lose heat, so it neither heats up nor cools down quickly. Hence, it can absorb a lot of heat from its surroundings without becoming very hot, making it an excellent cooling agent. Therefore, water is used in room coolers to absorb heat from air and in hot water bags because it stays warm for a longer time.

(a) When pressure is increased, the boiling point of water increases.

(b) When impurity is added, the boiling point of water increases.

Water is very precious for all living beings due to the following reasons:

1. Water is the source of life. It is the most essential substance for the existence of life.
2. It is required for everyday uses like drinking, cleaning, bathing, washing etc.
3. It provides a medium for all biochemical reactions inside our bodies to take place.
4. It regulates our body temperature through sweating and evaporation.

In lakes, as the temperature drops, the surface water cools, sinks and mixes with the deeper water, this process continues until the temperature reaches 4°C. Beyond that point, the surface water expands, becomes lighter and it does not sink. So, further cooling takes place only at the surface while temperature of lower layers of water does not change. The water at the surface freezes to form a layer of ice that insulates the deeper water and allows aquatic organisms to survive in the water beneath the ice.

oxygen and carbon dioxide

Reason — Many gases like oxygen, carbon dioxide and ammonia dissolve in water.

10^-9 to 10^-6 m

Reason — A colloid is a homogeneous looking heterogeneous mixture in which particles of intermediate size are dispersed in a continuous medium. These particles remain suspended throughout the medium and do not settle down. Among the given options, the colloidal particle-size range is 10^-9 to 10^-6 m.

Blue vitriol

Reason — Substances which contain water of crystallisation are called hydrated substances, e.g. Blue vitriol (CuSO₄.5H₂O).
Common salt is an anhydrous salt (does not contain water of crystallisation). Potassium nitrate is also an anhydrous salt. Ammonium chloride is an anhydrous salt.

silica gel

Reason — Certain substances absorb moisture (water vapour) from the atmosphere without dissolving in it. Such substances are called hygroscopic substances. E.g. silica gel.
Caustic soda (NaOH) is actually deliquescent, not just hygroscopic. Iron (III) chloride is also deliquescent, not simply hygroscopic. Calcium carbonate does not absorb moisture from air, so it is neither hygroscopic nor deliquescent.

hydrogen and oxygen

Reason — When water is subjected to electrolysis, it decomposes into hydrogen and oxygen gases at the cathode and anode respectively.

Solutions — Sugar and water, salt and water.

Colloids — milk, soap solution.

Suspensions — chalk and water, sand and water.

(a) Water gas [CO + H₂]

(b) Oxygen gas (O₂)

(c) Hydrogen gas (H₂)

(d) Carbon dioxide gas (CO₂)

(a) A solution is a homogeneous mixture of two or more substances whose composition can be varied.

(b) A solute is a substance that dissolves in other substances. It is in a smaller quantity in the solution.

(c) A solvent is a medium in which a solute dissolves. It is in a larger quantity in a solution.

(a) A solution in which more of the solute can be dissolved at a given temperature is called an unsaturated solution.

(b) A solution that cannot dissolve any more of the solute at a given temperature is called a saturated solution.

(c) A solution that contains more solute than it can normally hold at a given temperature is called a supersaturated solution.

(a) Solubility of a solid generally increases with an increase in temperature.
Solubility of a gas decreases with an increase in temperature.

(b) Pressure has practically no effect on the solubility of a solid in water.
In the case of gases, the amount of a gas dissolved in water increases with an increase in pressure and decreases with a decrease in pressure.

The fixed amount of water which is in a loose chemical combination with one molecule of a substance in its crystal form is called water of crystallization.

The two examples are:

(a) Blue vitriol (CuSO₄.5H₂O), Washing soda (Na₂CO₃.10H₂O)

(b) Common salt (NaCl), Potassium nitrate (KNO₃)

(c) Quicklime (CaO), Anhydrous calcium chloride (CaCl₂)

(d) Caustic soda (NaOH), Magnesium chloride (MgCl₂)

(e) Washing soda (Na₂CO₃.10H₂O), Glauber's salt (Na₂SO₄.10H₂O)

(f) Milk, blood

(g) Alcohol, Carbon tetrachloride

(a) When blue vitriol is heated, it loses its water of crystallisation. The blue crystalline solid of copper sulphate turns into white amorphous (powder) solid and drops of colourless liquid (which is water) condense on the upper cooler parts of the test tube.

