(a) CO₂
Ans. When carbon dioxide is passed into lime water, the lime water turns milky. This is the characteristic test for carbon dioxide.
(b) SO₂

 Ans. Sulphur dioxide smells like rotten egg.
(c) O₂
Ans. When a burning matchstick is brought near oxygen, the matchstick burns even more brightly. This is the characteristic test for oxygen.
(d) H₂
Ans. When a burning matchstick is brought near hydrogen gas, the matchstick burns with a pop sound

(a) zinc metal is added to copper sulphate solution?
Ans. Zinc displaces copper to form zinc sulphate. 
 Zn(s) + CuSO₄(aq)  → ZnSO₄(aq) +Cu
 
 
 (b) aluminium metal is added to dilute hydrochloric acid?
Ans. Aluminium reacts with dil.hydrochloric acid to form aluminium chloride. Hydrogen gas is evolved in this rection. 

2Al(s) + 6HCl(aq) → AlCl₃(aq) + 3H₂

(c) silver metal is added to copper sulphate solution?
 Ans. Silver is less reactive than copper. Hence, no reaction takes place. 

The reaction of Zn granules with
 (a) Dilute H₂SO₄ 
 
 Zn(s) +H₂SO₄(aq) → ZnSO₄(aq) + H₂(g) 
 
 (b) Dilute HCl
 
 Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g) 
 
 (c) Dilute HNO₃
 
 Reaction with dilute HNO₃  is different as compared to other acids because nitric acid is an oxidising agent and it oxidises H₂  gas evolved to H₂O. 
 
 4Zn(s)  + 10HNO₃(aq) → 4Zn(NO₃)₂(aq) + 5H₂O + N₂O(g)
 
 (d) NaCl solution
Zn(s) +  NaCl(aq)  → Noreaction
 
 (e) NaOH solution 
 
 Zn(s) + NaOH(aq) → NaZnO₂ (aq)     + H₂(g)
                                Sodium Zincate

 (a) Write a balanced chemical equation of the reaction involved.
Ans. Balanced chemical equation

NaSO₃(aq) + BaCl₂(aq) → BaSO₃(s) + 2NaCl(aq)

(b) What other name can be given to this precipitation reaction?
Ans. This reaction is also known as double displacement reaction.
(c) On adding dilute hydrochloric acid to the reaction mixture, white precipitate disappears. Why?
Ans. BaSO₃  is a salt of a weak acid (H₂SO₃) , therefore dilute acid such as HCl decomposes barium sulphite to produce sulphur dioxide gas which has the smell of burning sulphur
BaSO₃(s) + HCl → BaCl₂ + H₂O + SO₂(g)

Copper is a nobel metal. It does not react with hydrochloric acid or with nitric acid. But aluminium is more reactive than copper. So, these acids should be kept in copper container. 

Copper is less reactive than zinc and hence zinc chloride solution can be kept in copper container. But aluminium is more reactive than zinc and hence zinc chloride solution cannot be kept in aluminium container. For keeping water; both the containers can be used. However, copper is better.