Lesson: Chemical Reactions and Equations

# Question 1

Which of the following is not a physical change?

(a) Boiling of water to give water vapour

(b) Melting of ice to give water

(c) Dissolution of salt in water

(d) Combustion of Liquefied Petroleum Gas (LPG)

(d)

# Question 2

The following reaction is an example of a:

$\text{4}\text{\hspace{0.17em}}{\text{NH}}_{\text{3}}\left(\text{g}\right)\text{+5}\text{\hspace{0.17em}}{\text{O}}_{\text{2}}\left(\text{g}\right)\to \text{4}\text{\hspace{0.17em}}\text{NO}\left(\text{g}\right)\text{+6}\text{\hspace{0.17em}}{\text{H}}_{\text{2}}\text{O}\left(\text{g}\right)$

(i) Displacement reaction

(ii) Combination reaction

(iii) Redox reaction

(iv) Neutralisation reaction

(a) (i) and (iv)

(b) (ii) and (iii)

(c) (i) and (iii)

(d) (iii) and (iv)

(c)

# Question 3

Which of the following statements about the given reaction are correct?

$\text{3}\text{\hspace{0.17em}}\text{Fe}\left(\text{s}\right)\text{+4}\text{\hspace{0.17em}}{\text{H}}_{\text{2}}\text{O}\left(\text{g}\right)\to {\text{Fe}}_{\text{3}}{\text{O}}_{\text{4}}\left(\text{s}\right)\text{+4}\text{\hspace{0.17em}}{\text{H}}_{\text{2}}\left(\text{g}\right)$

(i) Iron metal is getting oxidised

(ii) Water is getting reduced

(iii) Water is acting as reducing agent

(iv) Water is acting as oxidising agent

(a) (i), (ii) and (iii)

(b) (iii) and (iv)

(c) (i), (ii) and (iv)

(d) (ii) and (iv)

(c)

# Question 4

Which of the following are exothermic processes?

(i) Reaction of water with quick lime

(ii) Dilution of an acid

(iii) Evaporation of water

(iv) Sublimation of camphor (crystals)

(a) (i) and (ii)

(b) (ii) and (iii)

(c) (i) and (iv)

(d) (iii) and (iv)

(a)

# Question 5

Three beakers labelled as A, B and C each containing 25 ml of water was taken. A small amount of $\text{NaOH}$, anhydrous ${\text{CuSO}}_{\text{4}}$ and $\text{NaCl}$ were added to the beakers A, B and C respectively. It was observed that there was an increase in the temperature of the solutions contained in beakers A and B, whereas in case of beaker C, the temperature of the solution falls. Which one of the following statement(s) is(are) correct?

(i) In beakers A and B, exothermic process has occurred.

(ii) In beakers A and B, endothermic process has occurred.

(iii) In beaker C exothermic process has occurred.

(iv) In beaker C endothermic process has occurred.

(a) (i) only

(b) (ii) only

(c) (i) and (iv)

(d) (ii) and (iii)

(c)

# Question 6

A dilute ferrous sulphate solution was gradually added to the beaker containing acidified permanganate solution. The light purple colour of the solution fades and finally disappears. Which of the following is the correct explanation for the observation?

(a) ${\text{KMnO}}_{\text{4}}$ is an oxidising agent, it oxidises ${\text{FeSO}}_{\text{4}}$

(b) ${\text{FeSO}}_{\text{4}}$ acts as an oxidising agent and oxidises ${\text{KMnO}}_{\text{4}}$

(c) The colour disappears due to dilution; no reaction is involved

(d) ${\text{KMnO}}_{\text{4}}$ is an unstable compound and decomposes in presence of ${\text{FeSO}}_{\text{4}}$ to a colourless compound.

(a)

# Question 7

Which among the following is(are) double displacement reaction(s)?

