ICSE Questions Set in Previous Years

I.C.S.E. Questions set in previous years

Question 1

Name a gas which reduces hot copper (II) oxide to copper.

Answer

Ammonia gas reduces hot copper (II) oxide to copper.

Question 2

Write correctly a balanced equation for the following 'word' equation :

Red lead ⟶ lead monoxide + oxygen

Answer

2Pb₃O₄ ⟶ 6PbO + O₂

Question 3

Name a gas which burns in air or oxygen forming H₂O.

Answer

Hydrogen

Question 4

Name a gas used along with acetylene for welding and cutting metals.

Answer

Oxygen

Question 5

Write correctly the balanced equation for the following :

When red lead, Pb₃O₄ is heated.

Answer

2Pb₃O₄ $\overset{\Delta}{\longrightarrow}$ 6PbO + O₂

Question 6

Write correctly balanced equations in the following cases :

(a) When lead (II) oxide (PbO₂) is heated.

(b) When phosphorous is burnt in a jar of oxygen.

Answer

(a) 2PbO₂ $\overset{\Delta}{\longrightarrow}$ 2PbO + O₂

(b) P₄ + 5O₂ ⟶ 2P₂O₅

Question 7

Name a gas which reduces hot copper (II) oxide to copper.

Answer

Ammonia gas reduces hot copper (II) oxide to copper.

Question 8

Name the products formed when a candle burns in oxygen.

Answer

Carbon dioxide and water vapour

Question 9

Give a reason for the following :

Like oxygen, nitrous oxide (N₂O) also supports combustion. A glowing splint introduced into a jar of N₂O, is rekindled.

Give a chemical test to distinguish oxygen from N₂O

Answer

The reason nitrous oxide (N₂O) supports combustion and rekindles a glowing splint is that nitrous oxide decomposes on heating into nitrogen and oxygen, which supports combustion.

2N₂O ⟶ 2N₂ + O₂

Chemical test to distinguish Oxygen from N₂O :

We can use nitric oxide (NO) to distinguish between nitrous oxide (N₂O) and oxygen (O₂).

Nitric oxide (NO) reacts with oxygen to produce nitrogen dioxide which can be easily identified by its reddish brown fumes.
2NO↑ + O₂↑ ⟶ 2NO₂↑ [Reddish brown vapours]

No reaction occurs between nitric oxide (NO) and nitrous oxide (N₂O). Hence, the absence of reddish brown fumes indicates nitrous oxide (N₂O).

Question 10

Name a non-metallic element which forms an acidic and a neutral oxide.

Answer

Nitrogen

Question 11

Name a non-metallic oxide which is a reducing agent.

Answer

Carbon monoxide

Question 12

Name (formula not acceptable) the gas produced in the following reaction: Burning of sulphur.

Answer

Sulphur dioxide

Question 13

Explain why chlorine turns moist starch iodide paper blue-black.

Answer

Chlorine reacts with potassium iodode to form iodine. This iodine reacts with starch to form blue black colour.

Cl₂ + 2KI ⟶ 2KCl + I₂

Starch + I₂ ⟶ Blue black colour

Question 14

State three tests by which you could identify a gas as being chlorine.

Answer

Tests for chlorine:

1. It turns moist blue litmus paper red and finally bleaches it.
2. It turns moist starch iodide paper (KI + starch) blue black
3. Pass the gas through silver nitrate solution, a white ppt. of silver chloride is formed.

Question 15

State what you observe when a piece of moist blue litmus paper is placed in a gas jar of chlorine.

Answer

Chlorine turns moist blue litmus paper red and finally bleaches it.

Question 16

For the elements sodium and phosphorous, state the following

(a) the formula of the chloride of each element,

(b) the physical state of each chloride at room temperature (i.e., solid, liquid or gas), and

(c) the nature of bonding of each chloride (i.e., ionic or covalent)

Answer

Question 17

Describe the two colour changes that take place when moist blue litmus is placed in a gas jar of chlorine. What are these colour changes that take place when chlorine water is exposed to sunlight?

Answer

Chlorine turns moist blue litmus red and finally bleaches it i.e., decolourizes it.

When chlorine water is exposed to sunlight, it changes to colourless from greenish yellow. This colour change happens because chlorine reacts with water to form hypochlorous acid which further breaks to form hydrochloric acid and oxygen gas.

