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1. Why should a magnesium ribbon be cleaned before burning in air?
Ans: So that the layer of magnesium oxide that it forms when reacting with air can be removed in order to get the desired chemical reaction.

2.
Write the balanced equation for the following chemical reactions.
(i) Hydrogen + chlorine  →  Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate  →  Barium sulphate + Aluminium chloride
(iii) Sodium + water  →  Sodium hydroxide + hydrogen

Ans: (i) Hydrogen + Chlorine  →  Hydrogen chloride                                                                                                                                   H2 (g)  +  Cl2 (g)  →  2 HCl (g)
(ii)  Barium chloride  +  Aluminium sulphate → Barium sulphate  +  Aluminium chloride
3 BaCl2(s)  +  Al2 (SO4)3 (s)  →  3 Ba (SO4) (s)  +  2 AlCl3 (s)
(iii) Sodium + Water  → Sodium Hydroxide + Hydrogen
2 Na(s) + 2 H2O (l)  →  2 NaOH(aq) + H2 (g)

3.
Write a balanced chemical equation with state symbols for the following reactions.
i) Solutions of Barium chloride and Sodium sulphate in water react to give insoluble Barium sulphate and the solution of Sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with Hydrochloric acid solution  to produce sodium chloride solution and water
Ans: i) Solutions of Barium chloride and Sodium sulphate in water react to give insoluble Barium sulphate and the solution of Sodium chloride.                                                                                                                                                                           BaCl2 (aq)  +  Na2 SO4 (aq)  →  Ba SO4 (s)  +  2 NaCl (aq)
(ii) Sodium hydroxide solution (in water) reacts with Hydrochloric acid solution
NaOH (aq)  +  HCl (aq)  →  NaCl (aq)  +  H2O (l)

4. A solution of a substance X is used for white washing.
(i) Name the substance X and write its formula.
Ans: The substance X is Calcium oxide. Its chemical formula is CaO.
(ii) Write the reaction of the substance X Named in (i) above with water.
Ans: Calcium oxide reacts vigorously with water to form Calcium hydroxide (slaked lime)                                                          CaO  + H2O (l)  →  Ca (OH)2 (aq)

5.
Why is the amount of gas collected in one of the test tubes in activity 1.7 ( in textbook) double of the amount collected in the other. Name this gas.
Ans. Water contains hydrogen and oxygen in the ratio of 2:1. The amount of hydrogen and oxygen produced during electrolysis of water is in a 2:1 ratio. During electrolysis of water, hydrogen goes to one test tube and oxygen goes to another (in the ratio 2:1) hence the amount of gas collected in one of the test tubes is double of the amount collected in the other.This gas is Hydrogen.

6:
Why does the colour of Copper sulphate solution change when an iron nail is dipped in it?
Ans: When an iron nail is dipped in a Copper sulphate solution, iron (which is more reactive than copper) displaces copper from Copper sulphate solution forming Iron sulphate, which is green in colour.                                                                               Fe (s) + CuSO4 (aq  ) →   FeSO4 (aq)   +  Cu (s)

7:
Give an example of a double displacement reaction other than the one given in activity 1.10.( textbook)
Ans: Sodium carbonate reacts with Calcium chloride to form Calcium carbonate and Sodium chloride.                                                Na CO3  +  CaCl2   →   CaCO3  +  2 NaCl
In this reaction, Sodium carbonate and Calcium chloride exchange ions to form two new compounds.                         Hence, it is a double displacement reaction.

8: Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(i) 4 Na (s)  +  O2 (g)   → 2 Na2O (s)
(ii) CuO (s)  +  H2 (g)  →  Cu(s)   + H2O (l)
Ans (i) Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.
(ii). Copper oxide (CuO) is reduced to copper (Cu) while hydrogen (H2) gets oxidised to water (H2O).