Selina Concise Chapter 3 Water ICSE Solutions Class 9 Chemistry

ICSE Solutions for Chapter 3 Water Class 9 Selina Chemistry

Exercise 3 A

1. Water exists in all three states. Discuss.

Solution

In the Free State, water exists in the form of solid, liquid and gaseous states. At room temperature, water exits in the form of liquid. When it freezes to 0°C, it turns into ice which is in solid form. When it boils to 100°C, it evaporates in the form of water vapour which is gaseous state.

(a) Solid state: A large amount of fresh water is found in the form of snow or ice.

(b) Liquid state: Most of the water present in oceans and found in streams, rivers, lakes, ponds and springs on land is water in the liquid state.

(c) Gaseous state: Water vapour present in the air is in the gaseous state. Water vapour condenses in the sky to form clouds. Mist and fog are also examples of water in the gaseous form

2. Why water is considered a compound?

Solution

Water is considered a compound because it is made of two elements hydrogen and oxygen combined in the ratio 1:8 by mass.

Mass ratio of elements H₂O

H:O, 2×1 : 16×1 = 1:8

(Atomic mass of H = 1, O = 16)

Components of water cannot be separated by physical methods but can be separated by electrolysis of water.

3. (a) Why does temperature in Mumbai and Chennai not fall as low as it does in Delhi?

(b) Give the properties of water responsible for controlling the temperature of our body.

Solution

(a) The temperature in costal cities like Mumbai and Chennai do not fall as low as in Delhi because these cities are situated in the coastal areas. Due to 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. So, the temperature does not fall as low as it does in Delhi.

(b) Our body is almost 65% of water, and it has the property of specific heat. Due to high specific heat capacity, the presence of a large amount of water is able to modify the climate of the body and control the temperature of our body, which is warm in winter and cool in summer.

4. ‘Water is the universal solvent’. Comment.

Solution

Water dissolves many substances forming an aqueous Solution. It can dissolve solids, liquids and gases. When a solid dissolves in water, the solid is the solute, the water is the solvent and the resultant liquid is the Solution. So, it is said that water is a universal solvent. In other words, water can dissolve nearly every substance.

5. What causes the violence associated with torrential rain?

Solution

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

6. (a) Which property of water enables it to modify the climate?

(b) Density of water varies with temperature. What are its consequences?

(c) What is the effect of impurities present in the water on the melting point and boiling point of water?

Solution

(a) Due to the high specific heat capacity, the presence of a large amount of water is able to modify the climate.

(b) The property of anomalous expansion of water enables marine life to exist in the colder regions of the world, because even when water freezes on the top, it is still liquid below the ice layer, as the density of water is greater than that of ice.

(c) The boiling point of water increases due to the presence of dissolved impurities.

The freezing point of water decreases due to the presence of dissolved impurities.

7. How do fishes and aquatic animals survive when the pond gets covered with thick ice?

Solution

Even though ponds covered with thick ice, beneath there will be water because of the maximum density level of water which helps the fishes and aquatic animals survive

8. The properties of water are different from the properties of the elements of which it is formed. Discuss.

Solution

Water is formed by the combination of Hydrogen and Oxygen in the ration of 2:1. When the two elements are joined chemically, the atoms lose their individual properties and have different properties from the elements by which they are made. Water remains liquid in room temperature whereas Hydrogen and Oxygen are gases which when combine changes the state due to the chemical reaction

9. How is aquatic life benefited by the fact that water has maximum density at 4^oC?

Solution

The property of anomalous expansion of water enables aquatic life to exist because water freezes on the surface of the water body, but it is still liquid below the ice layer.

10. What are the observations and conclusions when tap water is boiled and evaporated in a water glass?

Solution

When tap water is boiled and evaporated:

Observations:

(a) A number of concentric rings of solid matter are seen on the watch glass after evaporation of tap water.

Conclusion:

(b) Tap water contains dissolved salts, minerals and impurities.

11. What is the importance of dissolved salts in water?

Solution

Importance of dissolved salts in water:

• Dissolved salts provide specific taste to water.
• Dissolved salts act as micronutrients for the growth and development of living beings.

12. State the importance of the suitability of CO₂ and O₂ in water.

Solution

They add taste to water for drinking purposes.

13. How is air dissolved in water different from ordinary air?

Solution

Oxygen is more soluble in water than nitrogen. Air dissolved in water contains a higher percentage of oxygen (30-35%). Oxygen is only 21% in ordinary air. In this way, air dissolved in water is different from ordinary air.