Cl₂ + H₂O ⟶ HClO + HCl

HClO ⟶ HCl + [O]

[O] + colouring matter ⟶ bleached product

Question 18

Write correctly a balanced equation for the following 'word equation':

Calcium carbonate + Hydrochloric acid ⟶ Calcium chloride + water + Carbon dioxide

Answer

CaCO₃ + 2HCl ⟶ CaCl₂ + H₂O + CO₂

Question 19

In each case, describe one chemical test applied to the following substances, which would enable you to distinguish between them : carbon dioxide gas and hydrogen chloride gas

Answer

Carbon dioxide gas — It turns lime water milky due to the formation of white ppt. of calcium carbonate. The milkiness disappears when carbon dioxide is passed in excess.

Ca(OH)₂ + CO₂ ⟶ CaCO₃↓ + H₂O

CaCO₃↓ + H₂O + CO₂ [excess] ⟶ Ca(HCO₃)₂ [soluble]

Hydrogen chloride gas — When a glass rod dipped in ammonia solution is brought near the gas, dense white fumes of ammonium chloride are released.

NH₃ + HCl ⟶ NH₄Cl

Question 20

Give reasons for : 'It is dangerous to sleep in a closed room in which a coal fire is burning'.

Answer

The incomplete combustion of coal releases carbon monoxide gas into the environment. Carbon monoxide is a highly poisonous gas. If inhaled, it binds with haemoglobin 200 times more strongly than oxygen reducing the blood's ability to carry oxygen by converting haemoglobin into carboxyhaemoglobin. So, even in small quantities, it is fatal and can lead to death.

Question 21

Give the name of an acid salt found in 'Health Salts'.

Answer

Sodium bisulphate

Question 22

Name two important processes which generate or release carbon dioxide into the atmosphere.

Answer

Two important processes which generate or release carbon dioxide into the atmosphere are :

1. Burning of fossil fuels like coal, natural gas and petroleum.
2. Respiration by animals and plants.

Question 23

Name two processes which remove carbon dioxide from the atmosphere.

Answer

Two processes which remove carbon dioxide from the atmosphere are :

1. Photosynthesis.
2. Absorption by the oceans.

Question 24

Explain the following :

A white crust forms on the surface of lime water that has been exposed to the atmosphere.

Answer

Carbon dioxide present in the atmosphere turns lime water milky due to the formation of white ppt. of calcium carbonate.

CO₂ + Ca(OH)₂ ⟶ CaCO₃↓ + H₂O

Question 25

Give the biological importance of carbon dioxide dissolved in water.

Answer

Biological importance of carbon dioxide dissolved in water :

1. 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
2. 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 26

State your observations and give balanced equations of the reactions when carbon dioxide is passed through clear lime water (a) for a short time (b) for a long time.

Answer

(a) When carbon dioxide is passed through lime water for a short time, it turns lime water milky. This is due to the formation of insoluble calcium carbonate.

Ca(OH)₂ + CO₂ ⟶ CaCO₃ ↓ + H₂O

(b) When the gas is passed for a long time, the milkiness disappears. This is due to the formation of a soluble bicarbonate.

CaCO₃ + CO₂ + H₂O ⟶ Ca(HCO₃)₂ [soluble]

Question 27

XCl₂ is the chloride of a metal X. Write down the formula of the sulphate and the hydroxide of the metal X.

Answer

Chloride of a metal X is XCl₂

By interchanging subscript and writing as superscript:

$\underset{\phantom{1}{1}}{\text{X}} \space {\nearrow}\mathllap{\nwarrow} \space \underset{2}{\text{Cl}} \Rightarrow \overset{\phantom{2}{2}}{\text{X}} \space {\nearrow}\mathllap{\nwarrow} \space \overset{1}{\text{Cl}} \\[0.5em]$

Therefore, valency of metal X = 2.

