Water - Questions & Answers

Question – Emulsions [paint – oil in water] can passed through a filter paper, but sand in water cannot.
Emulsions are colloidal solutions where the dispersed particles are less than 100 nm in size, allowing them to pass through the pores of a filter paper. Sand in water forms a suspension with solid particles greater than 100 nm, which are too large to pass through a filter paper and settle down after some time.

Question – Compare the role of – bleaching powder, activated carbon & potash alum in making water potable.
Bleaching powder releases chlorine to destroy micro-organisms and kill harmful germs (chlorination). Activated carbon is used to reduce excess fluoride concentration by absorbing fluoride ions (defluoridation). Potash alum acts as a coagulant that reacts with alkalinity in water to bind suspended mud or clay into heavy particles that settle to the bottom, making filtration easier.

Complete and balance the equations

HARD & SOFT WATER
a. Formation of temporary hard water
CaCO3 + H2O + CO2 → Ca(HCO3)2
MgCO3 + H2O + CO2 → Mg(HCO3)2

b. Removal of temporary hardness in water – by boiling
Ca(HCO3)2 → CaCO3↓ + H2O + CO2 [g]
Mg(HCO3)2 → MgCO3↓ + H2O + CO2 [g]

c. Removal of temporary hardness in water – by addition of washing soda [Na2CO3]
Na2CO3 + Ca(HCO3)2 → CaCO3↓ + 2NaHCO3

d. Removal of permanent hardness in water – by addition of washing soda [Na2CO3]
Na2CO3 + CaSO4 → CaCO3↓ + Na2SO4

Short Answer Questions

1. Name – one substance which is 'deliquescent'.
Iron [III] chloride (or Calcium chloride / Sodium hydroxide)

2. Name – a salt (a) which contains water of crystallization, (b) which does not contain.
(a) Copper sulphate pentahydrate (or Washing soda). (b) Potassium nitrate (or Sodium chloride).

Long Answer Questions

1. How does an increase in temperature affect : i] the solubility of NaCl, ii] the solubility of CaSO4 in water?
i] The solubility of NaCl increases slightly with a rise in temperature. ii] The solubility of CaSO4 decreases with a rise in temperature.

2. Give reasons for the following: i] table salt becomes moist and sticky during the rainy season; ii] a white powder forms on the surface of washing soda crystals which are left exposed to the air.
i] Table salt contains deliquescent impurities like magnesium chloride and calcium chloride which absorb moisture from the air during the rainy/humid season. ii] Washing soda is an efflorescent substance; it loses its water of crystallization to the atmosphere on exposure and changes into an amorphous white powder (monohydrate).

3. Why is anhydrous CaCl2 – used in a desiccator. Name one substance which is 'efflorescent'.
Anhydrous CaCl2 is highly hygroscopic and deliquescent in nature. It readily absorbs moisture from other substances and acts as a strong drying/desiccating agent. Efflorescent substance: Washing soda (Na2CO3.10H2O).

4. Explain the following observations:
i] Washing-soda become coated with a white powder when left exposed to the atmosphere.
ii] In the expression anhydrous copper sulphate, what is meant by "anhydrous".
iii] Why is fused calcium chloride or conc. sulphuric acid used in a desiccator.
i] Washing soda undergoes efflorescence, losing its moisture (water of crystallization) to the atmosphere, changing into an amorphous white powder. ii] "Anhydrous" means the substance does not contain any fixed number of water molecules (water of crystallization) in loose chemical combination. iii] They are both powerful hygroscopic substances that readily absorb moisture without completely changing state, making them excellent drying agents for keeping a desiccator dry.

5. Complete the following : The solubility of a gas at constant pressure may be increased by decreasing the....
temperature

6. What is 'water of crystallization'? Name a crystalline salt which does not contain water of crystallization.
It is the fixed number of water molecules which enters into a loose chemical combination with the substance when the substance is crystallised from its hot saturated solution. Salt without it: Potassium nitrate (KNO3).

7. What would you observe – when the water of crystallization of a salt is removed by heating it.
The salt loses its definite geometric shape and specific colour, and changes into an amorphous powder (anhydrous state).

8. Define: i] Hygroscopy, ii] Efflorescence.
i] Hygroscopy is the phenomenon where certain substances absorb moisture from the atmosphere when exposed to it but do not change their state (solid remains solid, liquid remains liquid). ii] Efflorescence is the phenomenon where crystalline hydrated salts lose their moisture (water of crystallization) partly or completely to the atmosphere and change into the amorphous state.

