NCERT Class 10 very Important Frequently asked 5 mark questions of Science ch-1

Very Important 5 mark questions of Science Chemical reactions and equations:

1. What is Combination reaction?state two examples giving balanced chemical equations for the reactions.

Ans: The reaction in which two or more substances (elements or compounds) combine to form a single product is called a Combination reaction.

Examples:

Activity-1

— Clean a magnesium ribbon with the help of sand paper.

— Hold it with a pair of tongs. Burn it using a spirit lamp or burner and collect the ash so formed in a watch glass. 

—Note: Burn the magnesium ribbon keeping it away as far as possible from your eyes.

— We will observe that Magnesium ribbon burns with a dazzling white flame and changes into a white powder.

— This powder is Magnesium oxide.

— It is formed due to the Reaction between magnesium and oxygen present in air.

— The following reaction can be shown as:

          2Mg + O2 —–› 2MgO 

Activity-2

—Take a small amount of calcium oxide or quick lime in a beaker.

— Slowly add water to this.

— If we Touch the beaker we feel the change in temperature.

— Calcium oxide reacts vigorously with water to produce slaked lime [calcium hydroxide] releasing a large amount of heat. 

     CaO + H2O —–› Ca(OH)2 + Heat

2. (a) Define a Decomposition reaction. Why is this reaction always Endothermic? 

    (b) What is Thermal Decomposition, Electrolytic decomposition and photochemical decomposition. Give examples in each case. 

Ans: (a) The Reaction in which a compound splits up into two or more simpler substances is called decomposition reaction.

          In Decomposition Reactions either Heat, electricity or light is used. As energy is used in these reactions, therefore Decomposition Reactions are always Endothermic.

   (b) —Thermal Decomposition: In most of the decomposition Reactions heat is used to decompose the substances, therefore those Reactions are known as Thermal Decomposition.

E.g.,  Limestone (CaCO3) decomposes to calcium oxide and carbon dioxide on heating.

                  CaCO3 —–› CaO + CO2 

       — Electrolytic decomposition: The Reactions,in which electricity is used to break the substance, then it is referred as Electrolytic decomposition.

E.g.,  Water decomposes when electricity is passed through it.

                 2H2O —–› 2H2 + O2

       — Photo chemical decomposition: the reactions in which light is used to break down the substance then those Reactions are termed as photo chemical decomposition.

E.g.,  Silver Chloride when exposed to sunlight turns grey. This is due to decomposition of silver chloride into grey silver and Chlorine in the presence of light. This happens due to absorption of light energy.

            2AgCl —–› 2Ag + Cl2

3.  (a) Explain Displacement Reaction and Double Displacement Reaction, giving example in each case. 

     (b) Na2SO4 + BaCl2 —–› BaSO4 + 2NaCl

(i) Identify the type of reaction.

(ii) Name the precipitate if formed.

(iii) Suggest other name to this reaction.

Ans: (a) Displacement Reaction: The reaction in which an atom or a group of atoms in the molecule is displaced by another atom or a group of atoms is called displacement reaction. 

E.g., zinc displaces copper from copper sulphate solution.

      CuSO4 + Zn —–› ZnSO4 + Cu

         — Double Displacement Reaction: The Reaction in which two reacting molecules exchange their partner ions in aqueous solution is called Double Displacement Reaction.

E.g., when aqueous solution of AgNO3 is added to the solution of NaCl then precipitate of AgCl; and NaNO3 is formed.

       AgNO3 + NaCl—–› AgCl + NaNO3

      (b) (i) Double Displacement Reaction

           (ii) BaSO4

          (iii) precipitation reaction.