# Important Questions -Chemical Reactions and Equations

Find out the most important questions of the chapter chemical reaction and equation class 10 Chemistry. These important questions are designed by NTSE Guru Experts and are most likely to appear in exams. Chemical Reaction and Equation is one of the easiest chapters of chemistry if understood properly. With the help of these questions answers, a student can achieve an overall good score. Start your preparation of chemistry today with the help of chemical reaction and equation class 10 questions answers.

1.    (a) What is a chemical reaction?

(b) Write down four different observations which help us to determine whether a chemical reaction has taken place.

Ans.(a) The process in which a substance or substances undergo change to produce new substances with entirely different properties is known as a chemical reaction.

(b) Different observations which help us to determine whether a chemical reaction has taken place are;

(i)      Change in state,                          (ii)        Change in temperature,

(iii)    Change in colour,                      (iv)       Evolution of gas.

2.   (a) Which reaction occurs when quick lime is kept in water? Write chemical equation of the reaction and name the reaction.

(b) A newly white washed wall is said to give better look after 2-3 days. Do you agree with the statement? Give reason in support of your answer.

Ans. (a) When quick lime is kept in water, slaked lime (calcium hydroxide) is formed. This is an exothermic reaction in which heat is evolved. The reaction takes place as follows:

$\underset{Quick&space;Lime}{\mathop{CaO(s)}}\,+{{H}_{2}}O(l)\to&space;\underset{Slaked&space;Lime}{\mathop{Ca{{(OH)}_{2}}(aq)}}\,+Heat$

Reaction is known as ‘Slaking of Lime’ & it is a combination reaction.

(b) Yes, because the slaked lime applied on the walls combines with the carbon dioxide of the air to form a thin shining layer of calcium carbonate within 2-3 days.

$\underset{Slaked\,\,Lime}{\mathop{Ca{{(OH)}_{2}}(aq)}}\,+\underset{Carbondioxide}{\mathop{C{{O}_{2}}(g)}}\,\xrightarrow{{}}\underset{\begin{smallmatrix}&space;Calcium\,\,carbonate&space;\\&space;\left(&space;\text{White}\,\,\text{shiny}\,\,\text{substance}&space;\right)&space;\end{smallmatrix}}{\mathop{CaC{{O}_{3}}}}\,+{{H}_{2}}O(l)$

3.  Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Ans.  The colour changes because of the formation of FeSO4 (ferrous sulphate), which is green in colour.

$Fe(s)+\underset{\left(&space;Blue&space;\right)}{\mathop{CuS{{O}_{4}}(aq)}}\,\xrightarrow{{}}\underset{\left(&space;Green&space;\right)}{\mathop{FeS{{O}_{4}}(aq)}}\,+C{{u}_{\left(&space;s&space;\right)}}$

4.   Oil and fat containing food items are flushed with nitrogen. Why?

Ans. Oil and fat containing food items are flushed with nitrogen to prevent rancidity, which is the oxidation of oil or fat present in food. Nitrogen being an unreactive gas, does not react with food & also prevents the contact of food with air/ oxygen. In this way food remains fresh for much longer time.

5.   A magnesium ribbon is burnt in oxygen to give a white compound X accompanied by emission of light. If the burning ribbon is now placed 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.

Ans. $2Mg+{{O}_{2}}\xrightarrow{\,}\,2MgO$

$3Mg+{{N}_{2}}\xrightarrow{\,}M{{g}_{3}}{{N}_{2}}$

(A) X is MgO; Y is M ${{g}_{3}}{{N}_{2}}$

(B) $MgO+{{H}_{2}}O\xrightarrow{\,}\,Mg{{(OH)}_{2}}$

6.   Write the balanced chemical equations for the following reactions.

(a) Calcium hydroxide + Carbon dioxide Calcium carbonate + Water

(b) Zinc + Silver nitrate → Zinc nitrate + silver

(c) Aluminium + Copper chloride Aluminium chloride + copper

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Ans. (a) $Ca{{(OH)}_{2}}+C{{O}_{2}}\xrightarrow{{}}CaC{{O}_{3}}+{{H}_{2}}O$

