Water

Introduction

Question 1

Name a 'pure form' and an 'impure form' of water in the natural state.

Answer

Rain water is the purest form of natural water and river and lake water are impure form of natural water.

Potable water

Question 2

State what is potable water. Give its characteristics which make it fit for human consumption.

Answer

Potable water is water which is fit for human consumption & for drinking purposes.

Characteristics of potable water which make it fit for human consumption :

1. Potable water should be clear, colourless & odourless.
2. It should be free from harmful bacteria, germs, suspended impurities & harmful salts such as nitrates, nitrites, cyanides & urea.
3. It should contain small amounts of dissolved gases & minerals which give it a good taste. Dissolved air & salts of sodium, calcium, magnesium etc. impart taste to the water & are useful for metabolic functions of the human body.

Universal solvent

Question 3

Give a reason why water is called a 'universal solvent' but, an alkali is not.

Answer

Water is called a 'universal solvent' because it has the ability to dissolve a wide variety of substances. On the other hand, an alkali is not a universal solvent because it can only dissolve certain types of substances, specifically acids and some other specific compounds.

Question 4

Name the solute, solvent & solution in the statement - 'potassium chloride dissolves in water to give potassium chloride solution'. Define each of the terms in italics.

Answer

Solute — potassium chloride

The substance which dissolves or disappears in the solvent i.e., liquid to form a solution is called a solute.

Solvent — water

The liquid or medium of dissolution which allows the solute to dissolve in it so as to form a solution is called a solvent.

Solution — potassium chloride solution.

A homogeneous mixture of a solute in a solvent is called a solution.

Solubility — term

Question 5

Explain how each of the following factors affects the formation of a solution

(a) Proportion of - solute & solvent

(b) Particle size of the solute

(c) Temperature of the solvent.

Answer

(a) Proportion of solute & solvent — The solute must be in a small amount compared to the solvent. To dissolve the solute, the solvent must be in a much greater amount.

(b) Particle size of the solute — The solute must be finely powdered and not be present as large crystals. This initiates quicker & complete dissolution of the solute in the solvent.

(c) Temperature of the solvent — The solvent may be slightly heated since the solute may not be completely soluble in the solvent at ordinary temperatures.

Question 6

Define the term 'solubility' of a solute in water. If 10 g. of a solute is added to 100 g. of water at 50°C and stirred, it is seen that the total solute dissolves at that temperature giving an unsaturated solution. Is 10 g. the solubility of the solute in water. Give reasons.

Answer

Solubility of a solute in water is defined as the maximum amount of solute in grams that will saturate 100g. of water at t°C.

No, 10g is not the solubility of the solute as the solution is unsaturated. Solubility will be attained when the solution is saturated.

Solubility — solids, liquids & gases

Question 7

Give a reason why -

(a) Alcohol & water form a miscible mixture while oil & water do not.

(b) Boiled water tastes flat.

(c) On opening a bottle of soda the gas escapes out with a 'fizz'.

Answer

(a) As alcohol is soluble in water hence they form a miscible mixture whereas oil is not soluble in water hence they both form an immiscible mixture.

(b) 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, boiled water tastes flat.

(c) Soda bottles contains carbon dioxide dissolved in water under pressure. On opening the bottle, the pressure decreases hence solubility of dissolved carbon dioxide decreases & the gas escapes out with a 'fizz'.

Question 8

Give the importance of -

(a) Dissolved minerals & salts

(b) Dissolved air

in water.

Answer

(a) Dissolved minerals & salts in water — Provides minerals for living bodies for their growth & imparts taste to water.

(b) Dissolved air [O₂, CO₂, & NO₂] in water Solutions — like :

• Dissolved O₂ is useful for respiration in aquatic plants and animals.
• Dissolved CO₂ for photosynthesis
• Dissolved NO₂ is converted into nitrogenous compounds by bacteria and provides nourishment for plants.

Solutions

Question 9

How are solutions generally classified. What is the basis of this classification.

Answer

Solutions are generally classified as:

(a) True solutions

(b) Colloidal solutions

(c) Suspensions

The basis of above classification is size of the dissolved or dispersed particles in the medium.

