WATER - Questions & Answers

EXERCISE

Introduction

1. Name a 'pure form' and an 'impure form' of water in the natural state.
Pure form: Rain water is the purest form of natural water.
Impure form: Sea water is the most impure form of natural water.

Potable water

2. State what is potable water. Give its characteristics which make it fit for human consumption.
Potable water is water which is fit for human consumption and for drinking purposes.
Characteristics:
a] It should be clear, colourless, and odourless.
b] It should be free from harmful bacteria, germs, and suspended impurities.
c] It should be free from harmful salts such as nitrates, nitrites, cyanides, and urea.
d] It should contain small amounts of dissolved gases and minerals that give it a good taste and are useful for metabolic functions.

Universal solvent

3. Give a reason why water is called a 'universal solvent' but, an alkali is not.
Water is called a universal solvent because it has the ability to dissolve almost every common substance (solids, liquids, and gases) more than any other liquid. An alkali is not a universal solvent because it only dissolves a specific range of substances and often reacts chemically with them rather than just forming a physical solution.

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.
Solute: Potassium chloride.
Solvent: Water.
Solution: Potassium chloride solution.
Definitions:
Solute: The substance which dissolves or disappears in the solvent to form a solution.
Solvent: The liquid or medium of dissolution which allows the solute to dissolve in it.
Solution: A homogeneous mixture of a solute in a solvent.

Solubility - term

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.
a] Proportion of solute & solvent: The solute must be in a small amount compared to the solvent. A greater amount of solvent ensures complete dissolution.
b] Particle size of the solute: Smaller particle size (finely powdered solute) initiates quicker and complete dissolution compared to large crystals.
c] Temperature of the solvent: Increasing the temperature generally increases the solubility of most solids, as it provides more energy to break the bonds between solute particles.

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.
Solubility: It is the maximum amount of solute in grams that will saturate 100 g. of water at a specific temperature.
Reason: No, 10 g. is not the solubility because the resulting solution is unsaturated. Solubility refers to the amount of solute in a saturated solution; since an unsaturated solution can dissolve more solute, 10 g. is less than the actual solubility at 50°C.

Solubility - solids, liquids & gases

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'.
a] Alcohol and water are miscible because their molecules are attracted to each other and mix completely. Oil and water are immiscible because oil molecules do not attract water molecules, causing them to form separate layers.
b] Boiled water tastes flat because boiling removes the dissolved gases (like oxygen and carbon dioxide) that normally impart taste to the water.
c] Soda contains CO₂ dissolved under high pressure. On opening the bottle, the pressure decreases, which reduces the solubility of the gas, causing it to escape rapidly with a 'fizz'.

8. Give the importance of - a] Dissolved minerals & salts b] Dissolved air - in water
a] Dissolved minerals & salts: They provide essential minerals for the growth of living bodies and impart a pleasant taste to water.
b] Dissolved air: Dissolved oxygen is essential for the respiration of aquatic plants and animals. Dissolved carbon dioxide is used by aquatic plants for photosynthesis.

Solutions

9. How are solutions generally classified. What is the basis of this classification.
Solutions are classified into True solutions, Colloidal solutions, and Suspensions. The basis of this classification is the size of the dissolved or dispersed particles in the medium.

10. Draw a table to differentiate - True solutions, colloidal solutions & suspensions - with respect to i] Type of mixture ii] Solute particles in the mixture

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'.
Examples:
True Solutions: Potassium chloride in water, Sugar in water, Lead nitrate in water, Sodium carbonate in water.
Colloidal Solutions: Fog, Smoke, Paint, Blood.
Suspensions: Sand in water, Mud in water, Chalk powder in water, Flour in water.
Reason: A true solution does not exhibit the Tyndall effect because its particle size is extremely small (less than 1 nm), which is not large enough to scatter a beam of light.

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.
The solution is an unsaturated solution.
Reason: By definition, an unsaturated solution is one that can dissolve more of the solute at a given temperature. Since the statement says it "can dissolve more of the solute," it has not yet reached its saturation point.

13. Explain the meaning of the term 'saturated solution'. State two methods to convert a saturated solution to an unsaturated solution.
Saturated Solution: A solution in which no more solute can be dissolved in the solvent at a particular temperature.
Methods to convert:
1. By heating: Increasing the temperature allows the solvent to dissolve more solute.
2. By adding more solvent: Increasing the volume of the solvent allows more solute to be dissolved.

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.
A supersaturated solution contains more solute at a given temperature than what is present in a saturated solution.
Preparation: Prepare a saturated solution of potassium chloride in boiling water. When this hot solution is cooled slowly, it will contain more dissolved solute than a saturated solution can normally hold at that lower temperature, thus becoming supersaturated.

Crystals

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.
Water of crystallisation: It is a definite number of water molecules that enter into a loose chemical combination with a salt when it crystallises from its hot saturated solution.
Physical properties: It is responsible for the geometric shape and the colour of the crystal.

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.
Hydrated crystals: Crystals that contain a definite number of water molecules in loose chemical combination (e.g., CuSO₄·5H₂O).
Anhydrous crystals: Crystals that do not contain any water of crystallisation (e.g., KNO₃).
Chemical Formulas:
a] Washing soda: Na₂CO₃·10H₂O
b] Gypsum: CaSO₄·2H₂O
c] Blue vitriol: CuSO₄·5H₂O
d] Epsom salt: MgSO₄·7H₂O
e] Glauber's salt: Na₂SO₄·10H₂O
Pentahydrate: Blue vitriol (CuSO₄·5H₂O).