14. Identify A, B, C, and D first one is done for you.

Solution

When a solid changes into a liquid, it absorbs heat equal to the latent heat of fusion. When a liquid changes into a solid, it loses heat equal to the latent heat of solidification.

A liquid changes into a gas, it absorbs heat equal to the latent heat of vaporisation. When a gas condenses into a liquid, it loses heat equal to the latent heat of condensation.

15. Explain why:

(a) Boiled or distilled water tastes flat.

(b) Ice at zero degrees centigrade has greater cooling effect than water at 0^oC.

(c) Burns caused by steam are more severe than burns caused by boiling water.

(d) Rivers and lakes do not freeze easily?

(e) Air dissolved in water contains a higher proportion of oxygen.

(f) If distilled water is kept in a sealed bottle for a long time, it leaves etchings on the surface of the glass.

(g) Rain water does not leave behind concentric rings when boiled.

Solution

(a) Boiled water tastes flat because it does not contain dissolved matter such as air, carbon dioxide and other minerals.

(b) Ice at 0°C gives more cooling effect than water at 0°C because at 0°C ice absorbs 336 J per gram of energy to melt to 0°C water.

(c) Burns caused by steam are more severe than burns caused by boiling water because of high specific latent heat of condensation. 2268 J/g of heat is released when 1 g of steam condenses to form 1 gm of water.

(d) Due to the high specific latent heat of solidification of water, rivers and lakes do not freeze easily.

(e) Air dissolved in water contains a higher percentage of oxygen because the solubility of oxygen in water is more than that of oxygen in air.

(f) If distilled water is kept in a sealed bottle for a long time, it etches the surface of glass because substances which are apparently insoluble in water actually dissolve in minute traces in water.

(g) Rain water does not leave concentric rings when boiled because rain water does not contain dissolved solids.

Exercise 3 B

1. Explain the terms:

(a) Solution

(b) Solute

(c) Solvent

Solution

(a) Solution: A Solution is a homogeneous mixture of two or more substances, the components of which cannot be seen separately.

(b) Solute: A solute is the substance which dissolves in a solvent to form a Solution.

(c) Solvent: A solvent is the medium in which a solute dissolves. 

2. Explain why a hot saturated Solution of potassium nitrate forms crystals as it cools.

Solution

Solubility of nitrates decreases with a fall in temperature. Thus, when a hot saturated Solution of potassium nitrate cools, it forms crystals as it separates from the Solution.

3. Give three factors which affect the solubility of a solid solute in a solvent.

Solution

Three factors on which the solubility of a solid depend:

• Temperature
• Nature of the solid
• Nature of the solvent

4. (a) If you are given some copper sulphate crystals, how would you proceed to prepare its saturated Solution at room temperature?

(b) How can you show that your Solution is really saturated?

Solution

(a) Take 100 g of distilled water in a beaker. Add to this one gram of copper sulphate crystals.

(b) This mixture with the help of a glass rod and dissolve the copper sulphate crystals. Similarly, go on dissolving more copper sulphate (1 gram at a time) with constant and vigorous stirring. A stage is reached when no more copper sulphate dissolves. It is called a saturated Solution at this temperature.

5. (a) Define
(i) Henry’s law
(ii) Crystallisation
(iii) Seeding.

(b) State the different methods of crystallisation.

Solution

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

(ii) Crystallisation: It is the process by which crystals of a substance separate out on cooling its hot saturated Solution.

(iii) Seeding: This process of inducting crystallisation by adding a crystal of pure substance into saturated solution is called seeding.

(b) In the laboratory, crystals may be obtained by the following methods: 

• By cooling a hot saturated Solution gently
• By cooling a fused mass
• By sublimation
• By slowly evaporating a saturated Solution

6. What would you observe when crystals of copper (II) sulphate and iron (II) sulphate are separately heated in two test tubes strongly?

Solution

Action of heat on copper (II) sulphate crystals

When copper (II) sulphate crystals are heated in a hard glass test tube, the following observations are observed:

(i) The crystals are converted to a powdery substance.

(ii) The crystals lose their blue coloration on further heating.

(iii) Steaming vapors are produced inside the tube which condense near the mouth of the tube to form a colorless liquid.

(iv) On further heating, steam escapes from the mouth of the tube and water gets collected in a beaker placed under the mouth of the tube.

(v) On further heating, the residue changes to a white powder and steam stops coming out.

CuSO₄.5H₂O → CuSO₄ + 5H₂O

Action of heat on iron (II) sulphate

When iron (II) sulphate is heated in a test tube, the following is observed:

(i) The crystals crumble to a white powder and a large amount of steam and gas are given out.