(b) $Zn+2AgN{{O}_{3}}\xrightarrow{{}}Zn{{(N{{O}_{3}})}_{2}}+2Ag$

(c) $2Al+3CuC{{l}_{2}}\xrightarrow{{}}2AlC{{l}_{3}}+3Cu$

(d) $BaC{{l}_{2}}+{{K}_{2}}S{{O}_{4}}\xrightarrow{{}}BaS{{O}_{4}}+2KCl$

7.   A green coloured iron compound when heated, changes to reddish brown compound with a characteristic odour of burning sulphur. Name the compound & why does it give that characteristic odour?

Ans.     Green coloured iron compound is ferrous sulphate. On heating, it decomposes to Iron (III) Oxide (ferric oxide) which is reddish brown in colour. The characteristic odour of burning sulphur is of sulphur dioxide which is evolved during decomposition.

The reaction goes as

$\underset{\underset{\left(&space;Green&space;\right)}{\mathop{Hydrated\,\,Ferrous\,\,Sulphate}}\,}{\mathop{FeS{{O}_{4}}.7{{H}_{2}}O}}\,\xrightarrow{Heat}\underset{Ferrous\,\,Sulphate}{\mathop{2FeS{{O}_{4}}\left(&space;s&space;\right)}}\,\xrightarrow{Heat}\underset{\underset{\left(&space;Brown&space;\right)}{\mathop{ferric\,\,~oxide}}\,}{\mathop{F{{e}_{2}}{{O}_{3}}(s)}}\,+\underset{Sulphur\,\,dioxide}{\mathop{S{{O}_{2}}(g)}}\,+\underset{Sulphur\,\,trioxide}{\mathop{S{{O}_{3}}(g)}}\,$

8.   Study the following figure in which lead nitrate is heated in test tube.

(a) What does the reddish brown fume evolved from test tube indicate?

(b) What will remain in the test tube after heating?

(c) Give the chemical reaction involved.

(d) State the type of reaction.

Ans.  (a)  When lead nitrate is heated, brown fumes evolved from the test tube  are of nitrogen dioxide.

(b)  A yellow residue of lead-oxide is left behind in the test tube.

(c)   $\underset{\begin{smallmatrix}&space;Lead\,\,Nitrate&space;\\&space;\,\,\,\,\,\,\,\left(&space;White&space;\right)&space;\end{smallmatrix}}{\mathop{2Pb{{(N{{O}_{3}})}_{2}}(s)}}\,\xrightarrow{\Delta&space;}\underset{\begin{smallmatrix}&space;Lead\,\,Oxide&space;\\&space;\,\,\,\left(&space;Yellow&space;\right)&space;\end{smallmatrix}}{\mathop{2PbO(s)}}\,+\underset{\begin{smallmatrix}&space;Nitrogen\,\,dioxide&space;\\&space;\,\,\,\left(&space;Brown\,\,fumes&space;\right)&space;\end{smallmatrix}}{\mathop{4N{{O}_{2}}(g)}}\,+\underset{Oxygen}{\mathop{{{O}_{2}}(g)}}\,$

(d) It is an example of decomposition reaction (thermal decomposition reaction).

9.   Why respiration is considered an exothermic reaction? Explain.

Ans.  Respiration is considered as an exothermic reaction because in respiration, the glucose which acts as fuel for body cells combines with oxygen to release energy.

$\underset{{}}{\mathop{\underset{glu\cos&space;e}{\mathop{{{C}_{6}}{{H}_{12}}{{O}_{6\left(&space;aq&space;\right)}}}}\,+\underset{oxygen}{\mathop{6{{O}_{2\left(&space;g&space;\right)}}}}\,}}\,\xrightarrow{{}}\underset{Carbondioxide}{\mathop{6C{{O}_{2}}_{\left(&space;g&space;\right)}}}\,+6{{H}_{2}}{{O}_{\left(&space;l&space;\right)}}+Energy$

10.   (a) In the electrolysis of water, which gas is liberated at cathode & at anode?

