Water

Question 1

State the importance of water for all general uses.

Answer

Water is important for all natural processes and is vital for the growth of plants and animal life.
The human body needs water almost for every function carried out by each and every cell.
Industrial processes dependent on water are the agriculture, transportation and the power generation plants both hydro and thermoelectric.

Question 2

How does water occur in the free state and in the combined state.

Answer

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 3

State a reason to prove that water is a compound and not a element.

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 4

State why water is considered a universal solvent.

Answer

Water has the ability to dissolve a large number of substances e.g., salt, sugar etc. It also dissolves a large number of gases e.g., hydrogen chloride, ammonia etc. Hence water dissolves almost every common substance and is therefore called a 'universal solvent'.

Question 5

Define the terms :

(i) solute (ii) solvent (iii) solution

Answer

(i) Substances which dissolves or disappear in the solvent to form a solution is called a solute.

(ii) Liquid [generally water] or medium of dissolution - which allows the solute to dissolve in it so as to form a solution is called a solvent.

(iii) A homogenous mixture of a solute in a solvent [i.e., a uniform mixture of two or more substances] whose composition may be gradually changed by changing the relative amount of components is called solution.

Question 6

State the characteristics of a true solution.

Answer

Characteristics properties of a true solution are :

• Nature — It is clear, transparent and homogenous in nature.
• Particle size — Particle size is molecular and the particles -
  • can pass through the pores of filter paper.
  • cannot be seen under a microscope.
  • Do not settle down.
• Separation — Solute can be recovered from the solvent by physical and not by chemical means.

A true solution is therefore a mixture and not a compound.

Question 7

Differentiate between unsaturated, saturated & supersaturated solutions.

Answer

Question 8

How would you convert a saturated solution to an unsaturated solution and vice versa.

Answer

1. By heating the saturated solution slowly - The saturated solution can dissolve more of the solute at a higher temperature and hence becomes unsaturated.
2. By adding more solvent to the saturated solution - Increased amount of solvent can dissolve more of the solute at that temperature.

Question 9

Define solubility. Give the main steps with the calculations involved of the method to determine the solubility of a given salt 'X' in water.

Answer

The solubility of a solute in a solvent at a particular temperature is the maximum amount of the solute in grams that will saturate 100 g. of the solvent at that temperature.

Determination of Solubility of 'X' In Water :

Step 1 : Preparation of saturated solution of 'X'

• Take a boiling tube half filled with about 100 ml of distilled water.
• Add crystals of 'X' to distilled water and stir slowly.
• Continue stirring till the crystals dissolve. Repeat the process till no more salt can dissolve.
• Pour the saturated solution — in a clean dry test-tube.

Step 2 :

(ii) Calculations To Determine The Solubility Of Solute :

• Weigh a clean and dry evaporating dish = M g
• Add the above saturated solution to it and reweigh = M₁ g
• Heat the solution to dryness and reweigh the dish with residue = M₂g
• ∴ Weight of solute = (M₂ – M) g
• Note the temperature of saturated solution = t° C

Solubility = $\dfrac{\text{Weight of solute}}{\text{Weight of solvent}}$ x 100

Solubility = $\dfrac{[\text{M}_2 – \text{M]} \text{ g}}{\text{[M}_1 - \text{M}_2] \text{ g} }$ x 100 at t° C

Question 10

From the following list of salts : Na₂SO₄.10H₂O, NaCl, KClO₃, NaNO₃, Ca(OH)₂, NH₄Cl, KCI, CaSO₄.

State the salts whose solubility (a) increases, (b) decreases, (c) is fairly independent or slightly increases – with rise in temperature of water.

Answer

(a) KClO₃, NH₄Cl — solubility increases with increase in temperature.

(b) CaSO₄, Ca(OH)₂ above 70° C — solubility decreases with rise in temperature

(c) NaCl, KCl, Ca(OH)₂ below 70° C — slightly increases with rise in temperature.

Question 11

What is a solubility curve. State two applications and two benefits of the solubility curve.