9. What is the effect of temperature – on the solubility of KNO3 & calcium sulphate in water.
The solubility of KNO3 increases with a rise in temperature. The solubility of calcium sulphate decreases with a rise in temperature.

10. What test would you do – to find out whether a given solution is saturated or unsaturated.
Add a small amount of solute to the given solution at that temperature and stir. If it dissolves, the solution is unsaturated. If it does not dissolve and remains at the bottom, the solution is saturated.

11. How can you increase – the solubility of a given volume of gas in water.
By increasing the pressure on the surface of the water and decreasing the temperature of the water.

Short Answer Questions

1. Name – (a) a treated source of water (b) a solvent for grease stains.
(a) Distilled water (or pipe borne / chlorinated water). (b) Petrol (or kerosene / ammonia solution).

Long Answer Questions

1. How does water – occur in the free state & in the combined state.
In the free state, it occurs as solid (ice, snow, frost), liquid (river, sea, well, spring water, dew), and gas (water vapour, clouds, mist, fog). In the combined state, it is present in all living matter (plants and animals), in hydrated salts, and in certain minerals.

2. State – a reason to prove that water is a compound and not an element.
In water, the elements hydrogen and oxygen are combined in a fixed proportion by weight, its properties differ entirely from its individual elements, and its components can only be separated by chemical means (e.g. electrolysis) and not by physical means.

3. State why water is considered a universal solvent.
Water has the ability to dissolve a very large number of substances, including solids (like salts and sugar), liquids, and gases (like hydrogen chloride and ammonia), effectively dissolving almost every common substance.

4. Define the terms: (a) solute, (b) solvent, (c) solution.
(a) Solute: Substance which dissolves or disappears in the solvent to form a solution. (b) Solvent: Liquid or medium of dissolution which allows the solute to dissolve in it. (c) Solution: A homogenous mixture of a solute in a solvent.

5. State – the characteristics of a true solution.
It is clear, transparent and homogenous in nature. The particle size is molecular (less than 1 nm), allowing them to pass through filter paper. Particles cannot be seen under an ultra microscope and do not settle down. The solute can be recovered from the solvent by physical means only.

6. Differentiate – between unsaturated, saturated and supersaturated solutions.
Unsaturated solution: Can dissolve more of the solute at a given temperature. Saturated solution: Cannot dissolve more of the solute at a given temperature. Supersaturated solution: Contains more of the dissolved solute at a given temperature than is present in a saturated solution.

7. How would you convert – a saturated solution to an unsaturated solution and vice versa.
To convert saturated to unsaturated: Heat the saturated solution slowly, or add more solvent to it. To convert unsaturated to saturated: Add more solute to the solution at a given temperature until no more dissolves, or cool the solution.

8. Define solubility. Give the main steps with the calculations involved of the method to determine the solubility of a given salt 'X' in water.
Solubility is the maximum amount of a solute in grams that will saturate 100 g. of the solvent at that temperature. Main steps: 1. Prepare a saturated solution of salt 'X' at a given room temperature (t °C). 2. Weigh a clean, dry evaporating dish (M g). 3. Add the saturated solution to it and reweigh (M1 g). 4. Heat the solution to dryness and reweigh the dish with the solid residue (M2 g). Calculation: Solubility = [Weight of solute (M2 - M) / Weight of solvent (M1 - M2)] x 100 at t °C.

9. From the salts : NaCl, KClO3, NaNO3, Ca(OH)2, NH4Cl, KCl, CaSO4. State the salts whose – solubility (a) increases, (b) decreases, (c) is fairly independent or slightly increases – with rise in temperature of water.
(a) Increases: KClO3, NaNO3, NH4Cl. (b) Decreases: CaSO4, Ca(OH)2 [above 70°C]. (c) Fairly independent/slightly increases: NaCl, KCl, Ca(OH)2 [below 70°C].

10. State – what is a solubility curve. State two applications & two benefits of the solubility curve.
A solubility curve is a graph showing the qualitative effect of temperature on the solubility of a substance. Applications & Benefits: 1. Medical: Enables pharmacists to determine the amount of drugs that must be dissolved together at different temperatures to give a prescribed preparation. 2. Chemists & Research workers: Enables them to determine the most suitable solvent for extraction of essential chemicals. 3. Separation & purification: Allows separation of mixtures of solutes by fractional crystallization.

11. Give the influence of (a) pressure, (b) temperature on the solubility of gases in water.
(a) An increase in pressure causes an increase in solubility of gas in water. (b) An increase in temperature causes a decrease in solubility of gas in water.

