Class 10 - Chemistry - Chemical Reactions and Equations

Question 1:

Which of the statements about the reaction below are incorrect?

2PbO(s) + C(s) → 2Pb(s) + CO2 (g)

(a) Lead is getting reduced.

(b) Carbon dioxide is getting oxidised.

(c) Carbon is getting oxidised.

(d) Lead oxide is getting reduced.

(i) (a) and (b)

(ii) (a) and (c)

(iii) (a), (b) and (c)

(iv) all


Correct Option: - (i) (a) and (b)

Both lead is getting reduced and carbon dioxide is getting oxidised.



Question 2:

Fe2O3 + 2Al → Al2O3 + 2Fe

The above reaction is an example of a

(a) combination reaction.

(b) double displacement reaction.

(c) decomposition reaction.

(d) displacement reaction.


Correct Option: - (d) displacement reaction.

Displacement reaction as more reactive metal (Al) takes the place of less reactive metal in the compound.



Question 3:

What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.

(a) Hydrogen gas and iron chloride are produced.

(b) Chlorine gas and iron hydroxide are produced.

(c) No reaction takes place.

(d) Iron salt and water are produced.


Correct Option: - (a) Hydrogen gas and iron chloride are produced.

Fe(s) + 2HCl (aq) à FeCl2 (aq) + H2



Question 4:

What is a balanced chemical equation? Why should chemical equations be balanced?


A balanced chemical equation is the one in which the number of atoms of different elements on the reactants side (left side)

are same as those on product side (right side).

According to law of conservation of mass, mass cannot be destroyed or created.

Therefore total number of atoms of each element should be equal on both sides of the equation.

For Example:-

Fe(s) + 2HCl (aq) à FeCl2 (aq) + H2


Question 5:

Translate the following statements into chemical equations and then balance them.

(a) Hydrogen gas combines with nitrogen to form ammonia.

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.


  1. 3H2(g) + N2(g) à 2NH3(g)
  2. 2H2S(g) + 3O2(g) à 2H2O(l) + 2SO2(g)
  3. BaCl2(aq) + Al2(SO4)3(aq) à 2AlCl3(aq) + 3BaSO4(s)
  4. 2K(s) + 2H2O(l) à 2KOH(aq) +H2(g)



Question 6:

Balance the following chemical equations.

(a) HNO3 +Ca (OH) 2 → Ca (NO3) 2 + H2O

(b) NaOH + H2SO4 → Na2SO4 + H2O

(c) NaCl + AgNO3 → AgCl + NaNO3

(d) BaCl2 + H2SO4 → BaSO4 + HCl


(a) 2HNO3 +Ca (OH) 2 → Ca (NO3) 2 + 2H2O

(b)2NaOH + H2SO4 → Na2SO4 +2H2O

(c) NaCl + AgNO3 → AgCl + NaNO3

(d) BaCl2 + H2SO4 → BaSO4 + 2HCl


Question 7:

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


  • Ca(OH)2 + CO2 à CaCO3 + H2O
  • Zn + 2AgNO3 à Zn(NO3)2 + 2Ag
  • 2Al + 3CuCl2 à 2AlCl3 + 3Cu
  • BaCl2 + K2SO4 à BaSO4 + 2KCl

Question 8:

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

(a) Potassium bromide (aq) + Barium iodide (aq) → Potassium iodide (aq) +Barium bromide(s)

(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide (g)

(c) Hydrogen (g) + Chlorine (g) → Hydrogen chloride (g)

(d) Magnesium(s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)


  • 2KBr(aq) + BaI2(aq) à 2KI(aq) + BaBr2(s)

It is a double displacement reaction.

  • ZnCO3(s) à ZnO (s) + CO2(g)

It is a decomposition reaction.

  • H2(g) + Cl2(g) à 2HCl(s)

It is a combination reaction.

  • Mg(s) + 2HCl(aq) à MgCl2(aq) + H2(g)

It is a displacement reaction.



Question 9:

What does one mean by exothermic and endothermic reactions? Give examples.


 Exothermic reactions are those reactions in which energy is released in the form of heat.

All combination reactions are exothermic reactions.

For example: - Mixture of sodium and chlorine to give table salt.

Na(s) + (1/2) Cl2(s) à NaCl(s) + 411kJ of energy

All combustion reactions are also exothermic reactions.

For example: - CH4 + 2O2 à CO2 + 2H2O + Heat

Endothermic reactions are those reactions in which energy is absorbed in order to complete the reaction.

For example: - In the process of photosynthesis, plants use solar energy to convert carbon dioxide and water to glucose and oxygen.


6CO2 (g) + 6H2O (l)   à     C6H12O6 (aq) + 6O2 (g)

                                    Glucose        Oxygen                                                                                             



Question 10:

Why respiration is considered an exothermic reaction? Explain.


The world exothermic is the combination of two words: exo means out and therm means heat.

Therefore exothermic reaction is a reaction in which energy is liberated out.

Respiration is considered as exothermic reaction because in respiration oxidation of glucose takes place which produces large amount of heat energy.

This is shown in following chemical equation.

C6H12O6 (aq) + 6O2 (g) →   6CO2 (g)    +         6H2O (l) + ATP

Glucose        Oxygen   Carbon dioxide    Water    Energy

Question 11:

 Why decomposition reactions are called the opposite of combination reactions? Write equations for these reactions.


In Decomposition reaction a single substance decomposes to form two or more substances.

They require energy to complete the reaction.

This reaction is exact opposite of Combination reaction in which two or more reactants combine to form a single product with the release of energy.

