Ans) Magnesium is a very reactive metal. When stored, it reacts with oxygen to form a layer of magnesium oxide on its surface. This layer of magnesium oxide is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned before it is burnt in air, for removing this layer so that the underlying metal can be exposed to air.

Ans) A Chemical equation is balanced when the number of atoms of each element remains the same, before and after a chemical reaction. Chemical equation should be balanced because of law of conservation of mass. In a chemical reaction, mass can neither be created nor destroyed. That is, the total mass of the elements present in the products of a chemical reaction has to be equal to the mass of the elements present in the reactants.

Ans) 2KBr (aq) + Bal₂ (aq) → 2KI (aq) + BaBr₂ (s) {It is a double displacement reaction.}

Ans) ZnCO₃(s) → ZnO(s) + CO₂(g) {It is a dissociation reaction.}

Ans) H₂(g) + Cl₂(g) → 2HCl(g) {It is a combination reaction.}

Ans) Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) {It is a displacement reaction.}

Ans) Exothermic reaction— A reaction in which heat is released along with the formation of products is called an exothermic reaction. Ex –

(i). Burning of natural gas.

CH₄(g) + 202(g) → CO₂(g) + 2H₂O(g) + Energy

(ii). The decomposition of vegetable matter into compost is also an exothermic reaction.

Endothermic reaction— A reaction in which energy is absorbed is known as an endothermic reaction. Ex-

(i). Decomposition of calcium carbonate into calcium oxide (quick lime) and carbon dioxide.

CaCO₃(s) → Heat → CaO(s) + CO₂(g)

{Limestone → Quick lime}

(ii). Decomposition of lead nitrate into lead oxide and nitrogen dioxide.

2Pb(NO₃)₂(s) → Heat →  2PbO(s) + 4NO₂(g) + O₂(g)

{Lead nitrate → Lead oxide + Nitrogen dioxide + Oxygen }

Ans) In a displacement reaction, a more reactive element replaces a less reactive element from a compound. Example-

CuSO₄(aq) + Zn(s) → ZnSO₄(aq) + Cu(s)

In a double displacement reaction, there is an exchange of ions between the reactants. Example-

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

Ans) Combination reaction – “The chemical reactions in which two or more than two (2022) elements or compounds combine to form one compound are called combination reactions.”

Decomposition reactions – “The chemical reactions in which one compound breaks into two or more than two compounds or elements are called Decomposition reactions.”

Ans) Decomposition reactions are those in which a compound breaks down to form (NCERT) two or more substances. These reactions require energy to proceed. Example-

CaCO₃(s) → Energy → CaO(s) + CO₂(g) {Quick Lime}

2FeSO₄(s) → Energy → Fe₂O₃ + SO₂(g) + SO₃(g)

In a combination reaction, contrary to decomposition reaction, two or more substances combine to give a new substance with a release of energy. Hence, decomposition reactions are opposite of combination reactions. Example-

2H₂(g) + O₂(g) → 2H₂O (l)

C(s) + O₂(g) → CO₂(g)

Ans) A reaction in which an insoluble salt or a precipitate is formed, is called a precipitation reaction. Example-

(i). In the reaction of barium chloride (BaCl₂) and sodium sulphate (Na₂SO₄), a white precipitate of barium sulphate (BaSO₄) is formed. It is a precipitation reaction.

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

(ii). In the reaction of sodium carbonate (Na₂CO₃) and calcium chloride (CaCl₂), calcium carbonate is obtained as a precipitate.

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

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

Ans) Calcium oxide reacts vigorously with water to form calcium hydroxide:

CaO(s) + H₂O → Ca(OH)₂(aq)

Ans) The white suspension of slaked lime when applied on the walls, combines with carbon dioxide (in air) forming a thin shining layer of calcium carbonate.

Ca(OH)₂ + CO₂(g) → CaCO₃(s) + H₂O

Ans) A chemical equation conveys the following information:

• It represents the names of products and reactants.
• It tells the relative number of molecules (or atoms or moles or masses) of products and reactants.
• It represents the relative volumes of gaseous products and reactants.
• It represents the chemical reaction.
• It represents the physical states of the reactants and products.

Ans) Displacement reaction – “The reaction in which an element displaces another element from its compound is called the displacement reaction.” Example-

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

Double displacement reaction – The reaction in which two different atoms or groups of atoms (ions) are exchanged, is known as the double displacement reaction.” Example-

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

Ans) The potato chips are fried in oil. The oil and fats containing food items become rancid by oxidation and their smell and taste change. To prevent them from being oxidised and turned rancid. They are flushed with nitrogen. So their oxidation does not take places. Therefore the bags of potato chips are filled with nitrogen.