Answer

When the solubility of a solute - in a given solvent are plotted against their respective temperatures - a graph showing the effect of temperature on solubility of the substance is obtained. This graph is called the solubility graph or curve.

Two applications of the solubility curve :

1. Medical : Enables a pharmacist to determine the amount of drugs that must be dissolved together in a given quantity of solvent at different temperatures to give a prescribed drug preparation.
2. Chemists And Research Workers : Enables them to determine the most suitable solvent to be used at various temperatures for extraction of essential chemicals from their natural sources.

Two benefits of the solubility curve :

1. To determine solubility of a given solute at a particular temperature.
2. To compare solubilities of different solutes in a solvent at a given temperature.

Question 12

Give the influence of

(i) pressure (ii) temperature on the solubility of gases in water.

Answer

(i) Pressure — An increase in pressure on the surface of water causes increase in solubility of gases in water.

(ii) Temperature — An increase in temperature of water causes decrease in solubility of gases in water.

Question 13

State the reasons why

(i) boiled water tastes flat

(ii) a soda water bottle opens with a 'fizz'.

Answer

(i) 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, the water tastes flat.

(ii) Soda contains carbon dioxide dissolved in water under pressure.
Solubility of carbon dioxide under normal atmospheric pressure is very low, but when subjected to high pressure, it dissolves to a great extent as in case of soda water in which carbon dioxide is dissolved in water under pressure and the bottle is corked.
On opening the bottle, the gas rapidly bubbles out with a 'fizz', since the pressure on the surface of the water suddenly decreases and so does the solubility of carbon dioxide gas in water.

Question 14

What is meant by the terms :

(a) crystal, (b) crystallization, (c) seed crystal.

Explain with examples

Answer

(a) When a hot saturated solution is cooled the excess solid is generally thrown out in the form of particles having a definite geometric shape. These particles are called crystals.

(b) The process by which crystals are separated or deposited from a hot saturated solution of a substance on cooling gently is called crystallization.

(c) A well-formed crystal from the cooled filtrate of saturated solution used as seed for the formation of large sized crystal is called seed crystal.

E.g., A seed crystal of potassium nitrate (KNO₃) is used to prepare a large size crystal of potassium nitrate.

Question 15

Define the term 'water of crystallization'.

Answer

The fixed number of water molecules which enter into a loose chemical combination with the substance when the substance is crystallized from it's hot saturated solution is called water of crystallization.

Question 16

Differentiate between hydrated and anhydrous crystals with examples. State three defined changes which occur when hydrated copper sulphate is heated.

Answer

Changes that occur when hydrated copper sulphate is heated are :

1. White powder of anhydrous copper sulphate is formed from blue crystals of hydrated copper sulphate.
2. No geometric shape is seen.
3. Anhydrous copper sulphate is amorphous in nature.

Question 17

Washing soda and iron [III] chloride are separately exposed to the atmosphere. State

(i) the observations seen (ii) the phenomenon which occurs (iii) the reason for the phenomenon occurring in each case.

Would a similar phenomenon occur in case of exposure of common salt. Explain giving reasons.

Answer

When Washing soda is exposed to the atmosphere:

(i) Observations — It loses it's moisture [ i.e., water of crystallization] and becomes amorphous.

(ii) Phenomenon — Efflorescence

(iii) Reason for Efflorescence — Efflorescence occurs when the vapour pressure of the hydrated crystals exceeds the vapour pressure of the atmospheric humidity. Due to this, the substance loses its water of crystallization to the atmosphere.

When Iron (III) Chloride is exposed to the atmosphere:

(i) Observations — It absorbs moisture from the atmosphere and changes to liquid state.

(ii) Phenomenon — Deliquescence

(iii) Reason for Deliquescence — Deliquescence occurs when the vapour pressure of the deliquescent crystal solution is very low compared to the vapour pressure of the atmospheric humidity.

Exposure of common salt to the atmosphere:

No, in case of common salt (NaCl) no such phenomenon can occur as, it is anhydrous salt and does not contain water of crystallization.
Common salt is not deliquescent but contains small amount of calcium or magnesium chloride impurities which are deliquescent.
Hence, the impurities absorb moisture from the air, thereby turning common salt sticky and wet.