(ii) On strong heating, a brown residue of ferric oxide (Fe₂O₃) is produced and a mixture of SO₂ and SO₃ is given off.

7. Give the names and formulae of two substances in each case:

(a) Hydrated substance

(b) Anhydrous substance

(c) Liquid drying agent

(d) A basic drying agent

Solution

(a) (i) Washing soda crystals: Na₂CO₃.10H₂O
(ii) Blue vitriol: CuSO₄.5 H₂O

(b) (i) Table salt: NaCl
(ii) Nitre: KNO₃

(c) Sulphuric acid: H₂SO₄

(d) Quick lime: CaO

8. What is the effect of temperature on solubility of KNO₃ and CaSO₄ in water?

Solution

Solubility of potassium nitrate (KNO₃) in water increases with an increase in temperature.

Solubility of calcium sulphate (CaSO₄) in water decreases with an increase in temperature.

9. Solubility of NaCl at 40^oC is 36.5 g. What is meant by this statement?

Solution

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

10. Which test will you carry out to find out if a given Solution is saturated or unsaturated or supersaturated?

Solution

A Solution in which more of a solute can be dissolved at a given temperature is an unsaturated Solution.

The Solution in which no more solute can be dissolved at a given temperature is a saturated Solution at that temperature.

Solution in which some solute separates on cooling slightly is a super saturated Solution.

11. What is the effect of pressure on solubility of gases? Explain with an example.

Solution

With an increase in pressure, the solubility of a gas in water increases.

With an increase in temperature, the solubility of a gas in water decreases.

For example, the solubility of carbon dioxide in water under normal atmospheric pressure is low, but when the water surface is subjected to higher pressure, a lot more of CO₂ gas gets dissolved in it.

Similarly, in case of soda water, on opening the bottle, the dissolved gas rapidly bubbles out because the pressure on the surface of the water suddenly decreases.

12. State the term:

(a) A Solution where solvent is a liquid other than water.

(b) When a substance absorbs moisture on exposure to moist air and dissolves in the absorbed water and turned to Solution.

(c) A substance which contains water of crystallisation.

(d) When a substance absorbs moisture from the atmosphere but does not form a Solution.

(e) When a compound loses its water of crystallisation on exposure to dry air.

(f) The substance that can remove hydrogen and oxygen atoms in the ratio of 2:1(in the form of water) from the compound.

Solution

(a) Non-aqueous Solution

(b) Deliquescence

(c) Hydrated substance

(d) Hygroscope

(e) Efflorescence

(f) Dehydrating agent

13. Explain why:

(a) Water is an excellent liquid to use in cooling systems.

(b) A Solution is always clear and transparent.

(c) Lakes and rivers do not suddenly freeze in the winters.

(d) The solute cannot be separated from a Solution by filtration.

(e) Fused CaCl₂ or conc. H₂SO₄ is used in a desiccator.

(f) Effervescence is seen on opening a bottle of soda water.

(g) Table salts become sticky on exposure to humid air during the rainy season.

Solution

(a) Water is an excellent liquid to use in cooling systems because of its high specific heat.

(b) A water-soluble solid disappears in a Solution where the solvent is water, and water has the property of being clear and transparent. So, the Solution is also clear and transparent.

(c) Lakes and rivers do not freeze suddenly in winters because of the high specific latent heat of solidification, i.e. the amount of heat released when 1 g of water solidifies to form 1 g of ice at 0°C. It is about 336 J/g or 80 cal/g.

(d) The component which dissolves in a solvent is known as a solute. So, it cannot be separated from a Solution by filtration. However, filtration is used when the solute is insoluble in the Solution.

(e) Fused CaCl₂ or concentrated H₂SO₄ is deliquescent in nature, i.e. it absorbs moisture, and hence, these are used in desiccators as drying agents.

(f) Carbon dioxide is dissolved in soda water under pressure. On opening the bottle, the pressure on the surface of water suddenly decreases; therefore, the solubility of CO₂ in water decreases and the gas rapidly bubbles out.

(g) Table salt becomes sticky on exposure during the rainy season, because it generally contains a small percentage of magnesium chloride and calcium chloride as impurities. These impurities absorb moisture from the monsoon air due to their deliquescent nature, and thus, table salt become sticky.

14. Normally, solubility of crystalline solid increases with temperature. Does it increase uniformly in all cases? Name a substance whose solubility:

(a) Increases rapidly with temperature.

(b) solubility increases gradually with temperature.

(c) Increases slightly with temperature.

(d) Initially increases then decreases with rise in temperature.