(b) When washing soda is exposed to air the white crystalline substance changes into white powder.

(c) There is no change in colour of blue litmus solution when added to water as pure water is neutral.

The reaction of metals with water under different conditions indicates the reactive nature of metals and enables us to arrange the metals on the basis of their reactivity which is called metal activity series.

(a) 2Na + 2H₂O ⟶ 2NaOH + H₂ (g)

(b) 3Fe + 4H₂O $\xrightarrow{\text{steam}}$ Fe₃O₄ + 4H₂ (g)

(c) CO₂ + H₂O ⟶ H₂CO₃

(d) Na₂O + H₂O ⟶ 2NaOH

(a) Difference between Solution and suspension:

(b) Difference between Suspension and colloid:

Water is called a universal solvent because it can dissolve a large number of solids, liquids and gases.

Silica gel is a drying agent which absorbs moisture and keeps things dry. So, silica gel pouches are kept in unused water bottles.

Table salt contains impurities like magnesium chloride and calcium chloride which are deliquescent. They absorb water and become moist during rainy season.

Cold drink bottles contain dissolved carbon dioxide under high pressure. On opening the bottle, the pressure decreases and the gas is released producing a fizz sound.

Hydrated

Reason — Hydrated salts are salts that contain water molecules as part of their crystalline structure. When these salts are heated, they lose their water molecules, which evaporate as water vapour. For example, copper(II) sulphate pentahydrate (CuSO₄·5H₂O) gives off water vapour when heated.
Hygroscopic substances absorb moisture from air, but do not contain water of crystallisation. Deliquescent substances absorb moisture and dissolve in it, but they do not necessarily release water on heating. Anhydrous salts do not contain water of crystallisation, so they do not release water vapour.

Calcium sulphate

Reason — Calcium sulphate causes permanent hardness of water.
Calcium bicarbonate causes temporary hardness, which can be removed by boiling. Sodium chloride does not cause hardness of water. Magnesium nitrate is soluble, but not commonly responsible for water hardness in basic classification; hardness is mainly due to calcium and magnesium sulphates/chlorides.

sodium carbonate

Reason — Sodium carbonate can remove permanent hardness of water.
Sodium bicarbonate does not effectively remove permanent hardness of water. Sodium hydroxide is a strong base but not used for removing water hardness in this context. Sodium chloride does not affect hardness of water.

boiling

Reason — Temporary hardness of water can be removed by simple physical method, i.e. by boiling.
Filtration removes insoluble impurities, not dissolved salts causing hardness. Loading is used in water treatment to speed up sedimentation, not to remove hardness.

Both A and R are true but R is not the correct explanation of A.

Explanation — A is true as water is often called a "universal solvent" because it can dissolve many different substances, such as salts, sugars, gases, and a variety of other compounds.
R is true as we use water daily for drinking, washing, bathing, etc. However, these are two separate facts.

Therefore, Both A and R are true but R is not the correct explanation of A is the correct option.

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

Explanation — A is true as water can exist as a solid (ice), liquid (water), and gas (vapour) under different conditions of temperature and pressure.
R is true as on cooling water becomes ice and on heating it becomes vapour. Water changes between solid, liquid, and gas states depending on temperature.

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

A is true but R is false.

Explanation — Miscible liquids are those liquids which mix completely with each other to form a single, uniform layer. Hence, they can be said to "dissolve" in each other. So, Assertion (A) is true.
Petrol and water are immiscible; they do not mix and form two separate layers (petrol floats on water). So, Reason (R) is false.

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

A is false but R is true.

Explanation — A solution is a homogeneous mixture, i.e., its composition is uniform throughout. It is not a heterogeneous mixture. So, Assertion (A) is false.
In a solution, the substance which dissolves is called the solute, and the substance in which it dissolves is called the solvent. So, Reason (R) is true.

Therefore, Assertion (A) is false but Reason (R) is true.

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

Explanation — Pure water is tasteless, but the water we drink usually has a slight taste. So, Assertion (A) is true.
Drinking water contains dissolved minerals (salts like calcium and magnesium compounds) and dissolved gases, which impart taste to the water. So, Reason (R) is true.

Therefore, both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).

Both A and R are true but R is not the correct explanation of A.

