NCERT Solutions for Class 10 Science Chapter 1 (2025-2026)

Q.1 When sodium sulphate solution is treated with barium chloride solution, a white precipitate is formed. Name the reaction and also write the balanced chemical reaction.

Ans- The reaction involves the exchange of ions between two compounds to form two new compounds and thus it is a double displacement reaction. The balanced chemical equation for the given reaction can be written as follows:

${\text{Na}}_2{\text{SO}}_4\left(\text{aq}\right)+{\text{BaCl}}_2\left(\text{aq}\right)\to {\text{\hspace{0.33em}BaSO}}_4(\text{s})+2\text{NaCl}\left(\text{aq}\right)$

Q.2 In the following reaction, identify the species that undergoes –

$\text{Fe}\left(\text{s}\right)+{\text{Ni}}^{2+}\left(\text{aq}\right)\to {\text{Fe}}^{2+}\left(\text{aq}\right)+\text{Ni}\left(\text{s}\right)$

(i) Oxidation
(ii) Reduction
Give reasons also.

Ans-

$\text{Fe}\left(\text{s}\right)-2{\text{e}}^{-}\to {\text{\hspace{0.33em}Fe}}^{2+}\left(\text{aq}\right)$

Here, iron loses its two electrons to form Fe²⁺ ions. As we know that oxidation is the loss of electron(s), so iron undergoes oxidation.

${\text{Ni}}^{2+}\left(\text{aq}\right)+2{\text{e}}^{-}\to \text{\hspace{0.33em}Ni}\left(\text{s}\right)$

Here, Ni²⁺ ions gain two electrons to form nickel. As we know that reduction is gain of electron(s), so nickel undergoes reduction.

Q.3 Differentiate between combination reaction and decomposition reaction. Give one example of each of these reactions.

Ans-

Q.4 Write a balanced chemical reaction for each of the following reactions:
(a) Carbon dioxide is passed through lime water.
(b) A strip of copper metal is dipped in the solution of silver nitrate solution.

Ans- (a) When carbon dioxide gas is passed through lime water, the lime water turns milky due to the formation of white precipitate of calcium carbonate.

$\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)+{\mathrm{CO}}_2\left(\mathrm{g}\right)\to {\mathrm{CaCO}}_3\left(\mathrm{s}\right)\downarrow +{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

(b) When a strip of copper metal is dipped in the solution of silver nitrate solution, a greyish white deposit of silver metal is deposited on the strip.

$\mathrm{Cu}\left(\mathrm{s}\right)+2{\mathrm{AgNO}}_3\left(\mathrm{aq}\right)\to 2\mathrm{Ag}\left(\mathrm{s}\right)+\mathrm{Cu}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{aq}\right)$

Q.5 (a) Respiration is considered an exothermic reaction. Why?
(b) Write an example of exothermic reaction.

Ans- (a) During respiration, food is oxidised and energy is released. Therefore, it is considered an exothermic reaction.
(b) Reaction of quick lime with water is an exothermic reaction.

$2\mathrm{CaO}+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{Ca}(\mathrm{OH}{)}_2$

Q.6 How a double displacement reaction is different from simple displacement reaction? Explain with the help of an example.

Ans– In a double displacement reaction, exchange of ions takes place among the reactants, while in displacement reaction more reactive metal displaces less reactive metal from its salt.

Example of displacement reaction:

$\mathrm{Fe}\left(\mathrm{s}\right)+{\mathrm{CuSO}}_4\left(\mathrm{aq}\right)\to {\mathrm{FeSO}}_4\left(\mathrm{aq}\right)+\mathrm{Cu}\left(\mathrm{s}\right)$

Here, Fe displaces Cu from CuSO₄ because Fe is more reactive than Cu.

Example of double displacement reaction:

${\mathrm{Na}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)+{\mathrm{BaCl}}_2\left(\mathrm{aq}\right)\to {\mathrm{BaSO}}_4\left(\mathrm{s}\right)+2\mathrm{NaCl}\left(\mathrm{aq}\right)$

Here, the exchange of ions between reactants takes place.

Q.7 Explain the term combination reaction with the help of an example.

Ans- When two reactants combine to form a single product, the reaction is called combination reaction.

$2{\text{H}}_2(g)+{\text{O}}_2(g)\; \to 2{\text{H}}_2\text{O(g)}$

Q.8 Write balanced chemical reactions which represent the following:
(a) Ferrous sulphate is heated.
(b) Hydrogen gas combines with nitrogen to form ammonia.
(c) Magnesium metal reacts with water to give magnesium hydroxide and hydrogen.