Solution

(a) Potassium nitrate

(b) Potassium chloride

(c) Sodium chloride

(d)Calcium sulphate

15. What are drying or desiccating agents? Give examples.

Solution

These are substances which can readily absorb moisture from other substances without chemically reacting with them.

Examples: Phosphorous pentoxide (P₂O₅), quick lime (CaO)

16. Complete the following table:

Common Name	Chemical Name	Formula	Acid, base or salt	Efflorescent, hygroscopic or deliquescent substance
Solid caustic potash
Quick lime
Oil of vitriol
Washing soda
Solid caustic soda
Blue vitriol

Solution

Common Name	Chemical Name	Formula	Acid, base or salt	Efflorescent, hygroscopic or deliquescent substance
Solid caustic potash	Potassium hydroxide	KOH	Base	Deliquescent substance
Quick lime	Calcium oxide	CaO	Base	Hygroscopic substance
Oil of vitriol	Sulphuric acid	H2SO4	Acid	Hygroscopic substance
Washing soda	Hydrated sodium carbonate	Na2CO3.10H2O	Salt	Efflorescent substance
Solid caustic soda	Sodium hydroxide	NaOH	Base	Deliquescent substance
Blue vitriol	Copper sulphate	CuSO4	Salt	Efflorescent substance

17. In which of the following substances will there be

(a) Increase in mass

(b) Decrease in mass

(c) No change in mass when they are exposed to air?

Sodium chloride, Iron, Conc. sulphuric acid, Table salt, Sodium carbonate crystals

Solution

(a) Increase in mass: Iron and conc. sulphuric acid

(b) Decrease in mass: Sodium carbonate crystals

(c) No change in mass: Sodium chloride

18. State the methods by which hydrated salts can be made anhydrous.

Solution

Hydrated salts can be converted to anhydrous substances by heating and also when exposed to dry air.

Example:

Gaber Reaction: Gaber’s salt becomes powdery anhydrous sodium sulphate when exposed to dry air.

Na₂SO₄.10H₂O → Na₂SO₄ + 10H₂O

Exercise 3C

1. What is the composition of water? In what volume its elements combine?

Solution

The composition of water is 2 atoms of hydrogen with 1 atom of oxygen (H₂O).

By number of atoms, they combine in the ratio 2:1.

2. What is the use of solubility of oxygen and carbon dioxide in water?

Solution 

• Air dissolved in water is biologically very important.
• Oxygen dissolved in water is used by marine life like fish for respiration, and thus, marine life is sustained.
• Aquatic plants make use of dissolved carbon dioxide in photosynthesis to prepare food.
• Carbon dioxide dissolved in water reacts with calcium carbonate to form calcium bicarbonate.
• Marine organisms such as oysters and snails extract calcium carbonate from calcium bicarbonate to build their shells.

3. Hot saturated Solution of sodium nitrate forms crystals as it cools. Why?

Solution

Solubility of sodium nitrate decreases with a fall in temperature. Thus, when a hot saturated Solution of sodium nitrate cools, it forms crystals as it separates from the Solution.

4. What are hydrous substances? Explain with examples.

Solution

Substances which contain water molecules along with salt are hydrated substances.

Examples: Sodium carbonate dehydrate: Na₂CO₃.10H₂O

Copper sulphate pentahydrate: CuSO₄.5H₂O

5. Name three methods by which hydrous substances can be made anhydrous.

Solution

Methods by which hydrous substances can be made anhydrous:

• By heating
• By evaporation
• Exposure to dry air.

6. What is the importance of dissolved impurities in water?

Solution

• The dissolved impurities in water are salts and minerals.
• Dissolved salts provide specific taste to water.
• Salts and minerals are essential for growth and development.
• They supply the essential minerals needed by our body.

7. State two ways by which a saturated Solution can be changed to unsaturated Solution.

Solution

On heating, a saturated Solution becomes unsaturated and more solute can be dissolved in the Solution.

By adding more solvent, a saturated Solution can be made unsaturated.

8. What do you understand by?

(a) Soft water

(b) Hard water

(c) Temporary hard water

(d) Permanent hard water

Solution

(a) Water is said to be soft when the water containing sodium salts easily gives lather with soap.

(b) Water is said to be hard when it does not readily form lather with soap.

(c) Water which contains only hydrogen carbonates of calcium and magnesium is called temporary hard water.

(d) Water containing sulphates and chlorides of magnesium and calcium is called permanent hard water.

9. What are the causes for?

(a) Temporary hardness

(b) Permanent hardness

Solution

(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.