Explanation — Hard water contains dissolved calcium and magnesium salts. These ions react with soap to form an insoluble scum, therefore lather is not produced easily. So, Assertion (A) is true.
Temporary hardness (due to bicarbonates of Ca and Mg) can be removed by boiling. So, Reason (R) is true.
However, Reason (R) is not the correct explanation of Assertion (A) because it tells a method of removing temporary hardness, not the reason for poor lather formation.

Therefore, both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).

(a) maximum, minimum

(b) salt, ice

(c) decreases, increases

(d) Rain water

(e) fertilizers

(f) temporary

(g) blue

(h) scum

(a) False
Correct Statement — Hard water does not form lather easily with soap.

(b) False
Correct Statement — Temporary hardness is due to bicarbonate salts of calcium and magnesium.

(c) True

(d) False
Correct Statement — The reaction of steam with magnesium produces hydrogen gas and magnesium oxide.

Mg + H₂O $\xrightarrow{\text{steam}}$ MgO + H₂

(e) True

(a) Potable water

(b) Impurities

(c) Water cycle

(d) Water vapour

(e) Freezing mixture

(f) Anhydrous substance

(g) Hygroscopy

(h) Water of crystallisation

(a) Water which forms a lather with soap easily is called soft water.

(b) Water which does not forms a lather with soap easily rather forms a white sticky scum or precipitate is called hard water.

Typhoid, cholera, hepatitis are the three water borne diseases.

Alcohol is mixed with water and is used in car radiators to prevent it from freezing in cold weather because the presence of impurities in water lowers the freezing point.

The amount of gas dissolved in water increases with an increase in pressure. That is why carbonated drinks containing carbon dioxide are bottled under high pressure as they contain a large amount of gas dissolved in them.

(i) Bicarbonates of calcium and magnesium

(ii) Sulphates and chlorides of calcium and magnesium

(i) Boiling helps to remove the temporary hardness of water. When temporary hard water is boiled, the bicarbonates of calcium and magnesium break up to form their respective insoluble carbonates. These can be filtered out and the water becomes soft.

Ca(HCO₃)₂ $\xrightarrow{\text{boiled}}$ CaCO₃ + H₂O + CO₂ (g)

Mg(HCO₃)₂ $\xrightarrow{\text{boiled}}$ MgCO₃ + H₂O + CO₂ (g)

(ii) The permanent hardness of water is removed when it is treated with a small quantity of sodium carbonate. It reacts with soluble chlorides and sulphates of calcium and magnesium to form respective insoluble carbonates. These are filtered out and water becomes soft.

Na₂CO₃ + CaSO₄ ⟶ CaCO₃ ↓ + Na₂SO₄

Na₂CO₃ + MgCl₂ ⟶ MgCO₃ ↓ + 2NaCl

The main causes of water pollution are:

1. Chemical wastes from industrial and agricultural processes.
2. Thermal wastes from nuclear and thermal power plants.
3. Sewage and garbage disposed into water bodies.

It can be controlled by the following ways:

1. Domestic sewage should be treated before being discharged into water bodies.
2. Waste products of industries should be treated before being discharged into water bodies.
3. Use non-biodegradable materials such as detergents, pesticides and polythene responsibly to avoid pollution of water and the environment.

(a) During the rainy season, the air contains a large amount of water vapour (high humidity). Common salt (NaCl), especially if it contains impurities like magnesium chloride, absorbs moisture from the air. This makes the salt moist and sticky.

(b) Deliquescence is the property of certain substances to absorb moisture from the air and dissolve in it, forming a solution.

(c) We can keep the salt dry in our homes during monsoon by :

1. Storing salt in an airtight container.
2. Keeping it in a dry place.

(a) When soap solution is added to beaker A, no or very little lather is formed; instead, scum appears.
When soap solution is added to beaker B, a good amount of lather is formed easily.

(b) Beaker A contains hard water due to magnesium sulphate.
Beaker B contains comparatively soft water, as it forms lather easily with soap.

(c) Temporary hardness is caused by calcium bicarbonate and magnesium bicarbonate.
Permanent hardness is caused by calcium sulphate, magnesium sulphate, calcium chloride, and magnesium chloride.

(d) A simple method to remove temporary hardness at home is boiling water.

(e) Washing soda is used with hard water because washing soda reacts with calcium and magnesium ions and forms insoluble carbonates, which get precipitated and removed. This helps in softening hard water and increasing lather formation.