(i) $Pb+CuC{l}_{2}\to PbC{l}_{2}+Cu$

(ii) $N{a}_{2}S{O}_{4}+BaC{l}_{2}\to BaS{O}_{4}+2\text{\hspace{0.17em}}NaCl$

(iii) $C+{O}_{2}\to C{O}_{2}$

(iv) $C{H}_{4}+2{O}_{2}\to C{O}_{2}+2\text{\hspace{0.17em}}{H}_{2}O$

(a) (i) and (iv)

(b) (ii) only

(c) (i) and (ii)

(d) (iii) and (iv)

(b)

# Question 8

Which among the following statement(s) is(are) true? Exposure of silver chloride to sunlight for a long duration turns grey due to

(i) The formation of silver by decomposition of silver chloride

(ii) Sublimation of silver chloride

(iii) Decomposition of chlorine gas from silver chloride

(iv) Oxidation of silver chloride

(a) (i) only

(b) (i) and (iii)

(c) (ii) and (iii)

(d) (iv) only

(a)

# Question 9

Solid calcium oxide reacts vigorously with water to form calcium hydroxide accompanied by liberation of heat. This process is called slaking of lime.  Calcium hydroxide dissolves in water to form its solution called lime water. Which among the following is(are) true about slaking of lime and the solution formed?

(i) It is an endothermic reaction

(ii) It is an exothermic reaction

(iii) The pH of the resulting solution will be more than seven

(iv) The pH of the resulting solution will be less than seven

(a) (i) and (ii)

(b) (ii) and (iii)

(c) (i) and (iv)

(d) (iii) and (iv)

(b)

# Question 10

Barium chloride on reacting with ammonium sulphate forms barium sulphate and ammonium chloride. Which of the following correctly represents the type of the reaction involved?

(i) Displacement reaction

(ii) Precipitation reaction

(iii) Combination reaction

(iv) Double displacement reaction

(a) (i) only

(b) (ii) only

(c) (iv) only

(d) (ii) and (iv)

(d)

# Question 11

Electrolysis of water is a decomposition reaction. The mole ratio of hydrogen and oxygen gases liberated during electrolysis of water is

(a) 1:1

(b) 2:1

(c) 4:1

(d) 1:2

(b)

# Question 12

Which of the following(s) is(are) an endothermic process(es)?

(i) Dilution of sulphuric acid

(ii) Sublimation of dry ice

(iii) Condensation of water vapours

(iv) Evaporation of water

(a) (i) and (iii)

(b) (ii) only

(c) (iii) only

(d) (ii) and (iv)

(d)

# Question 13

In the double displacement reaction between aqueous potassium iodide and aqueous lead nitrate, a yellow precipitate of lead iodide is formed. While performing the activity if lead nitrate is not available, which of the following can be used in place of lead nitrate?

(a) Lead sulphate (insoluble)

(b) Lead acetate

(c) Ammonium nitrate

(d) Potassium sulphate

(b)

# Question 14

Which of the following gases can be used for storage of fresh sample of an oil for a long time?

(a) Carbon dioxide or oxygen

(b) Nitrogen or oxygen

(c) Carbon dioxide or helium

(d) Helium or nitrogen

(d)

# Question 15

The following reaction is used for the preparation of oxygen gas in the laboratory

$2\text{\hspace{0.17em}}KCl{O}_{3}\left(s\right)\underset{Catalyst}{\overset{Heat}{\to }}2\text{\hspace{0.17em}}KCl\left(s\right)+3\text{\hspace{0.17em}}{O}_{2}\left(g\right)$

Which of the following statement(s) is(are) correct about the reaction?

(a) It is a decomposition reaction and endothermic in nature.

(b) It is a combination reaction.

(c) It is a decomposition reaction and accompanied by release of heat.

(d) It is a photochemical decomposition reaction and exothermic in nature.

(a)

# Question 16

Which one of the following processes involves chemical reactions?

(a) Storing of oxygen gas under pressure in a gas cylinder.

(b) Liquefaction of air.

(c) Keeping petrol in a china dish in the open.

(d) Heating copper wire in presence of air at high temperature.

(d)

# Question 17

In which of the following chemical equations, the abbreviations represent the correct states of the reactants and products involved at reaction temperature?

(a) $2\text{\hspace{0.17em}}{H}_{2}\left(l\right)+{O}_{2}\left(l\right)\to 2\text{\hspace{0.17em}}{H}_{2}O\left(g\right)$

(b) $2\text{\hspace{0.17em}}{H}_{2}\left(g\right)+{O}_{2}\left(l\right)\to 2\text{\hspace{0.17em}}{H}_{2}O\left(l\right)$

(c) $2\text{\hspace{0.17em}}{H}_{2}\left(g\right)+{O}_{2}\left(g\right)\to 2\text{\hspace{0.17em}}{H}_{2}O\left(l\right)$

(d) $2\text{\hspace{0.17em}}{H}_{2}\left(g\right)+{O}_{2}\left(g\right)\to 2\text{\hspace{0.17em}}{H}_{2}O\left(g\right)$

(d)

# Question 18

Which of the following are combination reactions?