Question 18

Why is fused calcium chloride and not potassium chloride kept in a desiccator?

Answer

Fused calcium chloride is deliquescent in nature, absorbs moisture and hence used as drying agent or desiccating agent but KCl (potassium chloride) has no such property hence cannot be used as a desiccator.

Question 19

How does fused calcium chloride differ from iron [III] chloride when exposed to the atmosphere?

Answer

Fused calcium chloride is hygroscopic in nature, absorbs moisture from atmosphere but does not change it's state whereas iron [III] chloride is deliquescent in nature, it absorbs moisture from the atmosphere and changes into liquid state. Hence, the two substances differ.

Question 20

Conc. H₂SO₄ acts as a 'drying agent' & a 'dehydrating agent'. Explain and differentiate the words in italics.

Answer

Question 21

Explain the meaning of the terms – hard water & soft water.

Answer

Hard water — Water is said to be hard when it does not lather readily - with ordinary soap and hence wastes soap.

Soft Water — Water that lathers readily with ordinary soap and — hence does not waste soap.

Question 22

State the causes of hardness in water.

Answer

Substances that cause hardness in water are Calcium and magnesium salts. e.g., Calcium and magnesium - bicarbonates, sulphates and chlorides.

Question 23

Give two natural sources of hard water.

Answer

1. Water from springs
2. Water from rivers .

Question 24

Differentiate between temporary hard water & permanent hard water.

Answer

Question 25

State the cause of hardness in temporary & permanent hard water.

Answer

Cause of temporary hardness — presence of calcium and magnesium bicarbonates - Ca(HCO₃)₂, Mg(HCO₃)₂

Cause of permanent hardness — presence of calcium and magnesium chlorides and sulphates in water — CaCl₂, MgCl₂, CaSO₄, MgSO₄

Question 26

State the disadvantages of hardness in water.

Answer

Hard water is unfit for laundries, unsafe for drinking, not suitable for preparing solutions and for industrial uses since it forms a crusty boiler scale or fur in boilers.

Question 27

Temporary hardness in water can be removed by boiling. Give balanced equations to explain how, hardness in temporary hard water is removed by boiling.

Answer

Temporary hardness in water is due to the presence of calcium or magnesium bicarbonates.

On boiling temporary hard water, calcium and magnesium bicarbonates in water decompose to give insoluble carbonates, carbon dioxide and water.

The insoluble carbonates are filtered out, CO₂ gas escapes and water is rendered soft.

$\underset{\text{Calcium bicarbonate}}{\text{Ca(HCO}_3)_2}\xrightarrow{\text{Boil}} \underset{\text{Calcium carbonate}}{\text{CaCO}_3↓} + \text{H}_2\text{O} + \text{CO}_2 [\text{g}]$

$\underset{\text{Magnesium bicarbonate}}{\text{Mg(HCO}_3)_2}\xrightarrow{\text{Boil}} \underset{\text{Magnesium carbonate}}{\text{MgCO}_3↓} + \text{H}_2\text{O} + \text{CO}_2 [\text{g}]$

Question 28

Both temporary & permanent hardness in water can be removed by addition of washing soda. Give balanced equations for the same.

Answer

Below are the balanced equations for removal of temporary & permanent hardness in water by addition of washing soda:

Permanent Hard Water:

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

Temporary Hard Water:

$\underset{\text{Washing soda}}{\text{Na}_2\text{CO}_3} + \underset{\text{Calcium bicarbonate}}{\text{Ca(HCO}_3)_2}\longrightarrow \underset{\text{Calcium carbonate}}{\text{CaCO}_3↓} + 2\text{NaHCO}_3$

Question 29

A sample of water is given in a trough. State how would you prove experimentally whether the given sample is hard water or soft water.

Answer

A piece of ordinary soap is rubbed inside the sample of water. If the sample of water lathers, it is soft else it is hard.

