Chemical Reactions and Equations

Q1. Why should a magnesium ribbon be cleaned before burning in air?


Magnesium is a highly reactive metal which reacts with oxygen and forms a layer of magnesium oxide on its surface when exposed into air. The layer of magnesium oxide is stable which prevents its further reaction with oxygen. So, the magnesium ribbon is cleaned with the help of sand paper for cleaning so that the underlying metal can be exposed into air.


Q2. Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate +
Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen


i)H2(g) +Cl2(g) →2HCl(g)

ii)3BaCl2+ Al2(SO4)3  → 3BaSO4+2AlCl3

iii)2Na(s) +2H2O(l)  → 2NaOH(aq)+H2(g)


Q3. Write a balanced chemical equation with state symbols for the following reactions.
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.


(i)BaCl2(aq) +Na2SO4(aq) → BaSO4(s)+ 2Nacl(aq)

(ii)NaOH(aq)+Hcl(aq) →Nacl(aq)+H2O(l)


Q1. A solution of a substance ‘X’ is used for white washing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.


The substance ‘X’ is calcium oxide. Its chemical formula is CaO.

Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).


Q2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.


H2O consists of two parts of  hydrogen and one part of oxygen.Their ration is 2:1 during electrolysis process by weight.since hydrogen goes to one test tube and oxygen goes to another,hence the amount of gas collected in one of the test tubes is double of the amount collected in the other.


Q1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?


This takes place due to displacement reaction.

Here when iron reacts with copper it displaces copper from copper sulphate solution and form iron sulphate, which is green in colour.

Now, the blue colour of copper sulphate solution fades and green colour appears.


Q2. Give an example of a double displacement reaction other than the one given in Activity 1.10.


Here in this reaction, sodium carbonate and calcium chloride exchange ions to form two new compounds. Thus,it is called double displacement reaction.


Q3. Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) 4Na(s) + O2(g) → 2Na2O(s)
(ii) CuO(s) + H2(g) → Cu(s) + H2O(l)


(i) Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.

(ii) Copper oxide (CuO) is reduced to copper (Cu) while hydrogen (H2) gets oxidised to  form water (H2O).


Q1. 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


(i) (a) and (b)


Q2. 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.


(d) displacement reaction.


Q3. 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.


(a) Hydrogen gas and iron chloride are produced.


Q4. What is a balanced chemical equation? Why should chemical equations be balanced?


According to the law of conservation of mass which states that mass can neither be created nor destroyed. Hence, in a chemical reaction, the total mass of reactants should be equal to the total mass of the products.

It means the number of atoms of each element should remains the same, before and after a chemical reaction.

So for above reason the reaction should be balanced


Q5. 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 sulpur 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


(a)3H2(g)+N2(g)  → 2NH3(g)        

(b)2H2S(g)+3O2(g)  → 2H2O(l)+2SO2(g)

(c)BaCl2(aq)+Al2(SO4)3(aq)  → 2AlCl3(aq)+3BaSO4(s)

(d)2K(s)+2H2O(l)  → 2KOH(aq)+H2(g)


Q6. 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


Q7. 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


(a) Ca(OH)2+CO2 → CaCO3+H2O

(b)Zn+AgNO3 → Zn(NO3)2+2Ag

(c)2Al+3CuCl2 → 2AlCl3+3Cu

(d)BaCl2+K2SO4 → BaSO4+2KCl


Q8. 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)


(a)2KBr(aq)+BaI2(aq) → 2KI(aq)+BaBr2(g)           Double displacement reaction

(b)ZnCO3(s) → ZnO(s) +CO2(g)                         Decomposition reaction

(c)H2(g)+Cl2(g)  → 2HCl(g)                                Combination reaction

(d)Mg(s)+2HCl(aq)  → MgCl2(aq)+H2(g)                   Displacement reaction


Q9. What does one mean by exothermic and endothermic reactions? Give examples.


If during a chemical reactions energy is released in the form of heat, light, or sound,those reactions are called exothermic reactions.

Example: Mixture of sodium and chlorine to yield table salt

Na(s) +1/2 Cl2(s)  → NaCl(s)+ 411 kJof energy

Reactions in which energy get absorbed or required in order to proceed are called endothermic reactions.