(b) Write a balanced chemical equation for the electrolysis of water.

Ans.  (a) Hydrogen gas is liberated at cathode.

Oxygen gas is liberated at anode.

(b) $2{{H}_{2}}O\xrightarrow[{}]{electricity}2{{H}_{2}}+{{O}_{2}}$

11.   A compound when kept in sunlight changes to grey coloured compound. It is used in black & white photography.

(a) Write the chemical reaction involved.

(b) Can we call this reaction as exothermic reaction or is it endothermic? Why?

Ans.  (a)    $\underset{\begin{array}{*{35}{l}}&space;silver\text{&space;}bromide~&space;\\&space;~~~~~~~~\left(&space;Yellow&space;\right)&space;\\&space;\end{array}}{\mathop{2AgBr}}\,\xrightarrow{Sunlight}\underset{silver}{\mathop{2Ag}}\,+B{{r}_{2}}$

(b)  It is an endothermic reaction because in this reaction light energy is being used for decomposition

12.   What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Ans.

13.   Zinc & lead are more reactive than copper. Justify by giving suitable chemical reactions?

Ans.  Zinc & lead being more reactive displace copper from its salt solution as per following reaction;

$Zn(s)+\underset{Blue\text{&space;}colour}{\mathop{CuS{{O}_{4}}(aq)}}\,\xrightarrow{{}}\underset{colourless}{\mathop{ZnS{{O}_{4}}}}\,(aq)+Cu(s)$

$Pb(s)+CuC{{l}_{2}}(aq)\xrightarrow{{}}PbC{{l}_{2}}(aq)+Cu(s)$

Also, if copper is added to zinc sulphate or lead chloride solution, no reaction will take place. It is because copper is less reactive than zinc and lead.

14.   Give reason why?

(a)  Silver ornaments get blacken when in use.

(b)  Copper vessel turns greenish with time.

Ans.  (a) Silver ornaments get blacken because, on exposure to air it forms a coating of black silver sulphide (Ag2S) on its surface.

$2Ag(s)+\underset{\begin{smallmatrix}&space;Hydrogen\,\,Sulphide&space;\\&space;\,\,\,\,\,\,\,\,\,\,\,\left(&space;From\,\,Air&space;\right)&space;\end{smallmatrix}}{\mathop{{{H}_{2}}S(g)}}\,\xrightarrow{{}}\underset{\begin{smallmatrix}&space;silver\,\,sulphide&space;\\&space;\,\,\,\,\,\,\,\,\,\,\,\left(&space;Black&space;\right)&space;\end{smallmatrix}}{\mathop{A{{g}_{2}}S(s)}}\,+{{H}_{2}}(g)$

(b)  The copper vessel turns greenish because of the formation of basic copper carbonate, CuCO3.Cu(OH)2 on its surface, due to the attack of O2, CO2 and water vapour present in the air.

$2Cu(s)+C{{O}_{2}}(g)+{{O}_{2}}(g)+{{H}_{2}}O(l)\xrightarrow{{}}\underset{\begin{smallmatrix}&space;\text{Basic}\,\,\,\text{copper&space;carbonate&space;}&space;\\&space;\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\left(&space;\text{Green}&space;\right)&space;\end{smallmatrix}}{\mathop{CuC{{O}_{3}}.Cu{{(OH)}_{2}}}}\,$

15.  Consider the reactions

$Mn{{O}_{2}}+4HCl\to&space;MnC{{l}_{2}}+2{{H}_{2}}O+C{{l}_{2}}$

$ZnO+C\to&space;Zn+CO$

In the above reactions, name the substances which get oxidized & which get reduced.

Ans.   (i)  HCl has lost Hydrogen to give Cl2. Hence, HCl has been oxidized to ClMnO2  has been reduced to MnCl2 by losing oxygen.

(ii)  ZnO has lost oxygen to give Zn hence, ZnO is reduced. Carbon gets oxidized by gaining oxygen.

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