Water

Exercise 3(A)

Question 1

Water exists in all the three states. Discuss.

Answer

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%]

Question 2

Why is water considered a compound.

Answer

Henry Cavendish synthesized water from it's 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 ration 2:1.

Question 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.

Answer

(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.

Question 4

'Water is a universal solvent'. Comment.

Answer

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.

Question 5

What causes the violence associated with torrential rain?

Answer

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

Question 6(a)

Which property of water enables it to modify the climate ?

Answer

Due to it's 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.

Question 6(b)

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

Answer

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.

Question 6(c)

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

Answer

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

Question 7

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

Answer

The property of anomalous expansion 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.

Question 8

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

Answer

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:

Question 9

How is aquatic life benefitted by the fact that water has maximum density at 4°C?

Answer

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.

Question 10

What are your observations and conclusion when tap water is boiled and evaporated in watch glass?

Answer

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.

Question 11

What is the importance of dissolved salts in water?

Answer

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.

Question 12

State the importance of the solubility of CO₂, and O₂ in water.

Answer

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.

Question 13

How is air dissolved in water different from ordinary air?

Answer

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.

Question 14

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

Answer

(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.

Question 15

Explain why:

(a) Boiled or distilled water tastes flat.

(b) Ice at zero degree centigrade has greater cooling effect than water at 0°C.

(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.

Answer

(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.

Exercise 3(B)

Question 1

Explain the terms

(a) Solution

(b) solute

(c) solvent

Answer

(a) Solution — A solution is a homogenous 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.

Question 2

Explain why hot saturated solution of potassium nitrate forms crystals as it cools.

Answer

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.

Question 3

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

Answer

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.

Question 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?

Answer

(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.

Question 5

(a) Define

(i) Henry’s law

(ii) Crystallization

(iii) Seeding

(b) State any three methods of crystallization.

Answer

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

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

(b) Three methods of crystallization are:

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

Question 6

What would you observe when crystals of Copper(II) sulphate are heated in a test-tube strongly.

Answer

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

Question 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

Answer

(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

Question 8

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

Answer

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.

Question 9

Solubility of NaCl at 40°C is 36.5 g. What is meant by this statement

Answer

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.

Question 10

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

Answer

Add a few drops of solute in the solution and try to stir by 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.

Question 11

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

Answer

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.

Question 12

State the term : (Do not give examples)

(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 compounds.

Answer

(a) Non-aqueous solution

(b) Deliquescence

(c) Hydrated substance

(d) Hygroscopy

(e) Efflorescence

(f) Dehydrating agent

Question 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 salt becomes sticky on exposure to humid air during the rainy season.

Answer

(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 it's temperature because of it's high specific heat capacity. This is why radiators in car and generator 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 about 10^-10 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 about 10^-10 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 used as 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 absorb moisture in rainy season and turns sticky.

Question 14

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

(a) increases rapidly with temperature.

(b) increases gradually with temperature.

(c) increases slightly with temperature.

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

Answer

No, solubility of a crystalline solid does not increase 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

Question 15

What are drying or desiccating agents. Give examples.

Answer

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

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

Question 16

Complete the following table :

Answer

Question 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?

1. Sodium chloride
2. Iron
3. Conc. sulphuric acid
4. Table salt
5. Sodium carbonate crystals

Answer

(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 crystallization 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.

Question 18

State the methods by which hydrated salts can be made anhydrous?

Answer

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.

Exercise 3(C) — Multiple Choice Type

Question 1

Sodium sulphate is soluble in:

1. Ether
2. Water
3. Alcohol
4. Benzene

Answer

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.

Question 2

Water acts as a universal solvent because:

1. It is an organic compound
2. It is polar and has a high dielectric constant.
3. It is liquid at room temperature.
4. It boils at 100°C

Answer

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.

Question 3

Permanent hardness of water is removed by:

1. Adding calcium sulphate
2. Boiling with potassium chloride
3. Boiling
4. Adding sodium carbonate

Answer

Adding sodium carbonate

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

Permanent Hard Water:

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

Question 4

Solid solutions are called :

1. Allotropes
2. Isotopes
3. Alloys
4. Isotones

Answer

Alloys

Reason — A homogenous 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.

Question 5

With a rise in temperature, the solubility of sodium chloride will:

1. Increase rapidly
2. Decrease
3. Remain the same
4. Increase slightly

Answer

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.

Question 6

The solubility of which of the following substances decreases with a rise in temperature :

1. Potassium chloride
2. Hydrated sodium sulphate
3. Calcium hydroxide
4. Potassium nitrate

Answer

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.