$\begin{array}{l}\text{<strong>Ans- </strong>}\end{array}$ $\begin{array}{l}\text{(a)}\\ 2{\text{FeSO}}_4\stackrel{\text{Heat}}{\to }{\text{\hspace{0.33em}Fe}}_2{\text{O}}_3+{\text{SO}}_2+{\text{SO}}_3\end{array}$ $\begin{array}{l}{\text{(b) N}}_2+3{\text{H}}_2\to 2{\text{NH}}_3\end{array}$ $\begin{array}{l}\text{(c)}\\ \text{Mg}+2{\text{H}}_2\text{O\hspace{0.33em}}\to {\text{\hspace{0.33em}Mg(OH)}}_{\text{2}}+{\text{H}}_2\end{array}$

Q.9 Write the name of the chemical and its reaction which is used in black and white photography.

Ans- Silver bromide is used in black and white photography. When exposed to sunlight, it decomposes to form silver and bromine.

$2\mathrm{AgBr}\left(\mathrm{s}\right)\stackrel{\mathrm{Sunlight}}{\to }2\mathrm{Ag}\left(\mathrm{s}\right)+{\mathrm{Br}}_2\left(\mathrm{g}\right)$

Q.10 Give two examples of redox reactions.

$\begin{array}{l}\text{<strong>Ans- </strong>(a)\hspace{0.33em}ZnO}+\text{C\hspace{0.33em}}\to \text{\hspace{0.33em}Zn}+\text{CO}\\ {\text{(b)\hspace{0.33em}MnO}}_2+4\text{HCl\hspace{0.33em}}\to {\text{\hspace{0.33em}MnCl}}_2+2{\text{H}}_2\text{O}+{\text{Cl}}_2\end{array}$

Q.11 Write any four observations that help us to determine whether a chemical reaction is taking place.

Ans-

(a) Change in state
(b) Change in colour
(c) Evolution of a gas
(d) Change in tempreature

Q.12 Balance the following chemical equations:

$\begin{array}{l}\text{(a)\hspace{0.33em}Na}+{\text{O}}_2\to {\text{\hspace{0.33em}Na}}_2\text{O}\end{array}$ $\begin{array}{l}\text{(b)\hspace{0.33em}NaCl}+{\text{H}}_2\text{O\hspace{0.33em}}\to \text{\hspace{0.33em}NaOH}+{\text{Cl}}_2+{\text{H}}_2\end{array}$ $\begin{array}{l}\text{(c)\hspace{0.33em}}{\left({\text{NH}}_4\right)}_2{\text{CO}}_3\to 2{\text{NH}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2\end{array}$

Ans-

$\begin{array}{l}\text{(a)\hspace{0.33em}}4\text{Na(s)}+{\text{O}}_2\text{\hspace{0.33em}(g)}\to 2{\text{\hspace{0.33em}Na}}_2\text{O(s)}\end{array}$ $\begin{array}{l}\text{(b)\hspace{0.33em}}2\text{NaCl(aq)}+{\text{H}}_2\text{O(l)\hspace{0.33em}}\to \text{\hspace{0.33em}NaOH(aq)}+{\text{Cl}}_2\text{(g)}+{\text{H}}_2\text{(g)}\end{array}$ $\begin{array}{l}\text{(c)\hspace{0.33em}}{\left({\text{NH}}_4\right)}_2{\text{CO}}_3\text{(s)\hspace{0.33em}}\to 2{\text{NH}}_3\text{(g)}+{\text{H}}_2\text{O(g)}+{\text{CO}}_2\text{(g)}\end{array}$

Q.13 Na₂SO₄ (aq) +BaCl₂(aq) 2 NaCl (s) + BaSO₄ (aq)

Is this reaction an example of precipitation reaction? Also give reason for your answer.

Ans- When sodium sulphate is mixed with barium chloride, white cloured barium sulphate is formed which is insoluble in water. The insoluble substance is called precipitate and this reacion is called precipitation reaction.

Q.14 Identify the substance which is oxidised and which is reduced in the following reaction:

$\text{ZnO}+\text{C}\to \text{Zn}+\text{CO}$

Ans- In this reaction, ZnO is reduced to Zn and C is oxidised to CO.