(i) $2\text{\hspace{0.17em}}KCl{O}_{3}\stackrel{Heat}{\to }2\text{\hspace{0.17em}}KCl+3\text{\hspace{0.17em}}{O}_{2}$

(ii) $MgO+{H}_{2}O\to Mg{\left(OH\right)}_{2}$

(iii) $4Al+3\text{\hspace{0.17em}}{O}_{2}\to 2\text{\hspace{0.17em}}A{l}_{2}{O}_{3}$

(iv) $Zn+FeS{O}_{4}\to ZnS{O}_{4}+Fe$

(a) (i) and (iii)

(b) (iii) and (iv)

(c) (ii) and (iv)

(d) (ii) and (iii)

(d)

# Question 19

Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.

(a) Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773K to form ammonia gas.

(b) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water.

(c) Ethanol is warmed with ethanoic acid to form ethyl acetate in the presence of concentrated ${H}_{2}S{O}_{4}$.

(d) Ethene is burnt in the presence of oxygen to form carbon dioxide, water and releases heat and light.

## Solution:

(a)   ${N}_{2}\left(g\right)+3\text{\hspace{0.17em}}{H}_{2}\left(g\right)\underset{773\text{\hspace{0.17em}}\text{\hspace{0.17em}}K}{\overset{Catalyst}{\to }}2\text{\hspace{0.17em}}N{H}_{3}\left(g\right)$

Reaction type: Combination reaction

(b)   $NaOH\left(aq\right)+C{H}_{3}COOH\left(aq\right)\to C{H}_{3}COONa\left(aq\right)+{H}_{2}O\left(l\right)$

Reaction type: Double displacement reaction/Neutralisation reaction

(c)   ${C}_{2}{H}_{5}OH\left(l\right)+C{H}_{3}COOH\left(l\right)\stackrel{H+}{\to }C{H}_{3}COO{C}_{2}{H}_{5}\left(l\right)+{H}_{2}O\left(l\right)$

Reaction type: Double displacement reaction/Esterification reaction

(d)   ${C}_{2}{H}_{4}\left(g\right)+3{O}_{2}\left(g\right)\to 2C{O}_{2}\left(g\right)+2{H}_{2}O\left(g\right)+Heat+Light$

Reaction type: Redox reaction/Combustion reaction

# Question 20

Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.

(a) Thermite reaction, iron (III) oxide reacts with aluminium and gives molten iron and aluminium oxide.

(b) Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium nitride.

(c) Chlorine gas is passed in an aqueous potassium iodide solution to form potassium chloride solution and solid iodine.

(d) Ethanol is burnt in air to form carbon dioxide, water and releases heat.

## Solution:

(a)   $F{e}_{2}{O}_{3}\left(s\right)+2\text{\hspace{0.17em}}Al\left(s\right)\to A{l}_{2}{O}_{3}\left(s\right)+2\text{\hspace{0.17em}}Fe\left(l\right)+Heat$

Reaction type: Displacement reaction/Redox reaction

(b)   $3Mg\left(s\right)+{N}_{2}\left(g\right)\to M{g}_{3}{N}_{2}\left(s\right)$

Reaction type: Combination reaction

(c)   $2\text{\hspace{0.17em}}KI\left(aq\right)+C{l}_{2}\left(g\right)\to 2\text{\hspace{0.17em}}KCl\left(aq\right)+{I}_{2}\left(s\right)$

Reaction type: Displacement reaction

(d)   ${C}_{2}{H}_{5}OH\left(l\right)+3\text{\hspace{0.17em}}{O}_{2}\left(g\right)\to 2\text{\hspace{0.17em}}C{O}_{2}\left(g\right)+3\text{\hspace{0.17em}}{H}_{2}O\left(l\right)+Heat$

Reaction type: Redox reaction/Combustion reaction

# Question 21

Complete the missing components/variables given as x and y in the following reactions:

(a)   $Pb{\left(N{O}_{3}\right)}_{2}\left(aq\right)+2\text{\hspace{0.17em}}KI\left(aq\right)\to Pb{I}_{2}\left(x\right)+2\text{\hspace{0.17em}}KN{O}_{3}\left(y\right)$

(b)

(c)   $Zn\left(s\right)+{H}_{2}S{O}_{4}\left(aq\right)\to ZnS{O}_{4}\left(x\right)+{H}_{2}\left(y\right)$

(d)

## Solution:

(a)   $x\to \left(s\right)$

$y\to \left(aq\right)$

(b)   $x\to 2\text{\hspace{0.17em}}Ag$

(c)     $x\to \left(aq\right)$

$y\to \left(g\right)$

(d)   $x\to Heat$

# Question 22

Which among the following changes are exothermic or endothermic in nature?

(a) Decomposition of ferrous sulphate.

(b) Dilution of sulphuric acid.

(c) Dissolution of sodium hydroxide in water.

(d) Dissolution of ammonium chloride in water.

## Solution:

(b) and (c) are exothermic as heat is released in these changes.

(a) and (d) are endothermic as heat is absorbed in these changes.

# Question 23

Identify the reducing agent in the following reactions

(a)   $4\text{\hspace{0.17em}}N{H}_{3}+5\text{\hspace{0.17em}}{O}_{2}\to 4\text{\hspace{0.17em}}NO+6\text{\hspace{0.17em}}{H}_{2}O$

(b)

(c)   $F{e}_{2}{O}_{3}+3\text{\hspace{0.17em}}CO\to 2\text{\hspace{0.17em}}Fe+3\text{\hspace{0.17em}}C{O}_{2}$

(d)   $2\text{\hspace{0.17em}}{H}_{2}+{O}_{2}\to 2\text{\hspace{0.17em}}{H}_{2}O$

## Solution:

(a)   Ammonia $\left(N{H}_{3}\right)$

(b)   Water $\left({H}_{2}O\right)$ as ${F}_{2}$ is getting reduced to HF

(c)     Carbon monoxide $\left(CO\right)$

(d)   Hydrogen

# Question 24

Identify the oxidising agent (oxidant) in the following reactions

(a)   $P{b}_{3}{O}_{4}+8\text{\hspace{0.17em}}HCl\to 3\text{\hspace{0.17em}}PbC{l}_{2}+C{l}_{2}+4\text{\hspace{0.17em}}{H}_{2}O$

(b)   $2\text{\hspace{0.17em}}Mg+{O}_{2}\to 2\text{\hspace{0.17em}}MgO$

(c)   $CuS{O}_{4}+Zn\to Cu+ZnS{O}_{4}$

(d)   ${V}_{2}{O}_{5}+5\text{\hspace{0.17em}}Ca\to 2\text{\hspace{0.17em}}V+5\text{\hspace{0.17em}}CaO$

(e)   $3\text{\hspace{0.17em}}Fe+4\text{\hspace{0.17em}}{H}_{2}O\to F{e}_{3}{O}_{4}+4\text{\hspace{0.17em}}{H}_{2}$

(f)    $CuO+{H}_{2}\to Cu+{H}_{2}O$

## Solution:

(a)   $P{b}_{3}{O}_{4}$

(b)   ${O}_{2}$

(c)   $CuS{O}_{4}$

(d)   ${V}_{2}{O}_{5}$

(e)   ${H}_{2}O$

(f)    $CuO$

# Question 25

Write the balanced chemical equations for the following reactions:

(a) Sodium carbonate on reaction with hydrochloric acid in equal molar concentrations gives sodium chloride and sodium hydrogen carbonate.

(b) Sodium hydrogen carbonate on reaction with hydrochloric acid gives sodium chloride, water and liberates carbon dioxide.

(c) Copper sulphate on treatment with potassium iodide precipitates cuprous iodide $\left(C{u}_{2}{I}_{2}\right)$, liberates iodine gas and also forms potassium sulphate.