12. State the reasons why (a) boiled water tastes flat, (b) a soda water bottle opens with a 'fizz'.
(a) Water contains soluble gases which contribute to its taste; on boiling, the temperature increases, decreasing the solubility of dissolved gases (air) which are then expelled out, making it taste flat. (b) Soda water contains carbon dioxide dissolved under high pressure. On opening the bottle, the pressure suddenly decreases, causing the solubility of the gas to drop rapidly, and it escapes vigorously producing a 'fizz'.

13. State what is meant by the terms: (a) crystal, (b) crystallization, (c) seed crystal. Explain with examples.
(a) Crystal: Homogenous solids, arranged symmetrically, bounded by plane surfaces meeting at sharp edges at definite angles, having a regular definite shape (e.g. Cubic, Rhombic). (b) Crystallization: The process by which crystals are separated or deposited from a hot saturated solution of a substance on cooling gently (e.g. cooling hot saturated potassium nitrate solution). (c) Seed crystal: A well-formed crystal suspended in a cooling saturated solution to act as a nucleus for deposition of further crystals, allowing it to grow large.

14. Define the term – 'water of crystallization'
It is the fixed number of water molecules which enters into a loose chemical combination with the substance, when the substance is crystallised from its hot saturated solution. It is responsible for the geometric shape and colour of the crystal.

15. Differentiate between – hydrated and anhydrous crystals with examples. State three defined changes which occur when hydrated copper sulphate is heated.
Hydrated crystals contain a fixed number of water molecules in loose chemical combination (e.g., Hydrated copper sulphate - CuSO4.5H2O). Anhydrous crystals do not contain this fixed number of water molecules (e.g., Potassium nitrate - KNO3). Changes on heating hydrated copper sulphate: 1. Blue crystals change to a white powder. 2. Definite geometric shape is lost. 3. Changes from crystalline in nature to amorphous.

16. State – why is anhydrous calcium chloride & not potassium chloride, kept in a desiccator.
Anhydrous calcium chloride is a powerful hygroscopic substance and drying agent that actively absorbs moisture from the atmosphere to keep the desiccator dry. Potassium chloride does not possess these hygroscopic/desiccating properties.

17. How does – anhydrous calcium chloride differ from iron [III] chloride when exposed to the atmosphere.
Anhydrous calcium chloride is hygroscopic; it absorbs moisture from the atmosphere but does not change its solid state. Iron [III] chloride is deliquescent; it absorbs moisture from the atmosphere and dissolves into the absorbed moisture, changing into a liquid state.

18. Conc. H2SO4 – acts as a 'drying agent' & a 'dehydrating agent'. Explain and differentiate the words in italics.
As a drying agent, it acts by simply absorbing or removing moisture from other substances (like gases) without changing their chemical state. As a dehydrating agent, it chemically removes combined water or elements of water from compounds due to its strong affinity for water (e.g., turning glucose into carbon).

19. Explain the meaning of the terms – hard water & soft water.
Hard water is water that does not lather readily with ordinary soap and hence wastes soap. Soft water is water that lathers readily with ordinary soap and hence does not waste soap.

20. State the causes of – hardness in water.
Hardness is caused by the presence of dissolved calcium and magnesium salts, specifically their bicarbonates, sulphates, and chlorides.

21. Differentiate between – temporary hard water & permanent hard water.
Temporary hardness can be removed by boiling, as it is caused by bicarbonates. Permanent hardness cannot be removed easily by boiling, as it is caused by chlorides and sulphates.

22. State the cause – of hardness in temporary & permanent hard water.
Temporary hard water is mainly caused by the presence of calcium and magnesium bicarbonates: Ca(HCO3)2, Mg(HCO3)2. Permanent hard water is mainly caused by the presence of calcium and magnesium chlorides and sulphates: CaCl2, MgCl2, CaSO4, MgSO4.

23. State the disadvantages – of hardness in water.
Hard water is unfit for laundries (wastes soap), unsafe for drinking, not suitable for preparing solutions, and forms a crusty 'boiler scale' or fur in boilers, making it unsuitable for industrial uses.

24. Give balanced equations – to explain how, hardness in temporary hard water is removed by boiling.
Ca(HCO3)2 → CaCO3↓ + H2O + CO2
Mg(HCO3)2 → MgCO3↓ + H2O + CO2

25. Both temporary & permanent hardness in water – can be removed by addition of washing soda. Give balanced equations for the same.
Temporary: Na2CO3 + Ca(HCO3)2 → CaCO3↓ + 2NaHCO3
Permanent: Na2CO3 + CaSO4 → CaCO3↓ + Na2SO4

26. State – how would you prove experimentally, whether a given sample of water is, hard or soft water.
Take a sample of the water in a beaker or trough and rub a piece of ordinary soap inside it. If the sample lathers readily, it is soft water. If it does not lather readily, it is hard water.