Q.15 Represent the following by balanced chemical equations:
(a) Iron nail is kept in a copper sulphate solution.
(b) Silver chloride is kept in sunlight.
(c) Ferrous sulphate is heated at room temperature.
(d) Silver nitrate reacts with hydrochloric acid.
(e) Methane burns in presence of air.

Ans-

$\begin{array}{l}\text{(a) Fe(s)}+{\text{CuSO}}_4\text{\hspace{0.33em}(aq)}\to {\text{\hspace{0.33em}FeSO}}_4\text{(aq)}+\text{Cu(s)}\end{array}$

$\begin{array}{l}\text{(b)}2\text{AgCl(s)\hspace{0.33em}}\stackrel{\text{Sunlight}}{\to }2\text{Ag(s)}+{\text{Cl}}_2\text{(g)}\end{array}$

$\begin{array}{l}\text{(c)}2{\text{FeSO}}_4\text{(s)}\stackrel{}{\stackrel{\text{Heat}}{\to }}{\text{Fe}}_2{\text{O}}_3\text{(s)}+{\text{SO}}_2\text{(g)}+{\text{SO}}_3\text{(g)}\end{array}$

$\begin{array}{l}\left(\text{d}\right){\text{\hspace{0.33em}AgNO}}_3\text{(aq)}+\text{HCl\hspace{0.33em}(aq)}\to \text{\hspace{0.33em}AgCl(s)}+{\text{HNO}}_3\text{(aq)}\end{array}$

$\begin{array}{l}{\text{(e) CH}}_4\text{(g)}+2{\text{O}}_2\text{(g)\hspace{0.33em}}\to {\text{\hspace{0.33em}CO}}_2\text{(g)}+2{\text{H}}_2\text{O(g)}\end{array}$

Q.16 Balance the following chemical equations:

$\begin{array}{l}{\text{(a)\hspace{0.33em}HNO}}_3+{\text{Ca(OH)}}_{\text{2}}\to \text{Ca}{\left({\text{NO}}_3\right)}_2+{\text{H}}_2\text{O}\end{array}$ $\begin{array}{l}\text{(b)\hspace{0.33em}NaOH}+{\text{H}}_2{\text{SO}}_4\to {\text{Na}}_2{\text{SO}}_4+{\text{H}}_2\text{O}\end{array}$ $\begin{array}{l}\text{(c)\hspace{0.33em}NaCl}+{\text{AgNO}}_3\to \text{AgCl}+{\text{NaNO}}_3\end{array}$ $\begin{array}{l}{\text{(d)\hspace{0.33em}BaCl}}_2+{\text{H}}_2{\text{SO}}_4\to {\text{BaSO}}_4+\text{HCl}\end{array}$ $\begin{array}{l}\text{(e)\hspace{0.33em}Pb}{\left({\text{NO}}_3\right)}_2\stackrel{\Delta }{\to }\text{\hspace{0.33em}PbO}+{\text{NO}}_2+{\text{O}}_2\end{array}$

$\begin{array}{l}\text{<strong>Ans- </strong>(a)\hspace{0.33em}}2{\text{HNO}}_3+{\text{Ca(OH)}}_{\text{2}}\to \text{Ca}{\left({\text{NO}}_3\right)}_2+2{\text{H}}_2\text{O}\\ \text{(b)\hspace{0.33em}}2\text{NaOH}+{\text{H}}_2{\text{SO}}_4\to {\text{Na}}_2{\text{SO}}_4+2{\text{H}}_2\text{O}\\ \text{(c) The\hspace{0.33em}equation\hspace{0.33em}is\hspace{0.33em}already\hspace{0.33em}balanced.}\\ {\text{(d)\hspace{0.33em}BaCl}}_2+{\text{H}}_2{\text{SO}}_4\to {\text{BaSO}}_4+2\text{HCl}\\ \text{(e)\hspace{0.33em}}2\text{Pb}{\left({\text{NO}}_3\right)}_2\stackrel{\Delta }{\to }2\text{PbO}+4{\text{NO}}_2+{\text{O}}_2\end{array}$

Q.17 Write one difference between :
(a) Oxidation and reduction reactions
(b) Exothermic and endothermic reactions

(a)

(b)

Q.18 Write the chemical equation and its type for each of the following:
(a) Silver chloride is exposed to sunlight.
(b) Manganese dioxide reacts with hydrochloric acid.
(c) Zinc rod is dipped in copper sulphate solution.
(d) Solution of sodium sulphate and barium chloride are mixed.
(e) Calcium carbonate is heated.