Question 30

Two samples of water are placed in a beaker individually. State how you will determine experimentally which of the two samples contains permanent hard water.

Answer

Boil each sample of water separately. The boiled water is filtered. The clear residual sample left behind is rubbed with soap. If the sample of water does not lather after filtration, then the sample is permanent hard water.

Question 31

State what are synthetic detergents. Explain experimentally how you will determine the advantage of synthetic detergents over soap using a sample of hard water.

Answer

Synthetic detergents are sodium salts of sulphonic acid and their calcium and magnesium salts are soluble in hard water and do not form scum. Hence, synthetic detergents when rubbed in hard water lather easily and hence have an advantage over ordinary soap.

In order to study the advantage of synthetic detergents over soap experimentally, a sample of water is divided into two parts.

A synthetic soap is added to the first sample of hard water and rubbed slowly with the hand.

An ordinary soap is then added to the second sample of water and rubbed similarly.

Result : The sample of hard water lathers with a synthetic detergent but not with an ordinary soap.

Reason for the observation:

$\underset{\text{In hard water}}{\text{Ca(HCO}_3)_2} + \underset{\text{Soap}}{2\text{NaSt}} \longrightarrow \underset{\text{Scum}}{\text{Ca(St)}_2↓} + 2\text{NaHCO}_3$

Question 1

Complete and balance the equations:

Answer

Question 1(1984)

Name one substance which is 'deliquescent'.

Answer

Iron [III] chloride (FeCl₃)

Question 2(1984)

How does an increase in temperature affect :

(i) the solubility of NaCl
(ii) the solubility of CaSO₄ in water ?

Answer

(i) NaCl — Solubility increases slightly with increase in temperature.
(ii) CaSO₄ — Decreases with increase in temperature.

Question 1(1985)

Give reasons for the following :

(i) table salt becomes moist and sticky during the rainy season.

(ii) a white power forms on the surface of washing soda crystals which are left exposed to the air.

Answer

(i) Table salt is not deliquescent but contains a small amount of calcium and magnesium chloride impurities which are deliquescent. Hence, the impurities absorb moisture from the atmosphere, thereby turning table salt sticky and wet especially during humid conditions.

(ii) Washing Soda is a efflorescent substance and so when it is exposed to the atmosphere, it loses it's moisture [water of crystallization] partly and changes into amorphous state. Hence, a white power forms on the surface of washing soda crystals which are left exposed to the air.

Question 1(1986)

Why is anhydrous CaCl₂ used in a desiccator. Name one substance which is 'efflorescent'.

Answer

Anhydrous CaCl₂ is deliquescent in nature, absorbs moisture and hence used as drying agent or desiccating agent.

Washing soda [Na₂CO₃.10H₂O] is efflorescent in nature.

Question 2(1986)

Name a salt

(a) which contains water of crystallization

(b) which does not contain.

Answer

(a) Hydrated copper sulphate [CuSO₄.5H₂O] contain water of crystallization.

(b) Potassium nitrate [KNO₃] does not contain water of crystallization.

Question 1(1987)

Name a deliquescent substance.

Answer

Iron [III] chloride FeCl₃

Question 1(1988)

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.

Answer

(i) Washing Soda is an efflorescent substance and so when it is exposed to the atmosphere, it loses it's moisture [water of crystallization] partly and changes into amorphous state. Hence, a white power forms on the surface of washing soda crystals which are left exposed to the air.

(ii) When copper sulphate loses water of crystallization on being heated, it becomes white powder of CuSO₄ and is called anhydrous copper sulphate.

(iii) Fused calcium chloride or conc. sulphuric acid is deliquescent in nature, absorbs moisture and hence used as drying agent or desiccating agent.

Question 2(1988)

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

Answer

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

Question 1(1991)

What is 'water of crystallization' ? Name a crystalline salt which does not contain water of crystallization.

Answer

The fixed number of water molecules which enter into a loose chemical combination with the substance when the substance is crystallized from it's hot saturated solution is called water of crystallization.

KNO₃ is a substance that is crystalline but does not contain water of crystallization.