## Solution:

(a)   $N{a}_{2}C{O}_{3}+HCl\to NaCl+NaHC{O}_{3}$

(b)   $NaHC{O}_{3}+HCl\to NaCl+{H}_{2}O+C{O}_{2}$

(c)   $2\text{\hspace{0.17em}}CuS{O}_{4}+4\text{\hspace{0.17em}}KI\to C{u}_{2}{I}_{2}+2\text{\hspace{0.17em}}{K}_{2}S{O}_{4}+{I}_{2}$

# Question 26

A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of the chemical reaction?

$KCl\left(aq\right)+AgN{O}_{3}\left(aq\right)\to AgCl\left(s\right)+KN{O}_{3}\left(aq\right)$

## Solution:

It is a double displacement and precipitation reaction.

# Question 27

Ferrous sulphate decomposes with the evolution of a gas having a characteristic odour of burning sulphur. Write the chemical reaction involved and identify the type of reaction.

## Solution:

It is a thermal decomposition reaction.

# Question 28

Why do fire flies glow at night?

## Solution:

The protein present in fire flies oxidizes in the presence of an enzyme. As a result, light is emitted by the insect which appears as a glow in the darkness.

# Question 29

Grapes hanging from the plant do not ferment but after being plucked from the plant can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change?

## Solution:

When grapes are attached to the plant, oxygen reaches the cells of the grapes and so aerobic respiration takes place.

But when plucked, oxygen cannot reach the cells of the grapes. This facilitates the microbes to grow under anaerobic condition and it leads to fermentation.

It is a chemical change.

# Question 30

Which among the following are physical or chemical changes?

(a) Evaporation of petrol

(b) Burning of Liquefied Petroleum Gas (LPG)

(c) Heating of an iron rod to red hot.

(d) Curdling of milk

(e) Sublimation of solid ammonium chloride

## Solution:

Evaporation of petrol, heating of an iron rod to red hot and sublimation of solid ammonium chloride are physical changes.

Burning of Liquefied Petroleum Gas (LPG) and curdling of milk are chemical changes.

# Question 31

During the reaction of some metals with dilute hydrochloric acid, following observations were made.

(a) Silver metal does not show any change

(b) The temperature of the reaction mixture rises when aluminium (Al) is added.

(c) The reaction of sodium metal is found to be highly explosive

(d) Some bubbles of a gas are seen when lead (Pb) is reacted with

the acid.

Explain these observations giving suitable reasons.

## Solution:

(a) Silver metal does not react with dilute hydrochloric acid. Hence, there is no change.

(b) The reaction between aluminium metal and dilute hydrochloric acid is an exothermic reaction. Thus, the temperature of the reaction mixture rises when aluminium is added to hydrochloric acid.

(c) Sodium is one of the most reactive metals and when it reacts with dilute hydrochloric acid, the reaction is exothermic. Thus, the reaction of sodium metal and dilute hydrochloric acid is found to be highly explosive.

(d) When lead reacts with dilute hydrochloric acid, hydrogen gas is evolved. Given below is the chemical reaction:

$Pb+2\text{\hspace{0.17em}}HCl\to PbC{l}_{2}+{H}_{2}$

# Question 32

A substance X, which is an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and also write the chemical reactions involved.

## Solution:

The substance X is Calcium oxide.

$CaO\left(s\right)+{H}_{2}O\left(l\right)\to Ca{\left(OH\right)}_{2}\left(aq\right)$

# Question 33

Write a balanced chemical equation for each of the following reactions and also classify them.

(a) Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution.

(b) A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas.

(c) Iron (III) oxide on heating with carbon monoxide gas reacts to form solid iron and liberates carbon dioxide gas.

(d) Hydrogen sulphide gas reacts with oxygen gas to form solid sulphur and liquid water.

## Solution:

(a)$\text{Pb}{\left({\text{CH}}_{\text{3}}\text{COO}\right)}_{\text{2}}\text{+2}\text{\hspace{0.17em}}\text{HCl}\to {\text{PbCl}}_{\text{2}}{\text{+CH}}_{\text{3}}\text{COOH}$; Double displacement reaction

(b)   $\text{2}\text{\hspace{0.17em}}\text{Na+2}\text{\hspace{0.17em}}{\text{C}}_{\text{2}}{\text{H}}_{\text{5}}\text{OH}\to \text{2}\text{\hspace{0.17em}}{\text{C}}_{\text{2}}{\text{H}}_{\text{5}}{\text{ONa+H}}_{\text{2}}$; Displacement reaction.