Ans-

(a) It is decomposition reaction.

$2\text{AgCl}\left(\text{s}\right)\stackrel{\text{Sunlight}}{\to }2\text{Ag}\left(\text{s}\right)+{\text{Cl}}_2(\text{g})$

(b) It is oxidation reaction.

${\text{MnO}}_2\text{(s)}+4\text{HCl(aq)\hspace{0.33em}}\to {\text{\hspace{0.33em}MnCl}}_2\text{(aq)}+2{\text{H}}_2\text{O(l)}+{\text{Cl}}_2\text{(g)}$

(c) It is displacement reaction.

$\text{Zn(s)}+{\text{CuSO}}_4\text{(aq)\hspace{0.33em}}\to {\text{\hspace{0.33em}ZnSO}}_4\text{(aq)}+\text{Cu(s)}$

(d) It is double displacement reaction.

${\text{Na}}_2{\text{SO}}_4\left(\text{aq}\right)+{\text{BaCl}}_2\left(\text{aq}\right)\to {\text{\hspace{0.33em}BaSO}}_4(\text{s})+2\text{NaCl}\left(\text{aq}\right)$

(e) It is decomposition reaction.

${\text{CaCO}}_3(\text{s})\stackrel{\text{Heat}}{\to }\text{CaO}\left(\text{s}\right)+{\text{CO}}_2(\text{g})$

Q.19 When a compound of lead is heated, brown coloured fumes evolve from it.
(i) Name the compound.
(ii) Write the balanced chemical equation.
(iii) Write the chemical name of brown coloured fumes.
(iv) Name the type of reaction.
(v) Is this reaction exothermic or endothermic?

Ans-

(i) The compound is lead nitrate.

$\text{(ii)\hspace{0.33em}}2\text{Pb}{\left({\text{NO}}_3\right)}_2\stackrel{∆}{\to }2\text{PbO}+4{\text{NO}}_2+{\text{O}}_2$

(iii) Nitrogen dioxide
(iv) Decomposition reaction
(v) Endothermic reaction

${\text{<strong>Q.20 </strong>ASO}}_4+\text{B}\to {\text{BSO}}_4+\text{A}$

(i) Name this reaction.
(ii) Name the more reactive element.

Ans-

(i) The given reaction is an example of displacement reaction.
(ii) Element B is more reactive because it displaces element A from its salt.

Q.21 Name the type of following reactions:

$\begin{array}{l}{\text{(a)\hspace{0.33em}Na}}_2{\text{SO}}_4+{\text{BaCl}}_2\to {\text{\hspace{0.33em}BaSO}}_4+2\text{NaCl}\end{array}$ $\begin{array}{l}\text{(b)\hspace{0.33em}Pb}+{\text{CuCl}}_2\to {\text{\hspace{0.33em}PbCl}}_2+\text{Cu}\end{array}$ $\begin{array}{l}{\text{(c)\hspace{0.33em}MnO}}_2+4\text{HCl\hspace{0.33em}}\to {\text{\hspace{0.33em}MnCl}}_2+2{\text{H}}_2\text{O}+{\text{Cl}}_2\end{array}$ $\begin{array}{l}\text{(d)\hspace{0.33em}CaO}+{\text{H}}_2\text{O\hspace{0.33em}}\to {\text{Ca(OH)}}_{\text{2}}\end{array}$ $\begin{array}{l}\text{(e)\hspace{0.33em}}2\text{Pb}{\left({\text{NO}}_3\right)}_2\to 2\text{PbO}+4{\text{NO}}_2+{\text{O}}_2\end{array}$

Ans- (a) Double displacement reaction
(b) Displacement reaction
(c) Redox reaction
(d) Combination reaction
(e) Decomposition reaction

Q.22 What are exothermic and endothermic reactions? Give two examples for each.

Ans- Exothermic reactions: The reactions in which heat is evolved along with the products are known as exothermic reactions.

For example,
(1) Burning of natural gas:

${\text{CH}}_4(\text{g})+2{\text{O}}_2(\text{g})\to {\text{CO}}_2(\text{g})+2{\text{H}}_2\text{O}\left(\text{g}\right)+\text{Heat}$

(2) Formation of Ammonia in presence of Ni:

$\underset{\text{Nitrogen}}{{\text{N}}_{\text{2}}\text{(g)}}+\underset{\text{Hydrogen}}{{\text{3H}}_2\text{(g)}}\stackrel{\mathrm{Ni}}{\to }\underset{\text{Ammonia}}{{\text{2NH}}_3\text{(g)}}+\text{Heat}$

Endothermic reactions: The reactions in which heat is absorbed are known as endothermic reactions.

