Chemical equation is a method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it. In a chemical equation, the substances which combine or react are called reactants and new substances produced are called products. A chemical equation is a short hand method of representing a chemical reaction. A balanced chemical equation has equal number of atoms of different elements in the reactants and products side. An unbalanced chemical equation has unequal number of atoms of one or more elements in reactants and products. Formulae of elements and compounds are not changed to balance an equation.
(i) Consider the following reaction:
pMg₃N₂ + qH₂O ⇾ rMg(OH)₂ + sNH₃
When the equation is balanced, the coefficients p, q, r, s respectively are

(ii) Which of the following information is not conveyed by a balanced chemical equation?

(iii) The balancing of chemical equations is in accordance with

(iv) Which of the following chemical equations is an unbalanced one?

(v) Which of the following statements is/are correct?

In decomposition reactions, a single reactant breaks down to form two or more products. A decomposition reaction is opposite to combination reaction. Thermal decomposition reactions use the energy in form of heat for the decomposition of reactants. Electrolytic decomposition reactions involve the use of electrical energy for the decomposition of reactant molecules. Photolysis or photochemical decomposition involves the use of light energy for the purpose of decomposition.
(i) Which of the following reactions is a decomposition reaction?

\({ (ii) \ } 2 \mathrm{~Pb}\left(\mathrm{NO}_{3}\right)_{2} \longrightarrow 2 \mathrm{PbO}+n A+\mathrm{O}_{2}\)
What is nA in the given reaction?

(iii) Amino acid is formed by the decomposition of which component of our diet?

(iv) Silver chloride on exposure to sunlight for a long duration turns grey due to
(I) the formation of silver by decomposition of silver chloride
(II) sublimation of silver chloride
(III) decomposition of chlorine gas from silver chloride
(IV) oxidation of silver chloride
The correct statement(s) is/are

(v) What type of chemical reaction takes place when electricity is passed through water?

Redox reactions are those reactions in which oxidation and reduction occur Simultaneously. A redox reaction is made up of two half reactions. In the first half reaction, oxidation takes place and in second half reaction, reduction occurs. Oxidation is a process in which a substance loses electrons and in reduction, a substance gains electrons. The substance which gains electrons is reduced and acts as an oxidising agent. On the other hand, a substance which loses electrons is oxidised and acts as a reducing agent.
(i) Which of the following is a redox reaction?

(ii) Identify the reaction in which H2 02 is acting as a reducing agent.

(iii) For the following reactions, identify the one in which H₂S acts as a reducing agent.

(iv) For the following reaction, identify the correct statement.
\(\mathrm{ZnO}+\mathrm{CO} \longrightarrow \mathrm{Zn}+\mathrm{CO}_{2}\)

(v) In the following reaction, which substance is reduced?
\(\mathrm{PbS}+4 \mathrm{H}_{2} \mathrm{O}_{2} \longrightarrow \mathrm{PbSO}_{4}+4 \mathrm{H}_{2} \mathrm{O}\)

In a balanced chemical reaction, equal number of atoms are present on both sides of reaction. A balanced chemical reaction is based on law of conservation of mass which means that total mass of reactants and products participating in a reaction must be equal. For example, a balanced chemical equation of burning of magnesium in oxygen to form magnesium oxide is written as :
\(2 \mathrm{Mg}+\mathrm{O}_{2} \longrightarrow 2 \mathrm{MgO}\)
The mass of reactants (2 x 24 + 32 = 80) is equal to the mass of products [2 x (24 + 16) = 80]
(i) In a reaction, 35 g of reactant, PQ breaks down into 20 g of product, P and an unknown amount of product, Q. Using the law of conservation of mass, weight of products, Q will be

(ii) When solid mercury (II) oxide is heated, liquid mercury and oxygen gas are produced. Which of the following statements is true regarding the balanced chemical equation for this process?
(a) 1 mole of mercury (II) oxide produces two moles of mercury and one mole of oxygen gas
(b) 2 moles of mercury (II) oxide produce one mole of mercury and one mole of oxygen gas
(c) 1 mole of mercury (II) oxide produces half mole of mercury and half mole of oxygen gas
(d) 2 moles of mercury (II) oxide produce 2 moles of mercury and one mole of oxygen gas
(iii) Which of the following laws is satisfied by a balanced chemical equation?

(iv) In the given chemical reaction
\(\mathrm{C}_{6} \mathrm{H}_{6(l)}+15 \mathrm{O}_{2(g)} \longrightarrow m \mathrm{CO}_{2(g)}+n \mathrm{H}_{2} \mathrm{O}_{(l)}\)
The values of m and n are respectively

(v) Sulphur dioxide reacts with oxygen to form sulphur trioxide. What would be the molar ratio of sulphur dioxide to sulphur trioxide?

In a chemical reaction, reactants are converted into products. The conversion of reactants into products in a chemical reaction is often accompanied by some features which can be observed easily. These easily observed features which take place as a result of chemical reaction are known as characteristics of chemicals reactions. Some important characteristics of chemical reactions are: 
(I) Evolution of heat 
(II) Formation of precipitate
(III) Change in colour
(IV) Change in temperature
(V) Change in state
Anyone of these general characteristics can tell us whether a chemical reaction has taken place or not.
(i) Reaction of magnesium with air is a/an

(ii) In the following reaction
\(\mathrm{Ca}_{(a q)}^{2+}+2 \mathrm{OH}_{(a q)}^{-} \longrightarrow \mathrm{Ca}(\mathrm{OH})_{2(s)}\)
precipitate of calcium hydroxide will be of

(iii) In the given reaction,
\(\mathrm{S}_{(s)}+\mathrm{O}_{2(g)} \longrightarrow \mathrm{SO}_{2}\)
the physical state of SO₂ is

(iv) Which one of the following processes involve chemical reactions?

(v) In which of the following reactions, high amount of heat energy will be evolved?