17. Differentiate between an efflorescent, deliquescent & hygroscopic substance with suitable examples.
Efflorescent: Hydrated crystals that lose their water of crystallisation on exposure to the atmosphere and change into an amorphous powder. Example: Washing soda (Na₂CO₃·10H₂O).
Deliquescent: Water-soluble crystals that absorb moisture from the atmosphere and dissolve in it to form a saturated solution. Example: Ferric chloride (FeCl₃).
Hygroscopic: Substances (solids or liquids) that absorb moisture from the atmosphere but do not change their physical state. Example: Quicklime (CaO) or concentrated sulphuric acid (H₂SO₄).

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.
Drying agent: It removes physical moisture or water vapour from other substances (like gases) without changing their chemical composition.
Dehydrating agent: It removes chemically combined water molecules from a compound (like glucose).
Fused calcium chloride is a desiccating (drying) agent.
Reason: It is used in a desiccator to absorb moisture from the atmosphere or from other substances to keep them dry, but it does not remove chemically combined water from compounds.

Reactions of water

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.
Metals are arranged in an activity series to show their relative reactivity. This helps predict how a metal will react with water or acids.
a] Cold water: Sodium (Na) or Potassium (K).
b] Boiling water: Magnesium (Mg).
c] Steam: Iron (Fe) or Zinc (Zn).

Hard & soft water

20. Differentiate between - a] Hard & soft water b] Temporary & permanent hard water - with examples.
a] Hard water: Water that does not lather readily with soap (e.g., Spring water). Soft water: Water that lathers easily with soap (e.g., Rain water).
b] Temporary hard water: Hardness caused by dissolved bicarbonates of calcium and magnesium, which can be removed by boiling. Example: Ca(HCO₃)₂.
Permanent hard water: Hardness caused by chlorides and sulphates of calcium and magnesium, which cannot be removed by boiling. Example: MgSO₄.

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.
a] Entrance of hardness: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂
b] Removal by boiling: Ca(HCO₃)₂  boil → CaCO₃↓ + H₂O + CO₂
c] Removal by washing soda: CaSO₄ + Na₂CO₃ → CaCO₃↓ + Na₂SO₄

CRITICAL THINKING OBJECTIVE TYPE QUESTIONS

Q.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. C: Solvent
2. A solution which can pass through a filter paper but not through a semipermeable membrane. A: Colloidal
3. A monohydrate crystal. E: Washing soda (Note: Washing soda loses water to become a monohydrate).
4. A drying agent placed in desiccator. B: Fused calcium chloride
5. A heterogenous mixture of undissolved particles in dispersion medium, visible to the naked eye. D: Suspension

Q.2 Give a balanced equation for the following conversions:

1. Calcium sulphate in permanent hard water to calcium carbonate using sodium carbonate.
CaSO₄ + Na₂CO₃ → CaCO₃↓ + Na₂SO₄

2. Iron to triiron tetroxide using steam.
3Fe + 4H₂O ⇌ Fe₃O₄ + 4H₂

3. Sulphur dioxide to sulphurous acid using a neutral liquid.
SO₂ + H₂O → H₂SO₃

4. Potassium oxide to a strong alkali.
K₂O + H₂O → 2KOH

5. Magnesium bicarbonate in temporary hard water to magnesium carbonate by boiling.
Mg(HCO₃)₂  boil → MgCO₃↓ + H₂O + CO₂

Q.3 Complete the statements by filling the blanks with the correct word from the bracket.

1. Solubility of most solids increases [decreases/increases] with rise in temperature.
2. Kerosene & water form a immiscible [miscible/immiscible] mixture.
3. Solubility of a solute is the maximum [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 [do not change/change] their original state.
5. The ratio of hydrogen & oxygen in water is 2:1 [2:1 / 1:2].

Q.4 Give reasons for the following :

1. All solutions are homogenous mixtures of a solute in a solvent.
Because the solute particles are uniformly distributed throughout the solvent such that they cannot be seen separately or separated by simple filtration.

2. Hardness in temporary water can be removed by boiling, but hardness in permanent hard water cannot.
Boiling decomposes soluble bicarbonates into insoluble carbonates which can be filtered. However, boiling does not affect the soluble chlorides and sulphates that cause permanent hardness.

3. Colloidal solutions exhibit brownian movement.
This is due to the continuous bombardment of the molecules of the dispersion medium on the colloidal particles.

4. The percentage of oxygen, in air dissolved in water, is higher than the percentage of oxygen in ordinary air.
Oxygen is more soluble in water than nitrogen; hence, the air dissolved in water is relatively richer in oxygen (approx. 33%) compared to ordinary air (21%).

5. Washing soda can be used to remove both temporary and permanent hardness in water.
Washing soda reacts with both bicarbonates (temporary) and chlorides/sulphates (permanent) to form insoluble calcium and magnesium carbonates, which are then filtered out.

Q.5 Match the substances in List I with the appropriate answer in List II.

1. Green vitriol — D: Heptahydrate (FeSO₄·7H₂O)
2. Paint — E: Colloidal
3. Magnesium chloride — A: Permanent hardness in water
4. Magnesium bicarbonate — C: Temporary hardness in water
5. Calcium oxide — B: Hygroscopic