Question 10

Draw a table to differentiate - True solutions, colloidal solutions & suspensions - with respect to :

(i) Type of mixture

(ii) Solute particles in the mixture

Answer

Question 11

Give four examples of each of the above types of solutions i.e. true solution, colloidal solution & suspension. Give a reason why a true solution does not exhibit 'Tyndall effect'.

Answer

Examples of true solution

Examples of colloidal solution

Examples of suspension

In a true solution, the particles are neither visible to the naked eye nor under an ultra microscope. Hence, they cannot reflect light and cannot show 'Tyndall effect'.

Question 12

If 5 g. of a solute is added to 50 ml. of a solvent at a particular temperature and the solution can dissolve more of the solute at that temperature - is the solution obtained a saturated or an unsaturated solution. Give reasons.

Answer

As more of solute can be dissolved at that temperature, it is an unsaturated solution. A saturated solution cannot dissolve more of the solute at that particular temperature.

Question 13

Explain the meaning of the term 'saturated solution'. State two methods to convert a saturated solution to an unsaturated solution.

Answer

A saturated solution is one which cannot dissolve more of the solute at a given temperature.

Two methods to convert a saturated solution to an unsaturated solution are:

1. By heating the saturated solution slowly it can dissolve more of the solute at a higher temperature.
2. By adding more solvent to the saturated solution. Increased amount of solvent can dissolve more of the solute at that temperature.

Question 14

State how a 'supersaturated' solution differs from a 'saturated solution'. State briefly how you would prepare a supersaturated solution using potassium chloride and water.

Answer

A saturated solution of potassium chloride is prepared in boiling water. If this solution is cooled excess potassium chloride separates out from the hot saturated solution. The hot solution therefore contains more of the solute dissolved in than it can hold at that given temperature and is thus called a supersaturated solution.

Crystals

Question 15

'The process of separation or deposition of crystals from a hot saturated solution on gentle cooling of the solution is called crystallisation'. With reference to the statement explain what is meant by 'water of crystallisation'. State which physical properties of a crystal are correlated with the water of crystallisation.

Answer

Water of crystallisation — When a crystal is separated or deposited from a hot saturated solution on gentle cooling of the solution, it generally unites with a definite number of water molecules and enters into a loose chemical combination with them. This definite number of water molecules is called water of crystallisation.

The geometric shape and colour of the crystal is correlated with water of crystallization.

Question 16

Differentiate between 'hydrated' & 'anhydrous' crystals. Give the chemical formula of the following hydrated crystals:

(a) Washing soda

(b) Gypsum

(c) Blue vitriol

(d) Epsom salt

(e) Glauber's salt.

State which crystal from (a) to (e) is a pentahydrate.

Answer

Blue vitriol [CuSO₄.5H₂O] is a pentahydrate.

Question 17

Differentiate between an efflorescent, deliquescent & hygroscopic substance with suitable examples.

Answer

Question 18

Differentiate the function of concentrated sulphuric acid as a drying agent & as a dehydrating agent. Is fused calcium chloride a dehydrating or a desiccating agent. Give reasons.

Answer

Fused calcium chloride is deliquescent in nature, absorbs moisture and hence used as a desiccating agent.

Reactions of water

Question 19

Give a reason why metals are arranged in a series called - 'activity series of metals'. Name a metal which reacts with

(a) Cold water

(b) Boiling water

(c) Steam

to liberate hydrogen in each case.

Answer

Depending upon the reactivity of metals with water, they are arranged in a series called - 'activity series of metals' in a way such that the most reactive metal is at the top of the series, and the least reactive at the bottom.

(a) Potassium (K)

(b) Magnesium (Mg)

(c) Iron (Fe)

Hard & soft water

Question 20

Differentiate between -

(a) Hard & soft water

(b) Temporary & permanent hard water

with examples.

Answer

(a) Differences between Hard and soft water:

(b) Differences between temporary and permanent hard water:

Question 21

Give balanced equations to show how -

(a) Temporary hardness enters into water.

(b) Temporary hardness in water can be removed by boiling.

(c) Permanent hardness in water can be removed by addition of washing soda.