Question 7

The crystalline substance that does not contain water of crystallization is:

1. Plaster of paris
2. Potash alum
3. Potassium permanganate
4. Epsom salt

Answer

Potassium permanganate

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

Question 8

Table salt becomes sticky in the presence of moisture because :

1. It is deliquescent
2. It is hygroscopic
3. It contains impurities which are deliquescent
4. It is a good drying agent.

Answer

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.

Question 9

Which of the following substances can act as a dehydrating as well as a drying agent.

1. Oxalic acid
2. Conc. nitric acid
3. Washing soda
4. Conc sulphuric acid

Answer

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

Question 10

Which of the following is a deliquescent salt ?

1. CuSO₄
2. FeCl₃
3. KCl
4. ZnSO₄

Answer

FeCl₃

Reason — FeCl₃ absorbs moisture when exposed to the atmosphere and ultimately dissolves in the absorbed water. Hence, it is a deliquescent salt.

Question 11

If a salt on heating gives water vapour, then that salt is:

1. Hygroscopic
2. Deliquescent
3. Hydrated
4. Anhydrous

Answer

Hydrated

Reason — A hydrated salt on heating loses its water of crystallization and becomes anhydrous.

Question 12

The salt which is the cause of hardness in water:

1. Sodium sulphate
2. Magnesium bicarbonate
3. Sodium chloride
4. Calcium nitrate

Answer

Magnesium bicarbonate

Reason — Hardness in water is due to the presence of bicarbonates, chlorides or sulphates of calcium or magnesium.

Question 13

Temporary hardness of water can be removed by:

1. Adding sodium chloride
2. Boiling
3. Adding calcium carbonate
4. Leaving it for a few hours.

Answer

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 ↑

Exercise 3(C) — Very Short Type

Question 1

Name:

(a) The solute and solvent in sugar solution.

(b) The characteristic property which makes water the universal solvent.

(c) A substance whose solubility shows an anomalous behaviour.

(d) A substance whose solubility rapidly increases with the temperature.

Answer

(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

Question 2

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

Answer

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

Question 3

Name the substances which give water:

(a) temporary hardness

(b) permanent hardness

Answer

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

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

Question 4

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

Answer

Hydrous 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.

Exercise 3(C) — Short Answer Type

Question 1

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

Answer

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.

Question 2

Hot saturated solution of sodium nitrate forms crystals, as it cools. Why?

Answer

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.

Question 3

What do you understand by:

(a) Soft water

(b) Hard water

(c) Temporary Hard water

(d) Permanent hard water.

Answer

(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.

Question 4

What are the causes for

(a) Temporary hardness

(b) Permanent hardness

Answer

(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.

Question 5

State two ways, by which a saturated solution can be changed to unsaturated solution.

Answer

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

1. heating.
2. adding more solvent.

Question 6

What is a soap, what is it used for?

Answer

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.

Question 7

What is the advantage of a detergent over soap ?

Answer

Detergent 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.

Exercise 3(C) — Descriptive Type

Question 1

What are hydrous substances? Explain with examples.

Answer

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).

Question 2

What is the importance of dissolved impurities in water ?

Answer

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.

Question 3

What are the advantages of

(i) soft water

(ii) Hard water

Answer

(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.

Question 4

What are stalagmites and stalactites? How are they formed?

Answer

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.

Question 5

Give equations to show what happens when temporary hard water is

(a) boiled

(b) treated with slaked lime

Answer

(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

Question 6

State the disadvantages of using hard water.

Answer

1. Furring of tea kettles is caused by sediment formed from boiling hard water. This fur is carbonates of calcium amd 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.

Question 7

Why does the hardness of water render it unfit for use in a

(i) boiler

(ii) for washing purposes?

Answer

(i) The dissolved substance present in the hard water does not convert into steam and gets 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.

Question 8

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

(a) slaked lime

(b) washing soda

Answer

(a) In order to treat permanent hard water, slaked lime is used, magnesium hydroxide and calcium carbonate precipitates out and can be easily filtered.

MgSO₄ + Na₂CO₃ + Ca(OH)₂ ⟶ Mg(OH)₂ ↓ + CaCO₃ ↓ + Na₂SO₄

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

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

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

Question 9

What is permutit method, how can it be used for softening hard water ?

Answer

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 become 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 + CaI₂

Exercise 3(C) — Structured/Application/Skill Type

Question 1

The following figure shows the solubility curves of NaCl, KNO₃ and hydrated calcium sulphate.

(a) Identify and label the curves with the salt it represents.

(b) State the factors on which the solubility depends.

(c) Solubility of which salt(s) shows :

(i) Endothermic process

(ii) Exothermic process?

Answer

(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.