Question 2(1991)

What would you observe, when the water of crystallization of a salt is removed by heating it.

Answer

The salt loses it's geometric shape and becomes a white powder that is amorphous in nature.

Question 3(1991)

Define :

(i) Hygroscopy
(ii) Efflorescence.

Answer

(i) The phenomenon by which a substance absorbs moisture from the atmosphere when exposed to it, without changing its state [i.e., a solid substances remains a solid after absorption of moisture] is known as Hygroscopy.

(ii) Crystalline hydrated salts which on exposure to the atmosphere lose their moisture [water of crystallization] partly or completely to the atmosphere and change into the amorphous state are called efflorescent substances and the phenomenon is known as Efflorescence.

Question 4(1991)

What is the effect of temperature on the solubility of KNO₃, and calcium sulphate in water.

Answer

Solubility of KNO₃ increases with rise in temperature and that of calcium sulphate decreases with increase in temperature.

Question 1(1992)

What test would you do to find out whether a given solution is saturated or unsaturated.

Answer

When we add more of solute to a given solution and on stirring, the solute dissolves, it is unsaturated solution. If the solute settles down and does not dissolve, then it is a saturated solution.

Question 2(1992)

How can you increase the solubility of a given volume of gas in water.

Answer

Solubility of a given volume of gas can be increased by increasing the pressure on the surface of water.

Question 1

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

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

Answer

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

Question 2

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

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

Answer

1. An anhydrous crystal — lead chloride.
2. A substance which causes hardness in water. — CaCl₂.
3. A deliquescent salt of a divalent metal — CaCl₂
4. An anhydrate of a heptahydrate salt. — FeSO₄
5. A drying agent, deliquescent in nature used in a dessicator. — fused CaCl₂

Question 3.1

Give reasons for the following

Solubility curves find utility in separation and purification of solutes.

Answer

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.

Question 3.2

Give reasons for the following

Pressure and temperature influence the solubility of gases in water.

Answer

An increase in pressure on the surface of water causes increase in solubility of gas in water and an increase in temperature of water causes decrease in solubility of gas in water.

Question 3.3

Give reasons for the following

Heating a hydrated copper sulphate crystal is deemed a chemical change.

Answer

When a hydrated copper sulphate crystal is heated, it changes to a white powder of anhydrous copper sulphate from blue crystals and the geometric shape is lost, which cannot be restored. Hence, heating hydrated copper sulphate crystals is deemed a chemical change.

Question 3.4

Give reasons for the following

Efflorescence is minimum during humid conditions.

Answer

Efflorescence occurs when the vapour pressure of the hydrated crystals exceeds the vapour pressure of the atmospheric humidity. Hence, efflorescence is minimum during humid conditions.

Question 3.5

Give reasons for the following

A crusty 'boiler scale' is formed in boilers, when hard water is used.

Answer

As hard water contains bicarbonates of calcium and magnesium and on boiling CaCO₃ or MgCO₃ is formed in large scale which gets deposited as crusty 'boiler scale'.

Question 4

Name or state the following.

1. An efflorescent decahydrate salt.
2. A deliquescent salt of a trivalent metal.
3. A liquid hygroscopic substance.
4. A salt whose solubility decreases with rise in temperature of the solvent water.
5. A substance added to remove both temporary and permanent hardness in water.

Answer

1. Washing soda (Na₂CO₃.10H₂O)
2. Iron (III) chloride (FeCl₃)
3. Concentrated sulphuric acid (conc. H₂SO₄)
4. CaSO₄
5. Washing soda (Na₂CO₃)

Question 5.1

Differentiate between the following :

Natural water and treated water

Answer

Question 5.2

Differentiate between the following :

Saturated solution and super saturated solution

Answer

Question 5.3

Differentiate between the following :

Solubility and solubility curve

Answer

Question 5.4

Differentiate between the following :

Deliquescent salt and hygroscopic salt

Answer

Question 5.5

Differentiate between the following :

Solute and solvent [forming a solution].

Answer

Question 6

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

Answer