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

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


Q10. Why is respiration considered an exothermic reaction? Explain.


Exothermic reaction are the reactions in which energy is released during the process.

Example:During digestion, large molecules of food are broken down into simpler substances such as glucose.This combusion reaction is also termed as respiration.As during this process enery is released so is known as exothermic reaction.

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

Glucose        Oxygen           Carbon dioxide                   Water  


Q11. Why are decomposition reactions called the opposite of combination reactions?Write equations for these reactions.


Decomposition reactions:

Those reaction in which a compound breaks down to form two or more substances and require a source of energy to proceed.

Combination reactions:

Those reaction in which two or more substances combine to give a new substance with the release of energy.

Thus from the above definition it is clear that both are opposite to each other.


Decomposition reaction:

AB+Energy  àA+B

2H2O(l)  electrolysis  →  2H2(g)+O2(g)

Combination reaction:

2H2(g)+O2(g)  →  2H2O(l)+Energy


Q12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.


(a)Thermal decomposition:

2FeSO4(s)         +        Fe2O3(s)        ->     SO2(g)           +                 SO3(g)

Ferrous sulphate          Ferric oxide        Sulphur dioxide           Sulphur trioxide

(b)Decomposition by light:

2AgCl (s)                        light --->           2Ag(s)             +    Cl2(g)

Silver chloride                                  Silver               Chlorine

(c)Decomposition by electricity:

2Al2O3(aq)               electricity-->          4Al(s)          +           3O2(g)

Aluminium oxide                                  Aluminium                    Oxygen


Q13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.


Displacement reaction:In this type of reaction the more reactive element replaces the less reactive element from a compound.

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

Double Displacement reaction:In this type of reaction two atoms or a group of atoms switch places to form new compounds.


For example:

Displacement reaction:

CuSO4(aq)+Zn(s)   → ZnSO4(aq)+Cu(s)

Double displacement reaction:

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


Q14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.


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


Q15. What do you mean by a precipitation reaction? Explain by giving examples.


A reaction in which an insoluble solid (called precipitate) is formed is called a

precipitation reaction.

For example:

Na2CO3(aq)      +             CaCl2(aq)           →        CaCO3(s)              +            2NaCl(aq)

SodiumCarbonate        Calcium Chloride           Calcium Carbonate    Sodium Chloride

In this reaction, calcium carbonate is obtained as a precipitate. Hence, it is a precipitation reaction.


Q16. Explain the following in terms of gain or loss of oxygen with two examples each.
(a) Oxidation
(b) Reduction


a)Oxidation:In oxidation reaction there is  gain of oxygen.

b)Reduction:In reduction reaction there is  loss of oxygen.


(i) CO2 + H2 → CO + H2O

 (ii) 2Cu+O2  → 2CuO


 (i) CO2 + H2 → CO + H2O ---------(Removal of oxygen CO2 → CO)

 (ii) 2CuO+H2  → Cu +H2O---------(Loss of oxygen CuO→ Cu)

In equation (i), CO2 is reduced to CO and in equation

In eqation  (ii)CuO is reduced to Cu.


Q17. 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 copper oxide (CuO). The equation of the reaction involved on heating copper is given below.


2Cu                +                O2       Heat→                            2CuO

(Shiny brown in colour)                                             (Black in colour)


Q18. Why do we apply paint on iron articles?

Ans:Iron in presence of air and moisture iron corrodes. Iron react with the oxygen present in the atmosphere and forms ferric oxide(rust).To protect rusting we paint iron as it cuts off the contact of iron particles with the oxygen and moisture present in the atmosphere. It helps in preventing iron from rusting.

Q19. Oil and fat containing food items are flushed with nitrogen. Why?


flusing is performed to remove oxygen inside the pack.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. Thus, bags used in packing food items are flushed with nitrogen gas.


Q20. Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity


The process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc.

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

4Fe + 3O2+ nH2O    →      2Fe2O3.nH2O

                                        Hydrated iron oxide

This hydrated iron oxide is rust.


(b) 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.

For example, the taste and smell of an apple changes when kept for long.

Rancidity can be avoided by:

1. Storing food in air tight containers

2. Storing food in refrigerators

3. Adding antioxidants

4. Storing food in an environment of nitrogen.



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