For example,
(1) Formation of carbon disulphide:

$\underset{\text{Carbon}}{\text{C}}+\underset{\text{Sulphur}}{\text{S}}\stackrel{\mathrm{Heat}}{\to }\underset{\text{Carbon disulphide}}{{\text{CS}}_2}$

(2) Formation of nitric oxide:

$\underset{\text{Nitrogen}}{{\text{N}}_{\text{2}}}+\underset{\text{Oxygen}}{{\text{O}}_2}\stackrel{\mathrm{Heat}}{\to }\underset{\text{Nitric\hspace{0.33em}oxide}}{\text{2NO}}$

Q.23 Write one example of each of the following type of reaction:
(a) Double displacement reaction
(b) Redox reaction
(c) Decomposition reaction
(d) Combination reaction
(e) Displacement reaction

$\begin{array}{l}{\text{<strong>Ans- </strong>(a)\hspace{0.33em}Na}}_2{\text{SO}}_4\text{(aq)}+{\text{BaCl}}_2\text{\hspace{0.33em}(aq)}\to {\text{\hspace{0.33em}BaSO}}_4\text{(s)}+2\text{NaCl(aq)}\end{array}$ $\begin{array}{l}{\text{(b)\hspace{0.33em}MnO}}_2\text{(s)}+4\text{HCl\hspace{0.33em}(aq)}\to {\text{\hspace{0.33em}MnCl}}_2\text{(aq)}+2{\text{H}}_2\text{O(l)}+{\text{Cl}}_2\text{(g)}\end{array}$ $\begin{array}{l}\text{(c)\hspace{0.33em}}2\text{Pb}{\left({\text{NO}}_3\right)}_2\text{\hspace{0.33em}(s)}\to 2\text{PbO(s)}+4{\text{NO}}_2\text{(g)}+{\text{O}}_2\text{(g)}\end{array}$ $\begin{array}{l}\text{(d)\hspace{0.33em}CaO(s)}+{\text{H}}_2\text{O(l)\hspace{0.33em}}\to {\text{Ca(OH)}}_{\text{2}}\text{(aq)}\end{array}$ $\begin{array}{l}\text{(e)\hspace{0.33em}Pb(s)}+{\text{CuCl}}_2\text{(aq)\hspace{0.33em}}\to {\text{\hspace{0.33em}PbCl}}_2\text{(aq)}+\text{Cu(s)}\end{array}$

Q.24 Identify the substances that are oxidised and the substances that are reduced in the following reactions:
(a) Fe + CuSO₄ FeSO₄ + Cu
(b) Fe₂O₃ +2Al Al₂O₃ +2Fe
(c) MnO₂ + 4HCl MnCl₂ + 2H₂O +Cl₂

Ans- (a) Fe – Oxidised
Cu – Reduced
(b) Fe₂O₃ – Reduced
Al – Oxidised
(c) MnO₂ – Reduced
HCl – Oxidised

Q.25 In the following reaction between lead sulphide and hydrogen peroxide:
PbS(s) + 4H₂O₂(aq) → PbSO₄(s) + 4H₂O

(a) Which substance is reduced?
(b) Which substance is oxidised?

Ans-

(a) H₂O₂
(b) PbS

Q.26 Write the term used for the symbolic representation of a reaction.

Ans- Chemical equation

Q.27 Why do we apply paint on iron articles?

Ans- We paint iron articles to protection them from rust.

Q.28 Define the term corrosion.

Ans- Corrosion is defined as the process of slow and gradual conversion of a metal into its oxide, sulphide or sulphate by the attack of air and water present in the atmosphere. Rusting of iron is an example of corrosion.

Q.29 Define the term reduction.

Ans- Reduction is defined as
(i) the addition of hydrogen or electropositive element to a compound or
(ii) the removal of oxygen or electronegative element from a compound
Examples :

(i) 2Na(s) + H₂(g) → 2NaH(s) (ii) CuO(s) + H₂(g) → Cu(s) + H₂O(g)

Q.30 What are combination reactions?

Ans- Reactions in which two or more substances combine to form a single substance, are called combination reactions.