Formula of the sulphate:

$\text{X}^{2+} \phantom{\nearrow} \text{SO}_{4}^{2-} \\[0.5em] \overset{\phantom{2}{2}}{\text{X}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{S}}\text{O}_{4} \Rightarrow \underset{\phantom{2}{2}}{\text{X}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{S}}\text{O}_{4} \\[0.5em]$

As valency of both X and SO₄ is 2 so dividing by 2 we get 1 but 1 is never written so we get the formula as $\bold{XSO}_{\bold{4}}$

Formula of the hydroxide:

$\text{X}^{2+} \phantom{\nearrow} \text{OH}^{1-} \\[0.5em] \overset{\phantom{2}{2}}{\text{X}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{O}}\text{H} \Rightarrow \underset{\phantom{1}{1}}{\text{X}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{O}}\text{H} \\[0.5em]$

Dropping 1 and enclosing OH in brackets, we get the formula as $\bold{X(OH)}_{\bold{2}}$

Therefore, we get

Formula of Sulphate : $\bold{XSO}_{\bold{4}}$

Formula of Hydroxide : $\bold{X(OH)}_{\bold{2}}$

Question 28

Complete the following 'word' equations. Write the words 'No reaction' if none occurs.

(a) magnesium + copper sulphate ⟶

(b) iron (II) sulphide + hydrochloric acid ⟶

Answer

(a) magnesium + copper sulphate ⟶ magnesium sulphate + copper

(b) iron (II) sulphide + hydrochloric acid ⟶ iron (II) chloride + hydrogen sulphide

Question 29

Write the equation for the following and state whether it is a decomposition reaction : The action of heat on lead nitrate.

Answer

When lead nitrate is heated it decomposes to form lead oxide, nitrogen dioxide and oxygen. It is a decomposition reaction.

The reaction is as follows:

2Pb(NO₃)₂ $\overset{\Delta}{\longrightarrow}$ 2PbO + 4NO₂ + O₂

Question 30

What are the constituent units of crystals of :

(a) Iodine

(b) Sodium chloride

Answer

Constituent units of crystals of :

(a) Iodine — iodine

(b) Sodium chloride — sodium and chloride ions.

Question 31

(a) What is meant by the term 'atomicity of a gas'?

(b) Name a gas that is diatomic.

Answer

(a) The number of atoms in a molecule of an element is called its atomicity.

(b) Hydrogen (H₂) is a diatomic element.

Question 32

Name a common substance that exists in all the three states of matter.

Answer

Water exists in all the three physical states : as solid (ice), as liquid (water) and as gas (water vapour).

Question 33

Give an example of an endothermic reaction.

Answer

Heat is absorbed during the reaction of Nitrogen and Oxygen to produce nitric oxide, hence, the reaction is endothermic in nature.

N₂ + O₂ $\xrightarrow{\Delta}$ 2NO

Question 34

An element X is trivalent. Write the balanced equation for the combustion of X in oxygen.

Answer

Valency of X is 3+ and that of Oxygen is 2-

By interchanging the valency number and shifting it to the lower right side of the atom or radical, we get the formula : X₂O₃

Equation for the combustion of X in oxygen: 4X + 3O₂ ⟶ 2X₂O₃

Question 35

In which of the following four substances will there be :

(a) increase in weight

(b) decrease in weight

(c) no change in weight, when exposed to air.

(i) sodium chloride

(ii) sodium carbonate crystals

(iii) conc. H₂SO₄ and

(iv) iron?

Answer

(a) Increase in weight : Iron, conc. sulphuric acid

Reason — Increase in weight is due to the absorbed water in case of sulphuric acid. Gain in weight 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 weight : 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 weight : 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 36

'When stating the volume of a gas, the pressure and temperature should also be given'. Why ?

Answer

The change in any one of the parameters [pressure, volume, temperature] affects the other two parameters. Therefore, when stating the volume of a gas the pressure and temperature should also be given.

Question 37

Define or state :

(a) Absolute temperature

(b) Boyle's law

(c) Charles' law

Answer

(a) Absolute temperature — Absolute temperature refers to the temperature measured in Kelvin or absolute scale that has its zero at -273°C (absolute zero) and whose each degree is equal to one degree on the Celsius scale.

(b) Boyle's law — Temperature remaining constant the volume of a given mass of dry gas is inversely proportional to it's pressure.

V ∝ $\dfrac{1}{\text{P}}$ [T = constant]

(c) Charles' law — Charles law states that pressure remaining constant, the volume of a given mass of dry gas increases or decreases by $\dfrac{1}{273}$ of its volume at 0°C for each 1°C increase or decrease in temperature, respectively.

Question 38

A gas occupies 760 cm³ at 27°C and 70 cm of Hg. What will be its volume at STP?