Because of this reason decomposition reactions are called the opposite of combination reactions. For examples

Decomposition reaction:

AB + energy à A + B

CaCO3(s)à CaO(s) + CO2 (g)

Combination Reaction:

A + B à AB + energy

2H2 (g) + O2 (g) à 2H2O (l) + Energy

Question 12:

Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.


Chemical decomposition: A chemical reaction in which a single substance splits into two or more substances is called chemical decomposition.

For decomposition reaction where energy is supplied in the form of:

  1. Heat:


2FeSO4                  à    Fe2O3(s)   +         SO2 (g)                             +    SO3 (g)

Ferrous Sulphate       Ferric Oxide   Sulphur dioxide         Sulphur trioxide

  1. Light: Silver bromide decomposes to silver and bromine in sunlight.


2AgCl(s)                          à               2Ag(s) + Cl2 (g)

Silver chloride                        Silver     Chlorine

  1. Electricity: On passing the electricity, water dissociates to hydrogen and oxygen.


2H2O (l)       à            2H2 (g)           +     O2 (g)

           Water                      Hydrogen          Oxygen

Question 13:

What is the difference between displacement and double displacement reactions?

Write equations for these reactions.


 Displacement Reaction: - In, displacement reaction more reactive metal displace less reactive metal from its compound.

They are generally represented as:


A + BX à AX + B; where A is more reactive than B

A very reactive chemical species can replace an anion or a cation of a compound.

This displacement occurs according to the reactivity series. It is a slow reaction and change of colour takes place.

For Example: - (a) Zn + HCl à ZnCl2 + H2

In the above equation hydrogen is being displaced from HCl by Zn.

(b) Fe(s) + CuSO4 (aq) → FeSO4 (aq) + Cu(s)

   (Copper sulphate)       (Iron sulphate)

In this reaction, iron has displaced or removed another element, copper, from copper sulphate solution.

This reaction is known as displacement reaction.



Double Displacement reaction: - Two atoms or group of atoms interchange in order to form new compounds.

There is exchange of ions.

It is generally represented as;

AB + CD à AD + CB

 It is a fast process and a precipitate is formed in these types of reactions.

For Example: - (1) HCl + NaOH à NaCl + H2O

(2) Na2SO4 (aq)             + BaCl2 (aq) →                BaSO4(s)           + 2NaCl (aq)

(Sodium sulphate)  (Barium chloride) (Barium sulphate) (Sodium chloride)

 The white precipitate of BaSO4 is formed by the reaction of SO42– and Ba2+.

The other product formed is sodium chloride which remains in the solution.


Question 14:

In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal.

Write down the reaction involved.


The following reaction is of displacement reaction/ precipitation reaction, the more electro-positive Cu,

displaces lesser electro-positive Ag from the solution.

 2AgNO3 (aq) + Cu (s) → Cu (NO3)2 (aq) + 2Ag (s)

Silver Nitrate + Copper → Copper Nitrate + Silver




What do you mean by a precipitation reaction? Explain by giving examples.


A precipitate is a solid that forms out of solution.

Therefore in a reaction in which an insoluble solid (precipitate) is formed are known as precipitation reaction.

For example:-

When a solution of iron chloride and ammonium hydroxide is mixed, a brown precipitate of iron hydroxide is formed.

FeCl2 (aq) + 3NH3OH (aq) à Fe (OH) 2(s) ↓ + 3NH3 Cl (aq)

The down arrow shows it is a precipitate.


Question 16:

Explain the following in terms of gain or loss of oxygen with two examples each.

(a) Oxidation

(b) Reduction


  • Oxidation: -


In oxidation reaction either gain of oxygen or loss of hydrogen takes place.

The surface of copper powder becomes coated with black copper (II) oxide when copper powder is heated.

Black colour as oxygen is added to copper and copper oxide is formed.


2Cu + O2 → 2CuO

  • Reduction: - It is a chemical reaction in which loss of oxygen or gain of hydrogen takes place.

For example: - CO2 + H2 à CO2

(Removal of oxygen –reduction)

Carbon dioxide is reduced to Carbon monoxide

Question 17:

A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.


‘X’ is copper (Cu) and the black coloured compound formed is (CuO) copper oxide.

The equation of the reaction involved on heating copper is given below.


2Cu                                   + O2     à         2CuO

(Shiny colour copper)    Oxygen          Copper Oxide

Question 18:

Why do we apply paint on iron articles?


In order to prevent rusting of iron articles, we paint iron articles.

The paint layer provides a barrier against the metal and the atmosphere this is how painting helps in preventing rusting.


Question 19:

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


Nitrogen is an inert gas and does not easily react with these substances.

On the other hand, oxygen reacts with food substances and makes them rancid.

Oil and fat containing food items flushed with nitrogen because nitrogen acts as an antioxidant and it prevent them from being oxidised.

Therefore bags used in packing food items, and flushed with nitrogen gas to remove oxygen inside the pack.

As there will be no oxygen is present in inside the pack, food changing to rancid is avoided.

Question 20:

Explain the following terms with one example each.

(a) Corrosion

(b) Rancidity


  • Corrosion: - When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion.
  • The black coating on silver and the green coating on copper are other examples of corrosion.
  • Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, especially those of iron. Corrosion of iron is a serious problem.

For example: - iron in presence of moisture, reacts with oxygen to form hydrated iron oxide.

4Fe + 2O2 + nH2O à 2Fe2O3.nH2O (Hydrated Iron oxide)

 Rancidity: - The process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity.

Rancidity can be avoided by:-

  1. Storing food in air tight containers.
  2. Adding antioxidants
  3. Storing food in refrigerators.

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