(c)${\text{Fe}}_{\text{2}}{\text{O}}_{\text{3}}\text{+3}\text{\hspace{0.17em}}\text{CO}\to \text{2}\text{\hspace{0.17em}}\text{Fe+3}\text{\hspace{0.17em}}{\text{CO}}_{\text{2}}$; Redox reaction.

(d)   $\text{2}\text{\hspace{0.17em}}{\text{H}}_{\text{2}}{\text{S+O}}_{\text{2}}\to \text{2}\text{\hspace{0.17em}}\text{S+2}\text{\hspace{0.17em}}{\text{H}}_{\text{2}}\text{O}$; Redox reaction.

# Question 34

Why do we store silver chloride in dark coloured bottles?

## Solution:

On exposure to sunlight, the silver chloride may decompose as per the following reaction.

Therefore, it is stored in dark coloured bottles.

# Question 35

Balance the following chemical equations and identify the type of chemical reaction.

(a)   $Mg\left(s\right)+C{l}_{2}\left(g\right)\to MgC{l}_{2}\left(s\right)$

(b)   $HgO\left(s\right)\to Hg\left(l\right)+{O}_{2}\left(g\right)$

(c)

(d)   $TiC{l}_{4}\left(l\right)+Mg\left(s\right)\to Ti\left(s\right)+MgC{l}_{2}\left(s\right)$

(e)   $CaO\left(s\right)+Si{O}_{2}\left(s\right)\to CaSi{O}_{3}\left(s\right)$

(f)    ${H}_{2}{O}_{2}\left(l\right)\stackrel{UV}{\to }{H}_{2}O\left(l\right)+{O}_{2}\left(g\right)$

## Solution:

(a) The equation is already balanced; it is a combination reaction.

(b) ; it is a decomposition reaction.

(c) $2\text{\hspace{0.17em}}Na\left(s\right)+S\left(s\right)\stackrel{Fuse}{\to }N{a}_{2}S\left(s\right)$; it is a combination reaction.

(d) $TiC{l}_{4}\left(l\right)+2\text{\hspace{0.17em}}Mg\left(s\right)\to Ti\left(s\right)+2\text{\hspace{0.17em}}MgC{l}_{2}\left(s\right)$; it is a displacement reaction.

(e) The equation is already balanced; it is a combination reaction.

(f) $2\text{\hspace{0.17em}}{H}_{2}{O}_{2}\left(l\right)\stackrel{UV}{\to }2\text{\hspace{0.17em}}{H}_{2}O\left(l\right)+{O}_{2}\left(g\right)$; it is a decomposition reaction.

# Question 36

A magnesium ribbon is burnt in oxygen to give a white compound X accompanied by emission of light. If the burning ribbon is now laced in an atmosphere of nitrogen, it continues to burn and forms a compound Y.

(a) Write the chemical formulae of X and Y.

(b) Write a balanced chemical equation, when X is dissolved in water.

## Solution:

(a)   When magnesium ribbon burns in oxygen, the chemical equation of the reaction is:

$2\text{\hspace{0.17em}}Mg+{O}_{2}\to 2\text{\hspace{0.17em}}MgO$

Here the white compound X is magnesium oxide.

If magnesium ribbon burns in an atmosphere that has only nitrogen, the reaction would be:

$3\text{\hspace{0.17em}}Mg+{N}_{2}\to M{g}_{3}{N}_{2}$

The compound formed would be magnesium nitride.

(b)   When X, i.e. magnesium oxide is dissolved in water, magnesium hydroxide is formed. The chemical equation of the reaction is:

$MgO+{H}_{2}O\to Mg{\left(OH\right)}_{2}$

# Question 37

Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why?

## Solution:

In the activity series of metals, zinc is placed above hydrogen.  It means zinc is more reactive than hydrogen. Thus, zinc reacts with hydrochloric acid and hydrogen gas is evolved.

$Zn+HCl\to ZnC{l}_{2}+{H}_{2}$

Copper, on the other hand, is placed below hydrogen. It means copper is less reactive than hydrogen. Hence, copper does not displace hydrogen from dilute hydrochloric acid.

# Question 38

A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.

(a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.

(b) Name the black substance formed and gives its chemical formula.