Answer

Using gas laws:

$\dfrac{\text{P}_1{\text{V}_1}}{\text{T}_1}$ = $\dfrac{\text{P}_2{\text{V}_2}}{\text{T}_2}$

Substituting the values:

$\dfrac{70 \times 760}{300}$ = $\dfrac{76 \times {\text{V}_2}}{273}$

Therefore:

$\text{V}_2 =\dfrac{70 \times 760 \times 273}{300 \times 76} = 637 \text{cm}^3$

∴ The volume occupied by the gas is 637 cm³.

Question 39

At 0°C and 760 mm Hg pressure, a gas occupies a volume of 100 cm³. The Kelvin temperature (Absolute temperature) of the gas is increased by one-fifth while the pressure is increased by one-fifth times. Calculate the final volume of the gas.

Answer

Initial conditions [S.T.P.] :

P₁ = Initial pressure of the gas = 760 mm Hg
V₁ = Initial volume of the gas = 100 cm³
T₁ = Initial temperature of the gas = 0°C = 273 K

Final conditions:

P₂ (Final pressure) = increased by one-fifth of P₁
= (1 + $\dfrac{1}{5}$) of 760
= $\dfrac{6}{5}$ x 760
= 912
V₂ (Final volume) = ?
T₂ (Final temperature) = increased by one-fifth of 273 K = 1 + $\dfrac{1}{5}$ of 273
= $\dfrac{6}{5}$ x 273 = $\dfrac{1638}{5}$

By Gas Law:

$\dfrac{\text{P}_1\times\text{V}_1}{\text{T}_1} = \dfrac{\text{P}_2\times\text{V}_2}{\text{T}_2}$

Substituting the values :

$\dfrac{760 \times 100 }{273} = \dfrac{912\times \text{V}_2}{\dfrac{1638}{5}} \\[1em] \dfrac{760 \times 100 }{273} = \dfrac{5 \times 912 \times \text{V}_2}{1638} \\[1em] \text{V}_2 = \dfrac{760 \times 100 \times 1638}{273 \times 5 \times 912} \\[1em] \text{V}_2 = 100\text{ cm}^3$

∴ The final volume of the gas = 100 cm³

Question 40

The pressure of one mole of gas at STP is doubled and the temperature is raised to 546 K. What is the final volume of the gas ? [one mole of a gas occupies a volume of 22.4 litres at STP].

Answer

Initial conditions [S.T.P.] :

P₁ = Initial pressure of the gas = 1 atm
V₁ = Initial volume of the gas = 22.4 litres
T₁ = Initial temperature of the gas = 273 K

Final conditions:

P₂ (Final pressure) = 2 atm
V₂ (Final volume) = ?
T₂ (Final temperature) = 546 K

By Gas Law:

$\dfrac{\text{P}_1\times\text{V}_1}{\text{T}_1} = \dfrac{\text{P}_2\times\text{V}_2}{\text{T}_2}$

Substituting the values :

$\dfrac{1 \times 22.4}{273} = \dfrac{2\times \text{V}_2}{546}\\[0.5em] \text{V}_2 = \dfrac{1 \times 22.4\times 546}{273\times 2} \\[0.5em] \text{V}_2 = 22.4 \text{ lit}$

∴ Final volume of the gas = 22.4 lit.

Question 41

It is possible to change the temperature and pressure of a fixed mass of gas without changing it's volume. Give reasons for your answer.

Answer

No, it is not possible. Change in any one of the parameters [pressure, volume, temperature] affects the other two parameters.

Question 42

Complete the following equations in words and then write the balanced molecular equation in each case.

(a) Zinc nitrate + sodium carbonate ⟶ ............... + ...............

(b) Iron (II) sulphate (ferrous sulphate) + sodium hydroxide ⟶ ............... + ...............

Answer

(a) Zinc nitrate + sodium carbonate ⟶ zinc carbonate + sodium nitrate

Zn(NO₃)₂ + Na₂CO₃ ⟶ ZnCO₃ + 2NaNO₃

(b) Iron (II) sulphate (ferrous sulphate) + sodium hydroxide ⟶ Iron (II) hydroxide + sodium sulphate

FeSO₄ + 2NaOH ⟶ Fe(OH)₂ + Na₂SO₄

Question 43

Give one reason why magnetizing a piece of steel is a physical change.

Answer

Magnetization of a piece of steel is a physical change because no new substance is formed and the chemical composition of the piece of steel remains the same.

Question 44

Name a homogeneous mixture of :

(a) a liquid and a solid

(b) two liquids

Answer

(a) a liquid and a solid — sugar and water

(b) two liquids — water and alcohol

Question 45

If the formula of the nitride of a metal X is XN, what is the formula of

(i) it's sulphate

(ii) it's hydroxide.