Answer

(a) CaCO₃ + H₂O + CO₂ ⟶ Ca(HCO₃)₂
MgCO₃ + H₂O + CO₂ ⟶ Mg(HCO₃)₂

(b) Ca(HCO₃)₂ $\xrightarrow{\text{boil}}$ CaCO₃ ↓ + H₂O + CO₂ [g]
Mg(HCO₃)₂ $\xrightarrow{\text{boil}}$ MgCO₃ ↓ + H₂O + CO₂ [g]

(c) $\underset{\text{ washing soda} }{\text{Na}_2\text{CO}_3} + \underset{\text{ calcium sulphate}}{\text{CaSO}_4} \longrightarrow \text{CaCO}_3 ↓ + \text{Na}_2\text{SO}_4$

Objective Type Questions

Question 1

Select the correct answer from A, B, C, D & E for each statement given below:

A: Colloidal

B: Fused calcium chloride

C: Solvent

D: Suspension

E: Washing soda

1. The medium of dissolution which allows the solute to dissolve in it.

2. A solution which can pass through a filter paper but not through a semipermeable membrane

3. A monohydrate crystal.

4. A drying agent placed in desiccator.

5. A heterogenous mixture of undissolved particles in dispersion medium, visible to the naked eye.

Answer

1. C: Solvent

2. A: Colloidal

3. E: Washing soda

4. B: Fused calcium chloride

5. D: Suspension

Question 2

Give a balanced equation for the following conversions:

1. Calcium sulphate in permanent hard water to calcium carbonate using sodium carbonate

2. Iron to triiron tetroxide using steam.

3. Sulphur dioxide to sulphurous acid using a neutral liquid.

4. Potassium oxide to a strong alkali.

5. Magnesium bicarbonate in temporary hard water to magnesium carbonate by boiling.

Answer

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

2. 3Fe + 4H₂O ⇌ Fe₃O₄ + 4H₂ ↑

3. SO₂ + H₂O ⟶ H₂SO₃

4. K₂O + H₂O ⟶ 2KOH

5. Mg(HCO₃)₂ $\xrightarrow{\text{boil}}$ MgCO₃ ↓ + H₂O + CO₂ [g]

Question 3

Complete the statements by filling the blanks with the correct word from the bracket.

1. Solubility of most solids ............... (decreases/increases) with rise in temperature.

2. Kerosene & water form a ............... (miscible/immiscible) mixture.

3. Solubility of a solute is the ............... [minimum/maximum] amount of solute that will saturate 100 g. of water at t°C.

4. Hygroscopic substance absorb moisture from the atmosphere & ............... (do not change/change) their original state.

5. The ratio of hydrogen & oxygen in water is ............... (2:1/1:2)

Answer

1. Solubility of most solids increases with rise in temperature.

2. Kerosene & water form a immiscible mixture.

3. Solubility of a solute is the maximum amount of solute that will saturate 100 g. of water at t°C.

4. Hygroscopic substance absorb moisture from the atmosphere & do not change their original state.

5. The ratio of hydrogen & oxygen in water is 2:1.

Question 4

Give reasons for the following:

1. All solutions are homogenous mixtures of a solute in a solvent.

2. Hardness in temporary water can be removed by boiling, but hardness in permanent hard water cannot.

3. Colloidal solutions exhibit brownian movement.

4. The percentage of oxygen, in air dissolved in water, is higher than the percentage oxygen in ordinary air.

5. Washing soda can be used to remove both temporary and permanent hardness in water.

Answer

1. As solute dissolves in the solvent and forms a uniform composition throughout hence, all solutions are homogenous mixtures of a solute in a solvent.

2. On boiling, the calcium and magnesium bicarbonates responsible for temporary hardness in water, decompose into insoluble carbonates and carbon dioxide which are filtered out making water soft.
   The chlorides and sulphates of calcium and magnesium responsible for permanent hardness in water, are stable to heat hence, permanent hardness cannot be removed by boiling.

3. Colloidal solutions exhibit Brownian movement due to the constant bombardment of molecules of the dispersion medium on the colloidal particles.

4. Ordinary air contains 21% oxygen. As oxygen is more soluble in water as compared to air. Hence, air in water contains 33% oxygen which is more than that in ordinary air.

5. On adding washing soda to water containing calcium & magnesium chlorides, sulphates & bicarbonates an insoluble precipitate of calcium & magnesium carbonate is formed which is filtered out. The remaining soln. contains soluble sodium salts which do not cause hardness in water & hence the residual water is rendered soft. Hence, washing soda can be used to remove both temporary and permanent hardness in water.

Question 5

Match the substances in List I with the appropriate answer in List II.

Answer