Examples:

$$\begin{gathered} \left(\mathrm{a}\right)2\mathrm{Mg}\left(\mathrm{s}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to 2\mathrm{MgO}\left(\mathrm{s}\right) \\ \left(\mathrm{b}\right)2\mathrm{Na}\left(\mathrm{s}\right)+{\mathrm{Cl}}_2\left(\mathrm{g}\right)\to 2\mathrm{NaCl}\left(\mathrm{s}\right) \end{gathered}$$

Q.31 What are double displacement reactions? Give an example.

Ans- Double displacement is a reaction in which compounds exchange their ions, i.e. two atoms or ions displace each other.

$\underset{\text{Barium\hspace{0.33em}nitrate}}{\text{Ba}{\left({\text{NO}}_3\right)}_2\left(\text{aq}\right)}+\underset{\text{Ammonium\hspace{0.33em}carbonate}}{{\left({\text{NH}}_4\right)}_2{\text{CO}}_3\left(\text{aq}\right)}\to \underset{\text{Barium\hspace{0.33em}carbonate}}{{\text{BaCO}}_3}+\underset{\text{Ammonium\hspace{0.33em}nitrate}}{ 2{\text{NH}}_4{\text{NO}}_3\left(\text{aq}\right)}$

Q.32 Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Ans- Iron displaces copper from copper sulphate solution and form iron sulphate solution. The colour of iron sulphate solution is green.

Q.33 What type of reaction is represented by the digestion of food in our body?

Ans- Decomposition reaction

${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6\left(\mathrm{aq}\right) + 6{\mathrm{O}}_2\left(\mathrm{aq}\right)\to 6{\mathrm{CO}}_2\left(\mathrm{aq}\right)+6{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)+\mathrm{Energy}$

Q.34 Define rancidity.

Ans- The condition produced by aerial oxidation of fats and oils in foods marked by unpleasant smell, taste and makes food unfit for eating is called rancidity.

Q.35 What is a chemical reaction?

Ans- A chemical reaction is defined as the overall change in the energy of the system and the generation of new products by breaking and formation of chemical bonds of reactants.

Q.36What is meant by oxidation?

Ans- Oxidation is defined as (i) the addition of oxygen or electronegative atom to an element or compound (ii) the removal of hydrogen or electropositive atom from a compound.

Q.37 What are displacement reactions?

Ans- The reactions in which more reactive element displaces less reactive element from its salt solution are called displacement reactions.

              2KI (aq)  +  Cl₂ (g)    →  2KCl (aq)  +  I₂(g)

Q.38 What is catalytic decomposition?

Ans- The reactions in which a compound splits up into two or more simpler substances in the presence of catalyst are called catalytic decomposition reactions. Example:- Digestion of food in human body

Q.39 Which of the following is a combination and which is a displacement reaction?                 

(a)      Cl₂   +   2KI   →    2KCl   +   I₂

(b)      2K   +   Cl₂     →   2KCl     

Ans-

(a) Displacement reaction
(b) Combination reaction

Q.40 Define oxidising agent.

Ans- The substance which gives oxygen for oxidation or removes hydrogen is called an oxidising agent.

Q.41 What do you understand by a balanced chemical equation? Why should the chemical equation be balanced?

Ans- A chemical equation which consists of an equal number of atoms of each element on both sides of the equation is known as balanced chemical equation.

The chemical equation should be balanced due to the following reasons:

(a)  The number of various types of atoms in reactants must be equal to the number of same type of atoms in products.

(b)  The total mass of all the reactants must be equal to the total mass of the product

So, the chemical equation is balanced to satisfy the law of conservation of mass in chemical reaction.

Q.42 Write the balanced equation for the following chemical reactions-

$\begin{array}{l}\text{(a)\hspace{0.33em}Sodium\hspace{0.33em}+\hspace{0.33em}Water\hspace{0.33em}}\to \text{\hspace{0.33em}Sodium\hspace{0.33em}hydroxide\hspace{0.33em}+\hspace{0.33em}Hydrogen}\\ \text{(b)\hspace{0.33em}Barium\hspace{0.33em}chloride\hspace{0.33em}+\hspace{0.33em}Aluminium\hspace{0.33em}sulphate\hspace{0.33em}}\to \text{\hspace{0.33em}Barium\hspace{0.33em}sulphate\hspace{0.33em}+\hspace{0.33em}Aluminium chloride}\\ \text{(c)\hspace{0.33em}Hydrogen\hspace{0.33em}sulphide\hspace{0.33em}+\hspace{0.33em}Oxygen\hspace{0.33em}}\to \text{\hspace{0.33em}Water\hspace{0.33em}+\hspace{0.33em}Sulphur\hspace{0.33em}dioxide}\end{array}$