Answer

Nitride of a metal X is XN.

Since valency of nitrogen is 3- so valency of X is 3+

Formula of the sulphate:

$\text{X}^{3+} \phantom{\nearrow} \text{SO}_{4}^{2-} \\[0.5em] \overset{\phantom{2}{3}}{\text{X}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{S}}\text{O}_{4} \Rightarrow \underset{\phantom{2}{2}}{\text{X}} \space {\searrow}\mathllap{\swarrow} \space \underset{3}{\text{S}}\text{O}_{4} \\[0.5em]$

So, we get the formula as $\bold{X}_{\bold{2}}({\bold{SO}}_{\bold{4}})_{\bold{3}}$

Formula of the hydroxide:

$\text{X}^{3+} \phantom{\nearrow} \text{OH}^{1-} \\[0.5em] \overset{\phantom{2}{3}}{\text{X}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{O}}\text{H} \Rightarrow \underset{\phantom{1}{1}}{\text{X}} \space {\searrow}\mathllap{\swarrow} \space \underset{3}{\text{O}}\text{H} \\[0.5em]$

Dropping 1, we get the formula as $\bold{X(OH)}_{\bold{3}}$

Therefore, we get

Formula of Sulphate : $\bold{X}_{\bold{2}}({\bold{SO}}_{\bold{4}})_{\bold{3}}$

Formula of Hydroxide : $\bold{X(OH)}_{\bold{3}}$

Question 46

What is the valency of nitrogen in

(a) NO

(b) N₂O

(c) NO₂

Answer

$\text{(a)} \space \text{NO} = \underset{\phantom{1}{1}}{\text{N}} \space {\nearrow}\mathllap{\nwarrow} \space \underset{1}{\text{O}} \Rightarrow \overset{\phantom{1}{1}}{\text{N}} \space {\nearrow}\mathllap{\nwarrow} \space \overset{1}{\text{O}} \\[0.5em]$

But valency of O is 2. Multiplying by 2, we get:

$\overset{{2 \times 1}}{\text{N}} \space {\nearrow}\mathllap{\nwarrow} \space \overset{2 \times 1}{\text{O}} \Rightarrow \overset{\phantom{2}{2}}{\text{N}} \space {\nearrow}\mathllap{\nwarrow} \space \overset{2}{\text{O}} \\[0.5em]$

Therefore, valency of Nitrogen is 2.

$\text{(b)} \space \text{N}_2\text{O} = \underset{\phantom{2}{2}}{\text{N}} \space {\nearrow}\mathllap{\nwarrow} \space \underset{1}{\text{O}} \Rightarrow \overset{\phantom{1}{1}}{\text{N}} \space {\nearrow}\mathllap{\nwarrow} \space \overset{2}{\text{O}} \\[0.5em]$

Therefore, valency of Nitrogen is 1.

$\text{(c)} \space \text{NO}_2 = \underset{\phantom{1}{1}}{\text{N}} \space {\nearrow}\mathllap{\nwarrow} \space \underset{2}{\text{O}} \Rightarrow \overset{\phantom{2}{2}}{\text{N}} \space {\nearrow}\mathllap{\nwarrow} \space \overset{1}{\text{O}} \\[0.5em]$

But valency of O is 2. Multiplying by 2, we get:

$\overset{{2 \times 2}}{\text{N}} \space {\nearrow}\mathllap{\nwarrow} \space \overset{2 \times 1}{\text{O}} \Rightarrow \overset{\phantom{4}{4}}{\text{N}} \space {\nearrow}\mathllap{\nwarrow} \space \overset{2}{\text{O}} \\[0.5em]$

Therefore, valency of Nitrogen is 4.

Question 47

Name a non-metallic element, which :

(a) is liquid at ordinary temperature

(b) is a conductor of electricity

Answer

(a) Bromine

(b) Graphite

Question 48

Name a metal oxide which is yellow in colour.

Answer

Lead (II) oxide (PbO)

Question 49

Name the process of change of state by which napthalene changes into vapour. Name an element that can undergo the same change of state.

Answer

Sublimation is the name of the process.

Iodine also undergoes sublimation process.

Question 50

Dilute hydrochloric acid is added first to a mixture of iron and sulphur and then to a compound formed by iron and sulphur. Name the gases formed in each case.