## Solution:

a) When kept in the open for a few days, the silver $\left(Ag\right)$ reacts with the moisture, oxygen and gases present in the air. This phenomenon is called corrosion. This black coating is formed when silver reacts with hydrogen sulphide.

(b) When silver reacts with hydrogen sulphide $\left({H}_{2}S\right)$ present in the air, the black substance formed is silver sulphide; and its chemical formula is $A{g}_{2}S$.

# Question 39

On heating, blue coloured powder of copper (II) nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas X is formed.

(a) Write a balanced chemical equation of the reaction.

(b) Identity the brown gas X evolved.

(c) Identity the type of reaction.

(d) What could be the pH range of aqueous solution of the gas X?

## Solution:

(a)   When cupric nitrate is heated in a boiling tube, copper oxide, oxygen gas and nitrogen dioxide are formed. Balanced chemical equation of the referred reaction is:

(b) Nitrogen dioxide is a brown coloured gas evolved in the reaction.

(c) This is an example of a decomposition reaction.

(d) When nitrogen dioxide dissolves in water, it forms nitric acid. Water has a pH value of 7 while pH of an acid is less than 7. Therefore, the pH of the aqueous solution of the gas X will be less than 7.

# Question 40

Give the characteristic tests for the following gases:

(a)   $C{O}_{2}$

(b)   $S{O}_{2}$

(c)   ${O}_{2}$

(d)   ${H}_{2}$

## Solution:

(a)   On reacting with carbon dioxide $\left(C{O}_{2}\right)$, lime water turns milky. This is due to the formation of insoluble calcium carbonate.

$Ca{\left(OH\right)}_{2}+C{O}_{2}\to CaC{O}_{3}+{H}_{2}O$

(b)   Sulphur dioxide $\left(S{O}_{2}\right)$ gas reacts with potassium permanganate solution (purple in colour) and makes it colourless. This is due to the action of the reducing agent sulphur dioxide $\left(S{O}_{2}\right)$

$2\text{\hspace{0.17em}}KMn{O}_{4}+2\text{\hspace{0.17em}}{H}_{2}O+5\text{\hspace{0.17em}}S{O}_{2}\to {K}_{2}S{O}_{4}+2\text{\hspace{0.17em}}MnS{O}_{4}+2\text{\hspace{0.17em}}{H}_{2}S{O}_{4}$

(c) When we bring a burning candle near the mouth of the test tube containing oxygen $\left({O}_{2}\right)$, the intensity of the flame increases. This confirms the presence of oxygen, as oxygen supports burning.

(d) When a burning candle is brought near hydrogen, it burns with a pop sound. This is the test for the presence of hydrogen gas.

# Question 41

What happens when a piece of:

(a) Zinc metal is added to copper sulphate solution?

(b) Aluminium metal is added to dilute hydrochloric acid?

(c) Silver metal is added to copper sulphate solution?

Also, write the balanced chemical equation if the reaction occurs.

## Solution:

(a)   When a piece of zinc metal is added to copper sulphate solution, it displaces copper from its solution. This happens because zinc is more reactive than copper. Due to this reaction, a solution of zinc sulphate is obtained.

$Zn\left(s\right)+CuS{O}_{4}\left(aq\right)\to ZnS{O}_{4}\left(aq\right)+Cu\left(s\right)$

(b)   Aluminium is more reactive than hydrogen. So, when a piece of aluminium metal is added to dilute hydrochloric acid, it displaces hydrogen from the dilute hydrochloric acid solution and forms aluminium chloride and hydrogen gas.

$2\text{\hspace{0.17em}}Al\left(s\right)+6\text{\hspace{0.17em}}HCl\left(aq\right)\to 2\text{\hspace{0.17em}}AlC{l}_{3}\left(aq\right)+3\text{\hspace{0.17em}}{H}_{2}\left(g\right)$

(c)   As per the activity series, silver metal is less reactive than copper. Silver cannot displace copper from its salt solution. Therefore, no reaction takes place when a piece of silver is added to copper sulphate solution.

# Question 42

What happens when zinc granules are treated with dilute solution of  also write the chemical equations if reaction occurs.