27. State – how you will determine experimentally, which of the two samples of water, contains permanent hard water.
Take both hard water samples and boil them for some time, then filter them after all gases have escaped. Rub the residual samples with soap. The sample that still does not lather after filtration contains permanent hardness, because permanent hardness cannot be removed by boiling.

28. Differentiate between – chlorination & defluoridation.
Chlorination is the addition of chlorine or chlorine compounds to water to destroy harmful micro-organisms and make water potable. Defluoridation is a technique used to reduce excess fluoride concentration in drinking water (often by absorbing fluoride ions in activated carbon) to prevent health issues like dental damage.

29. State – the function of: (a) sand & gravel (b) alum – in the purification of water.
(a) Sand and gravel act as a filtration bed to remove suspended impurities from water. (b) Alum acts as a coagulant that binds suspended mud or clay particles, forming heavy particles that settle to the bottom, making the water more acceptable for filtration.

Multiple Choice Questions [MCQs] – Select the correct answer

1. A salt whose solubility in water increases & then decreases as the temperature rises.
(a) Anhydrous copper sulphate (b) Glaubers salt (c) Potassium chloride (d) Calcium sulphate
(b) Glaubers salt

2. A white hydrated crystalline salt – which turns into amorphous state on exposure to atmosphere.
(a) FeSO4.7H2O (b) Na2SO4.10H2O (c) CuSO4.5H2O (d) Ca(NO3)2.4H2O
(b) Na2SO4.10H2O

3. A substance both – hygroscopic & deliquescent.
(a) Calcium chloride (b) Green vitriol (c) Silica gel (d) Rock salt
(a) Calcium chloride

4. A substance with a monovalent negative radical which causes – temporary hardness in water.
(a) Calcium chloride (b) Calcium bicarbonate (c) Calcium sulphate (d) Calcium carbonate
(b) Calcium bicarbonate

5. An efflorescent substance, which removes – both temporary & permanent hardness in water.
(a) Magnesium sulphate [heptahydrate] (b) Sodium carbonate [decahydrate] (c) Calcium sulphate [dihydrate] (d) Copper sulphate [pentahydrate]
(b) Sodium carbonate [decahydrate]

6. Boiling of water cannot be used to remove hardness in water containing.
(a) Magnesium chloride (b) Magnesium bicarbonate (c) Magnesium sulphate (d) Calcium chloride
(a) Magnesium chloride / (c) Magnesium sulphate / (d) Calcium chloride (All three cause permanent hardness which cannot be removed by boiling)

High Order Thinking Questions

1. Washing soda & iron [III] chloride are separately exposed to the atmosphere. State (a) the observations seen (b) the phenomenon which occurs, (c) the reason for the phenomenon occurring in each case.
Would a similar phenomenon occur in case of exposure of common salt. Explain giving reasons.
(a) Observations: Washing soda changes into an amorphous solid powder. Iron [III] chloride dissolves into a liquid state.
(b) Phenomenon: Washing soda undergoes efflorescence. Iron [III] chloride undergoes deliquescence.
(c) Reason: Efflorescence occurs because the vapour pressure of the hydrated crystals of washing soda exceeds the vapour pressure of the atmospheric humidity. Deliquescence occurs because the vapour pressure of the aqueous tension of the deliquescent crystal solution is very low compared to the vapour pressure of the atmospheric humidity.
Common Salt: Common salt is not deliquescent itself, but it turns moist/sticky on exposure to air because it contains small amounts of deliquescent impurities like calcium and magnesium chlorides which absorb moisture from the air.

2. Fill in the blanks with the correct words from the choices in bracket.
(a) Water of crystallization is [is/is not] responsible for the geometric shape of blue vitriol crystals.
(b) At any given temperature the mass of gas dissolved in a fixed volume of liquid is directly proportional to the pressure on the surface of the liquid. [pressure/concentration/temperature]
(c) A chilled soda water on opening produces less [less/more] effervescence compared to one kept at room temperature on opening.
(d) Particles of potassium chloride dissolved in water to form a true solution cannot [cannot/can] be removed, both by filtration and chemical means.

