State the type of chemical reaction in each of the following:
i)When electric current is passed through water, hydrogen and oxygen gases are produced.
ii)When oxygen is passed over red, hot copper, copper oxide is produced.
iii)Calcium carbonate produces calcium oxide and carbon dioxide on heating.
iv)Calcium reacts with water to give calcium hydroxide and hydrogen gas.
Answers
1. Why should a magnesium ribbon be cleaned before it is burnt in air?
Ans: Magnesium is an 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 by sand paper for removing this layer so that the underlying metal can be exposed to air.
2. Write the balanced equation for the following chemical reactions.
(a) Hydrogen + Chlorine → Hydrogen chloride
(b) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(c) Sodium + Water → Sodium hydroxide + Hydrogen
Ans: (a) H2 (g) + Cl2 (g) → 2HCl (g)
(b) 3BaCl2 (s) + Al2(SO4)3 (s) → 3BaSO4(s) + 2AlCl3 (s)
(c) (c) 2Na(s) + 2H2O (l) → 2NaOH (aq) + H2 (g)
3. Write a balanced chemical equation with state symbols for the following reactions.
(a) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(b) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Ans: (a) BaCl2 (aq) + Na2SO4 (aq) → BaSO4 (s) + 2NaCl (aq)
(b) NaOH (aq) + HCl (aq) → NaCL (aq) + H2O (l)
Page No: 10
1. A solution of a substance ‘X’ is used for white washing.
(a) Name the substance ‘X’ and write its formula.
(b) Write the reaction of the substance ‘X’ named in (a) above with water.
Ans: (a) The substance ‘X’ is calcium oxide. Its chemical formula is CaO.
(b) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).
CaO (s) + H2O (l) → Ca(OH)2 (aq)
Calcium Oxide (Quick Lime) + Water → Calcium Hydroxide (Slaked Lime)
2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.
Ans: Water contains two parts of hydrogen and one part oxygen. Therefore, during the electrolysis of water the amount of hydrogen gas collected in one of the test tubes is double than that of the oxygen produced and collected in the other test tube.
Page No: 13
1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Ans: When an iron nail dipped in the copper sulphate solution than iron displaces copper from the copper sulphate because iron is more reactive than copper. Therefore the colour of the copper sulphate solution changes.
The reaction involved here is:
Fe (s) + CuSO4 (aq) → FeSO4 (aq) + Cu (s)
2. Give an example of a double displacement reaction other than the one given in Activity 1.10.
Ans: 2KBr (aq) + BaI2 (aq) → 2KI (aq) + BaBr2 (aq)
3. Identify the substances that are oxidised and the substances that are reduced in the following reactions.
(a) 4Na (s) + O2 (g) → 2Na2O (s)
(b) CuO (s) + H2 (g) → Cu (s) + H2O (l)
Ans: (a) Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.
(b) Copper oxide (CuO) is reduced to copper (Cu) while hydrogen (H2) gets oxidised to water (H2O).
Excercise
Page No: 14
1. Which of the statements about the reaction below are incorrect?
2PbO (s) + C (s) → 2Pb (s) + CO2 (g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all
Ans: (a) (a) and (b)
2. Fe2O3 + 2Al → Al2O3 + 2Fe
The above reaction is an example of a
(a) combination reaction.
(b) double displacement reaction.
(c) decomposition reaction.
(d) displacement reaction.
Ans: (d) displacement reaction.