$\begin{array}{l}\text{<strong>Ans-</strong>}\end{array}$

$\begin{array}{l}\text{(a)\hspace{0.33em}}2\text{Na(s)}+2{\text{H}}_2\text{O(l)\hspace{0.33em}}\to 2\text{NaOH(aq)}+{\text{H}}_2\text{(g)}\\ \text{(b)\hspace{0.33em}}3{\text{BaCl}}_2\text{(aq)}+{\text{Al}}_2{\left({\text{SO}}_4\right)}_3\text{(aq)}\to 3{\text{BaSO}}_4\text{(s)}+2{\text{AlCl}}_3\text{(aq)}\\ \text{(c)\hspace{0.33em}}2{\text{H}}_2 \text{S(g)}+3{\text{O}}_2\text{\hspace{0.33em}(g)}\to 2{\text{H}}_2\text{O(l)}+2{\text{SO}}_2\text{(g)}\end{array}$

Q.43 A solution of a substance ‘X’ is used for white washing.
(a) Name the substance ‘X’ and write its formula.
(b) Write the reaction of the substance ‘X’ named in (a) above with water.

Ans-

(a) The substance ‘X’ is calcium oxide and its formula is CaO.

$\text{(b) }\underset{\text{(Calcium oxide)}}{\text{CaO (s)}}+ \underset{\text{(Water)}}{{\text{H}}_2\text{O}\left(\mathrm{l}\right)}\to \underset{\begin{array}{l}\text{(Calcium hydroxide)}\\ \text{\hspace{0.33em}\hspace{0.33em}\hspace{0.33em}Or (Slaked lime)}\end{array}}{\text{Ca}{\left(\text{OH}\right)}_2\left(\text{aq}\right)}$

Q.44 Why should magnesium ribbon be cleaned before burning in air?

Ans- Magnesium ribbon should be cleaned before burning in air due to the presence of a thin layer of oxide on the surface of magnesium at ordinary temperature. This layer prevents magnesium from further oxidation.

Q.45 (i) What is respiration?
(ii) Is respiration exothermic reaction or endothermic? Explain with reason.

Ans-

(i) Respiration is a metabolic process in which the food consumed is broken down into smaller and simpler complexes through chemical reaction with oxygen and results in the release of energy to carry out life processes and other activities.

$\underset{\text{Glucose}}{{\text{C}}_6{\text{H}}_{12}{\text{O}}_6\text{(aq)}}+6{\text{O}}_2( \text{g})\to 6{\text{CO}}_2( \text{g})+6{\text{H}}_2\text{O}\left(\mathrm{l}\right)+\text{ energy }$

(ii) Since energy is released during the process, respiration is considered as an exothermic reaction.

Q.46 What is the difference between combination and decomposition reaction? Give one example of each type.

Ans-

Decomposition reactions are opposite to combination reactions.

Q.47 Write two uses of decomposition reactions.

Ans-

1.These reactions are used to extract several metals from their oxides or other salts. When the oxide or salt is electrolysed, metal is obtained. For example, aluminium metal is obtained by passing electricity in molten aluminium oxide.

2. Digestion of food in our body is also an example of decomposition reaction. The starch (carbohydrates) present in food is decomposed to give simple sugar (glucose) and the proteins are broken down into amino acids.

Q.48 What is the difference between displacement and double displacement reactions? Write example of each reaction.

Ans-

Q.49 What is observed when a solution of potassium iodide is added to a solution of lead nitrate taken in a test tube? What type of reaction is this? Write a balanced chemical equation to represent the above reaction.

Ans- When a solution of potassium iodide (KI) is added to a solution of lead nitrate [Pb(NO₃)₂], then a yellow precipitate of lead iodide (PbI₂) is formed. This is a double displacement reaction.