3. Name the substance in each case.
(a) The soluble salt formed when an efflorescent crystals of sodium carbonate is added to hard water, where hardness is due to presence of sulphates in water.
Sodium sulphate (Na2SO4)
(b) A deliquescent salt present in impure common salt, which causes it to turn sticky in humid conditions.
Magnesium chloride / Calcium chloride
(c) The black charred mass obtained when a liquid dehydrating agent is added to a carbohydrate.
Carbon

UNIT TEST PAPER – 3

Q.1 Select the correct word from the words in brackets to complete each sentence :

1. If pressure on the surface of water increases, its boiling point increases and freezing point decreases. [increases/decreases].
2. A saturated solution can be converted to an unsaturated solution by increasing [increasing/decreasing] the amount of the solvent.
3. Dissolved air in water contains a higher [higher/lower] percentage of oxygen than ordinary air.
4. At low temperatures the solubility of a gas in water is more [less/more] compared to that at ordinary temperatures.
5. Efflorescence occurs when the vapour pressure of the hydrated crystals is more [more/less] than the vapour pressure of the atmospheric humidity.

Q.2 Select the correct answer from the choice given in the brackets.

1. An anhydrous crystal. [blue vitriol/epsom salt/lead chloride]
lead chloride
2. A substance which causes hardness in water. [NH4Cl/CaCl2/NaCl]
CaCl2
3. A deliquescent salt of a divalent metal. [CuCl/CaCl2/FeCl2/PbCl2]
CaCl2
4. An anhydrate of a heptahydrate salt. [Cu(NO3)2/Ca(NO3)2/FeSO4/CaSO4]
FeSO4
5. A drying agent, deliquescent in nature used in a desiccator. [conc. H2SO4/fused CaCl2/FeCl3]
fused CaCl2

Q.3 Give reasons for the following.

1. Solubility curves find utility in separation and purification of solutes.
When a saturated solution of a given mixture of solutes is cooled, those fractions with very low solubilities will be the first to crystallize out from the solution, allowing for separation.

2. Pressure and temperature influence the solubility of gases in water.
An increase in pressure on the surface directly increases the mass of gas dissolved (Henry's Law), while an increase in temperature expels dissolved gases, decreasing solubility.

3. Heating a hydrated copper sulphate crystal is deemed a chemical change.
Upon heating, it loses its fixed number of water molecules (water of crystallization) in loose chemical combination, permanently losing its blue color and geometric shape to become a white amorphous powder.

4. Efflorescence is minimum during humid conditions.
Efflorescence relies on the vapour pressure of hydrated crystals exceeding atmospheric humidity. During humid conditions, atmospheric humidity is high, making this pressure difference minimal.

5. A crusty 'boiler scale' is formed in boilers, when hard water is used.
Hard water contains calcium and magnesium salts which form insoluble crusty deposits on heating, coating the inside of boilers.

Q.4 Name or state the following.

1. An efflorescent decahydrate salt.
Washing soda [Na2CO3.10H2O] (or Glauber's salt)
2. A deliquescent salt of a trivalent metal.
Iron [III] chloride [FeCl3]
3. A liquid hygroscopic substance.
Concentrated sulphuric acid [H2SO4]
4. A salt whose solubility decreases with rise in temperature of the solvent water.
Calcium sulphate [CaSO4]
5. A substance added to remove both temporary & permanent hardness in water.
Washing soda (Sodium carbonate)

Q.5 Differentiate between the following :

1. Natural water and treated water
Natural water is found in the natural state (e.g. river, spring water). Treated water has received some form of chemical or physical treatment (e.g. distilled, pipe borne water).

2. Saturated solution and a super saturated solution
A saturated solution cannot dissolve more solute at a given temperature. A supersaturated solution contains more of the solute at a given temperature than is present in a normal saturated solution.

3. Solubility and solubility curve
Solubility is the maximum amount of a solute that will saturate 100g of a solvent at a particular temperature. A solubility curve is a graphical representation showing the effect of temperature on the solubility of a substance.

4. Deliquescent salt and hygroscopic salt
Deliquescent salts absorb moisture from the atmosphere and dissolve into the liquid state. Hygroscopic salts absorb moisture from the atmosphere but do not change their physical state.

5. Solute and solvent [forming a solution]
A solute is the substance which dissolves or disappears in the solvent. A solvent is the liquid medium of dissolution which allows the solute to dissolve in it.

Q.6 Match the terms in List I with the correct answers in List II.

1. Solubility in a given solvent decreases with increase in temperature
E: CaSO4

2. A white efflorescent crystal
A: Na2SO4.10H2O

3. A hygroscopic dehydrating agent
D: conc. H2SO4

4. A deliquescent alkali
B: NaOH

5. A substance present in temporary hard water
C: Ca(HCO3)2