Answer

Hydrogen gas is evolved when dil. hydrochloric acid is added first to a mixture of iron and sulphur

Fe + 2HCl (dil.) ⟶ FeCl₂ + H₂ ↑

Hydrogen sulphide (H₂S) gas is evolved when dil. hydrochloric acid is added to compound formed by iron and sulphur.

FeS + 2HCl ⟶ FeCl₂ + H₂S

Question 51

State whether the following conversations are examples of oxidation or reduction :

(a) Na ⟶ Na⁺ + e^-

(b) PbO₂ ⟶ PbSO₄

Answer

(a) Oxidation, loss of electrons

(b) Oxidation, gain of oxygen

Question 52

Complete the following equations and state in each case if the reaction represents oxidation or reduction :

(a) Fe²⁺ ⟶ Fe³⁺

(b) Cl^- ⟶ Cl

(c) Cu²⁺ ⟶ Cu

(d) Ag ⟶ Ag⁺

(e) H ⟶ H⁺

(f) Al ⟶ Al³⁺

Answer

(a) Fe²⁺ ⟶ Fe³⁺ + e^-
Oxidation, loss of electrons

(b) Cl^- ⟶ Cl + e^-
Oxidation, loss of electrons

(c) Cu²⁺ + 2e^- ⟶ Cu
Reduction, gain of electrons

(d) Ag ⟶ Ag⁺ + e^-
Oxidation, loss of electrons

(e) H ⟶ H⁺ + e^-
Oxidation, loss of electrons

(f) Al ⟶ Al³⁺ + 3e^-
Oxidation, loss of electrons

Question 53

Reactions can be classified as follows :

Direct combination, decomposition, simple displacement, double decomposition, redox (oxidation-reduction) reactions.

State which of the above types take place in the reactions given below :

(a) Cl₂ + 2KI ⟶ 2KCl + I₂

(b) SO₂ + 2H₂O + Cl₂ ⟶ 2HCl + H₂SO₄

(c) 4HNO₃ ⟶ 4NO₂ + 2H₂O + O₂

(d) 2Mg + O₂ ⟶ 2MgO

(e) AgNO₃ + HCl ⟶ AgCl + HNO₃

Answer

(a) Simple displacement

(b) Redox reaction

(c) Decomposition reaction

(d) Direct combination

(e) Double decomposition

Question 54

Express Kelvin Zero in °C.

Answer

Kelvin zero or absolute zero = -273°C.

Question 55

Study the reaction scheme below and then answer (a), (b) and (c) which follow :

(a) Give the chemical name and formula of (i) marble (ii) gas 'A' (iii) solid 'B'

(b) What would you expect to see (or observe) when drops of water are added to solid calcium oxide?

(c) (i) Rewrite the following sentences filling in the blanks :

Solution C is normally called ...............

When gas 'A' is bubbled through 'C'., a white precipitate of ............... is formed.

(ii) On bubbling excess of gas A through the resulting suspension, the white precipitate dissolves and then reappears on boiling. Suggest an explanation for these observations.

Answer

(a) CaCO₃ ⟶ CaO + CO₂ [Gas A]

CaO + H₂O ⟶ Ca(OH)₂

Ca(OH)₂ + H₂O [excess] ⟶ CaO + H₂O

(i) marble — Calcium carbonate [CaCO₃]

(ii) gas 'A' — Carbon dioxide [CO₂]

(iii) solid 'B' — Calcium hydroxide [Ca(OH)₂]

(b) When drops of water are added to solid calcium oxide, a large amount of heat is liberated and a hissing sound is heard along with the formation of calcium hydroxide [Ca(OH)₂]. It is an exothermic reaction.

(c) (i) Solution C is normally called lime water

When gas 'A' is bubbled through 'C'., a white precipitate of calcium hydroxide is formed.

(ii) When excess of gas A [Carbon dioxide] is passed through the resulting suspension, the white ppt. of calcium carbonate dissolves due to the formation of calcium hydrogen carbonate which is soluble in water.

CaCO₃ + H₂O + CO₂ [excess] ⟶ Ca(HCO₃)₂ [soluble]

On boiling, calcium hydrogen carbonate decomposes to form calcium carbonate due to which, the white ppt. reappears.

Ca(HCO₃)₂ $\xrightarrow{\space \Delta \space}$ H₂O + CO₂ + CaCO₃↓