## Solution:

(a)   When zinc reacts with dilute ${H}_{2}S{O}_{4},$ zinc sulphate is formed. The reaction is:

$Zn\left(s\right)+{H}_{2}S{O}_{4}\left(aq\right)\to ZnS{O}_{4}\left(aq\right)+{H}_{2}\left(g\right)$

(b)   When zinc reacts with dilute HCl, zinc chloride is formed. The reaction is:

$Zn\left(s\right)+2\text{\hspace{0.17em}}HCl\left(aq\right)\to ZnC{l}_{2}\left(aq\right)+{H}_{2}\left(g\right)$

(c)   When zinc reacts with dilute $HN{O}_{3}$, zinc nitrate, water and nitrous oxide are formed. Reaction with dilute $HN{O}_{3}$ is different as compared to other acids because nitric acid is an oxidising agent and it oxidizes ${H}_{2}$ gas to form ${H}_{2}O$.

$4\text{\hspace{0.17em}}Zn\left(s\right)+10\text{\hspace{0.17em}}HN{O}_{3}\left(aq\right)\to 4\text{\hspace{0.17em}}Zn{\left(N{O}_{3}\right)}_{2}\left(aq\right)+5\text{\hspace{0.17em}}{H}_{2}O\left(l\right)+{N}_{2}O\left(g\right)$

(d)   Zinc does not react with NaCl solution.

(e)   When zinc reacts with dilute NaOH solution, sodium zincate is formed. The reaction is:

$Zn\left(s\right)+2\text{\hspace{0.17em}}NaOH\left(aq\right)\to N{a}_{2}Zn{O}_{2}\left(aq\right)+{H}_{2}\left(g\right)$

# Question 43

On adding a drop of barium chloride solution to an aqueous solution of sodium sulphite, a white precipitate is obtained.

(a) Write a balanced chemical equation of the reaction involved.

(b) What other name can be given to this precipitation reaction?

(c) On adding dilute hydrochloric acid to the reaction mixture, white precipitate disappears. Why?

## Solution:

(a)   When a drop of barium chloride solution is added to an aqueous solution of sodium sulphite, barium sulphite and sodium chloride are formed. The balanced chemical equation is:

$N{a}_{2}S{O}_{3}\left(aq\right)+BaC{l}_{2}\left(aq\right)\to BaS{O}_{3}\left(s\right)+2\text{\hspace{0.17em}}NaCl\left(aq\right)$

(b)   This reaction is also known as double displacement reaction.

(c)   $BaS{O}_{3}$ is the salt of a weak acid ${H}_{2}S{O}_{3}$ (sulfurous acid). When a dilute acid such as $HCl$ is added to barium sulphite, it produces a white precipitate of barium chloride $\left(BaC{l}_{2}\right),$ water and sulphur dioxide. Since $BaC{l}_{2}$ is soluble in water, the white precipitate disappears.

# Question 44

You are provided with two containers made up of copper and aluminium. You are also provided with solutions of dilute  In which of the above containers these solutions can be kept?

## Solution:

A) In copper container

(a)   Dilute $HCl$: The solution of dilute $HCl$ can be kept in a copper container as it does not react with dilute $HCl$.

(b)   Dilute $HN{O}_{3}$: The solution of dilute nitric acid ( $HN{O}_{3}$ ) cannot be kept in a copper container as nitric acid acts as a strong oxidising agent and reacts with copper.

(c)   $ZnC{l}_{2}$: Zinc chloride can be kept in a copper container as zinc is more reactive than copper (Cu) and so there is no possibility of a displacement reaction.

(d)   ${H}_{2}O$: ${H}_{2}O$ can be kept in a copper container as copper does not react with water.

(B) In aluminium containers

(a)   Dilute $HCl$: The solution of dilute $HCl$ cannot be kept in an aluminium container as aluminium will undergo reaction with the solution of dilute $HCl$.

(b)   Dilute $HN{O}_{3}$: The solution of dilute $HN{O}_{3}$ can be kept in an aluminium container as aluminium gets oxidised by dilute $HN{O}_{3}$ to form a layer of $A{l}_{2}{O}_{3}$.

(c)   $ZnC{l}_{2}$: $ZnC{l}_{2}$ cannot be kept in an aluminium container as aluminium is more reactive than zinc; so, it can displace zinc ion from the solution.

(d)   ${H}_{2}O$: Aluminium does not react with cold or hot water. Therefore, water can be kept in an aluminium container.