(a) Pb₃O₄ + 8HCl → 3PbCl₂ + Cl₂ + 4H₂O
 
 Ans : Pb₃O₄
 
(b) 2Mg + O₂ → 2MgO

Ans O₂

(c) CuSO₄ + Zn → Cu + ZnSO₄

Ans ; CuSO₄

(d) V₂O₅ + 5Ca → 2V + 5CaO

Ans : V₂O₅

(e) 3Fe + 4H₂O → Cu +H₂O

Ans : H₂O

(f) CuO + H₂ → Cu + H₂O

Ans : CuO

(a) Sodium carbonate on reaction with hydrochloric acid in equal molar concentrations ves sodium chloride and sodium hydrogen carbonate.

Ans: Na₂CO₃ + HCl  → NaCl + NaHCO₃

(b) Sodium hydrogen-carbonate on reaction with hydrochloric acid gives sodium chloride, water and liberates carbon dioxide.
Ans:
NaHCO₃ + HCl → NaCl + H₂O + CO₂

(c) Copper sulphate on treatment with potassium iodide precipitates cuprous iodide Cu₂, I₂  liberates iodine gas and also forms potassium sulphate. 
Ans: 
 2CuSO₄ + 4 KI → Cu₂I₂ + I₂ + k₂SO₄

KCl+ AgNO₃ → AgCl + KNO₃
 This is a precipitation reaction. This is double displacement reaction. 

2FeSO₄ + Heat → Fe₂O₃ + SO₂  + SO₃
 It is decomposition reaction 

Firefly secrete the enzyme luciferase. This enzyme reacts with luciferin which carries out oxidation of magnesium. This happens in the presence of magnesium and oxygen. Light is emitted because of oxidation of magnesium. Due to this, fireflies glow at night.

When grapes are still on plants, their self-defense mechanism is at work. This prevents fermentation of grapes. When grapes are plucked form plants, then yeast (a type of fungi) carry out fermentation. During fermentation, sugar changes into alcohol. So, it is a chemical change

(a) Evaporation of petrol
 (b) Burning of Liquefied Petroleum Gas (LPG)
 (c) Heating of an iron rod to red hot.
 (d) Curdling of milk
 (e) Sublimation of solid ammonium chloride
Ans. (a), (c) and (e) are physical changes.
(b) and (d) are chemical changes. 

(a) Silver metal does not show any change
(b) The temperature of the reaction mixture rises when aluminium (Al) is added.
(c) The reaction of sodium metal is found to be highly explosive
(d) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid.

Explain these observations giving suitable reasons.
Ans. (a) Silver metal does not react with dilute HCl.
(b) The temperature of the reaction mixture rises when aluminium is added because it is an exothermic reaction.
(c) Reaction of sodium metal is found to be highly explosive because it is an exothermic reaction
(d) When lead is treated with hydrochloric acid, bubbles of hydrogen gas are evolved.
 2Pb + HCl → PbCl₂ + H₂