10. What are the advantages of (i) soft water and (ii) hard water?

Solution

Advantages of soft water:

• When the water is soft, you use much less soap and fewer cleaning products. Your budget will reflect your savings.
• Plumbing will last longer. Soft water is low in mineral content and therefore does not leave deposits in the pipes.
• Clothes last longer and remain bright longer if they are washed in soft water.

Advantages of hard water:

• Water free from dissolved salts has a very flat taste. The presence of salts in hard water makes it tasty. So, hard water is used in making beverages and wines.
• Calcium and magnesium salts present in small amounts in hard water are essential for bone and teeth development.
• Hard water checks the 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 lead poisoning.

11. What are stalagmites and stalactites? How are they formed?

Solution

In some limestone caves, conical pillar-like objects hang from the roof and some rise from the floor. These are formed by water containing dissolved calcium hydrogen carbonate continuously dropping from the cracks in the rocks. Release of pressure results in the conversion of some hydrogen carbonate to calcium carbonate.

Ca (HCO₃)₂ → CaCO₃ + CO₂ + H₂O

This calcium carbonate little by little and slowly deposit on both roof and floor of the cave.

The conical pillar which grows downwards from the roof is called stalactite and the one which grows upward from the floor of the cave is called stalagmite.

These meet after a time. In a year, some grow less than even a centimetre, but some may be as tall as 100 cm.

CaCO₃ + CO₂ + H₂O → Ca (HCO₃)₂

MgCO₃ + CO₂ + H₂O → Mg (HCO₃)₂

If the water flows over beds of gypsum (CaSO₄.2H₂O), a little bit of gypsum gets dissolved in water and makes it hard.

12. Name the substance which makes water (i) temporarily hard and (ii) permanently hard.

Solution

(i) Hydrogen carbonates of calcium and magnesium

(ii) Sulphates and chlorides of magnesium and calcium

13. Give equations to show what happens when temporary hard water is

(a) Boiled

(b) Treated with slaked lime

Solution

(a) when boiled

Ca (HCO₃)₂ → CaCO₃ + H₂O + CO₂↑

Mg (HCO₃)₂ → MgCO₃ + H₂O + CO₂↑

(b) when Treated with slaked lime

Ca(HCO₃)₂+ Ca(OH)₂ → 2CaCO₃ + 2H₂O

Mg (HCO₃)₂+ Ca(OH)_2→ MgCO₃ + 2H₂O

14. State the disadvantages of using hard water.

Solution

Disadvantages of using hard water:

• It is more difficult to form lather with soap.
• Scum may form in a reaction with soap, wasting the soap.
• Carbonates of calcium and magnesium form inside kettles. This wastes energy whenever you boil a kettle.
• Hot water pipes ‘fur up’. Carbonates of calcium and magnesium start to coat the inside of pipes which can eventually get blocked.

15. What is soap? For what is it used?

Solution

Soap is chemically a sodium salt of stearic acid (an organic acid with the formula C₁₇H₃₅COOH) and has the formula C₁₇H₃₅COONa.

Soap is used for washing purposes.

16. What is the advantage of a detergent over soap?

Solution

Detergents are more soluble in water than soap and are unaffected by the hardness of water as their calcium salts are soluble in water.

17. Why does the hardness of water render it unfit for use in a (i) boiler and (ii) for washing purposes.

Solution

Steam is usually made in boilers which are made of a number of narrow copper tubes surrounded by fire. As the cold water enters these tubes, it is immediately changed into steam, while the dissolved solids incapable of changing into vapour deposit on the inner walls of the tubes.

This goes on and makes the bore of the tubes narrower. The result is that less water flows through the tubes at one time and less steam is produced. When the bore of the tube becomes very narrow, the pressure of the steam increases so much that at times the boiler bursts.

If hard water is used, 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.

18. Explain with equation, what is noticed when permanent hard water is treated with

(a) Slaked time

(b) Washing soda

Solution

(a) Slaked lime

Lime is first thoroughly mixed with water in a tank and then fed into another tank containing hard water. Revolving paddles thoroughly mix the two Solutions. Most of the calcium carbonate settles down. If there is any solid left over, it is removed by a filter. This is known as Clarke’s process.

(b) Washing soda

When washing soda or soda ash is added to hard water, the corresponding insoluble carbonates settle down and can be removed by filtration.

19. Explain the permit method, how can it be used for softening hard water.

Solution

Permit is an artificial zeolite. Chemically, it is hydrated sodium aluminium or thio silicate with 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 permit. When hard water containing calcium and magnesium ions percolates through these lumps, ions exchange. Sodium permit is slowly changed into calcium and magnesium permit, and the water becomes soft with the removal of calcium and magnesium ions.

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

CaP + 2NaCl → Na₂P + CaCl₂