$\underset{\text{Lead nitrate}}{\text{Pb}{\left({\text{NO}}_3\right)}_2\left(\text{aq}\right)}+\underset{\text{Potassium iodide}}{2\text{KI}\left(\text{aq}\right)}\to \underset{\begin{array}{l}\text{Lead iodide}\\ \text{(yellow ppt.)}\end{array}}{{\text{PbI}}_2(\text{s})\downarrow }+\underset{\text{Potassium nitrate}}{2{\text{KNO}}_3\left(\text{aq}\right)}$

Q.50 Write the type of chemical reaction represented by the following equations

$\begin{array}{l}\text{(i)\hspace{0.33em}}2\text{Al}\left(\text{s}\right)+6\text{HCl}\left(\text{ag}\right)\to 2{\text{AlCl}}_3\left(\text{aq}\right)+3{\text{H}}_2(\text{g})\end{array}$

$\begin{array}{l}\text{(ii)\hspace{0.33em}}2\text{KBr}\left(\text{aq}\right)+{\text{Cl}}_2(\text{g})\to 2\text{KCl}\left(\text{g}\right)+{\text{Br}}_2(\text{g})\end{array}$

$\begin{array}{l}\text{(iii)\hspace{0.33em}}2\text{Zn}\left(\text{s}\right)+{\text{O}}_2(\text{g})\to 2\text{ZnO}\left(\text{s}\right)\end{array}$

Ans-

Q.51 What happens when an iron nail is dipped in copper sulphate solution?

Ans- When an iron nail is dipped in copper sulphate solution, the blue colour of copper sulphate changes to light green due to the formation of ferrous sulphate and copper is formed. This happens because iron is more reactive than copper, so, it displaces copper from its salt solution.

$\underset{\left(\text{Iron}\right)}{\text{Fe}\left(\text{s}\right)}+\underset{\left(\text{Blue Solution}\right)}{{\text{CuSO}}_4\left(\text{ag}\right)}\to \underset{\left(\text{Light Green}\right)}{{\text{FeSO}}_4\left(\text{ag}\right)}+\underset{\left(\text{Copper}\right)}{\text{Cu}\left(\text{s}\right)}$

Q.52 Explain the terms oxidising and reducing agents with an example of each.

Ans- Oxidising agent: The substance, which oxidises elements or compounds, is known as an oxidising agent.
For example:

$4\text{Fe(s)}+3{\text{O}}_2\text{(g)\hspace{0.33em}}\to 2{\text{Fe}}_2{\text{O}}_3\text{(s)}$

Hence, oxygen is an oxidising agent. It oxidises iron (Fe³⁺ ion) to ferric oxide (Fe₂O₃ ).

Reducing agent: The substance which reduces an element or a compound is known as reducing agent.
For example:

${\text{SnCl}}_2\text{(aq)}+2{\text{FeCl}}_3\text{\hspace{0.33em}(aq)}\to {\text{\hspace{0.33em}SnCl}}_4\text{(aq)}+2{\text{FeCl}}_2\text{(aq)}$

Stannous chloride is a reducing agent. It reduces ferric chloride (or Fe³⁺ ion) to ferrous chloride (or Fe²⁺).

Q.53 Packets of potato chips are filled with a gas. Name this gas and explain its use.>

Ans- Packets of potato chips are filled with nitrogen gas. Oil and fat containing food items are flushed with nitrogen to prevent them from being oxidised. They become rancid and their smell and taste change.

Q.54 Translate the following statements into chemical equations and then balance them.
(a) Sodium chloride with sulphuric acid giving sodium sulphate and hydrochloric acid
(b) Zinc reacting with silver chloride to give zinc chloride and silver

Ans-

$\begin{array}{l}\text{(a)\hspace{0.33em}}2\text{NaCl(aq)}+{\text{H}}_2{\text{SO}}_4\text{(aq)\hspace{0.33em}}\to {\text{\hspace{0.33em}Na}}_2{\text{SO}}_4\text{(aq)}+2\text{HCl(aq)}\end{array}$

$\begin{array}{l}\text{(b)\hspace{0.33em}}2\text{Zn(s)}+{\text{AgCl}}_2\text{(aq)\hspace{0.33em}}\to 2{\text{ZnCl}}_2\text{(aq)}+\text{Ag(s)}\end{array}$

Q.55 What do you understand by corrosion? Give an example.

Ans- The process of slow and gradual conversion of a metal into their compounds by the attack of air and water present in atmosphere is known as corrosion. Corrosion, in case of iron is called rusting. Rust is hydrated iron (III) oxide [Fe₂O₃ .H₂O]. The presence of oxygen and water (moisture) is essential for rusting to take place.

Q.56 Identify the component oxidised in the following reaction:
H₂S + Cl₂ S + 2HCl

Ans- H₂S

In the given reaction, hydrogen sulphide is oxidised to sulphur by the removal of hydrogen and chlorine is reduced to hydrogen chloride by the addition of hydrogen.