Intext I
1. Why should a magnesium ribbon be cleaned before it is burnt in the air?
Answer: Magnesium is a 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 sandpaper for removing this layer so that the underlying metal can be exposed to air.
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
Answer: (i) H2 (g) + Cl2 (g) → 2HCl (g)
(ii) 3BaCl2 (s) + Al2(SO4)3 (s) → 3BaSO4 (s) + 2AlCl3 (s)
(iii) 2Na(s) + 2H2O (l) → 2NaOH (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 (in water) to produce sodium chloride solution and water.
Answer: (i) BaCl2 (aq) + Na2SO4 (aq) → BaSO4 (s) + 2NaCl (aq)
(ii) NaOH (aq) + HCl (aq) → NaCL (aq) + H2O (l)
Intext II
1. A solution of a substance ‘X’ is used for whitewashing.
(i) Name the substance ‘X’ and write its formula.
(ii) Write the reaction of the substance ‘X’ named in (i) above with water.
Answer: (i) The substance ‘X’ is calcium oxide. Its chemical formula is CaO.
(ii) 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.
Answer: 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.
Intext III
1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Answer: When an iron nail is dipped in the copper sulphate solution, 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.
Answer: 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.
(i) 4Na (s) + O2 (g) → 2Na2O (s)
Answer: Sodium (Na) is oxidised as it gains oxygen and oxygen gets reduced.
(ii) CuO (s) + H2 (g) → Cu (s) + H2O (l)
Answer: Copper oxide (CuO) is reduced to copper (Cu) while hydrogen (H2) gets oxidised to water (H2O).
An example of a displacement reaction is: CuSo4 (aq) + Zn (s) → ZnSO4 (aq) + Cu (s). In a double displacement reaction, atoms or groups of atoms switch places to form new compounds. An example of a double displacement reaction is: Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s) + 2NaCl (aq)
2H2 (g) + O2 (g) → 2H2O (l) + Heat Energy
Endothermic reactions are reactions that absorb energy or require energy in order to proceed. Examples of endothermic reactions include:
N2 (g) + O2 (g) → 2NO + Heat Energy
H2O (l) + Heat Energy → H2O (g)
10. Why is a combustion reaction an oxidation reaction?
Answer: A combustion reaction is an oxidation reaction because it involves the combination of a substance with oxygen. In a combustion reaction, a fuel (such as wood, gasoline, or natural gas) reacts with oxygen to produce heat, light, and often, other products such as water or carbon dioxide. The process of adding oxygen to a substance is known as oxidation, so a combustion reaction can be considered an oxidation reaction. The process of removing oxygen from a substance, on the other hand, is known as reduction, and is often associated with the opposite of an oxidation reaction.
11 (i) What happens when copper is heated in air?
Answer: When copper is heated in air, it combines with oxygen to form copper oxide, a black oxide. The chemical equation for this reaction is: 2Cu + O2 → 2CuO
(ii) What happens when the product obtained in the above reaction is heated in hydrogen?
Answer: If hydrogen gas is passed over heated copper oxide, then the black copper oxide is reduced and copper metal is obtained. The chemical equation for this reaction is: CuO (s) + H2 (g) → Cu (s) + H2O (g)
12. An aluminium can was used to store ferrous sulphate solution. It is observed that in lbw days holes appeared in the can. Explain the reason and write the chemical equation to support your answer.
Answer: Aluminium is more reactive than iron, so when the aluminium can is used to store ferrous sulphate solution, the aluminium will displace the iron from the solution, forming holes in the can. The chemical equation for this reaction is: 2FeSO4 + 2Al → Al2(SO4)3 + 3Fe
13. A metal ‘P’ when exposed to moist air for longer period of time, loses its shiny brown surface and gains a green coat. Why this happened? Identify the metal. Write the name and chemical formula of this green coloured compound. List any two ways to prevent this process.
Answer: The metal has corroded because of being exposed to moist air. The green compound formed is basic copper carbonate. The metal is copper. Two ways to prevent this process are painting and oiling.
14. Can oxidation or reduction take place alone? Why or why not? What are such reactions called?
Answer: Oxidation and reduction cannot occur alone, they must always occur together in a reaction. These reactions are called redox reactions.
15. Why is respiration considered an exothermic reaction? Explain.
Answer: Respiration is considered an exothermic reaction because energy is released in this process. During respiration, glucose combines with oxygen present in our cells to form carbon dioxide and water along with energy. The chemical equation for this reaction is: C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O (l) + Energy
16. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer: Decomposition reactions are called the opposite of combination reactions because they involve the breakdown of a compound into two or more substances, while combination reactions involve the combination of two or more substances to form a new compound. An example of a decomposition reaction is: CaCO3 → CaO + CO2, and an example of a combination reaction is: CaO + H2O → Ca(OH)2
17. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Answer: (Heat) CaCO3 (s) → CaO (s) + CO2 (g), (Light) 2AgCl (s) → 2Ag (s) + Cl2 (g), (Electricity) 2H20 (I) → 2H2 (g) + 02 (g)
18. What is the difference between displacement and double displacement reactions? Write equations for these reactions.
Answer: In a displacement reaction, a more reactive element replaces a less reactive element in a compound. An example of a displacement reaction is: CuSo4 (aq) + Zn (s) → ZnSO4 (aq) + Cu (s). In a double displacement reaction, atoms or groups of atoms switch places to form new compounds. An example of a double displacement reaction is: Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s) + 2NaCl (aq)
19. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Answer: The chemical equation for the recovery of silver from silver nitrate solution through displacement by copper metal is: 2AgNO3 (aq) + Cu (s) → Cu(NO3)2 (aq) + 2Ag (s)
20. What do you mean by a precipitation reaction? Explain by giving examples.
Answer: A precipitation reaction is a chemical reaction in which an insoluble solid (precipitate) is formed from two soluble reactants in aqueous solution. An example of a precipitation reaction is the reaction between silver nitrate and sodium chloride solutions, which forms silver chloride precipitate: AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq). Another example is the reaction between potassium chloride and sodium sulfate solutions, which forms potassium sulfate precipitate: KCl (aq) + Na2SO4 (aq) → K2SO4 (s) + 2NaCl (aq)
21. (a) Explain the following in terms of gain or loss of oxygen with two examples each.
(i) Oxidation
(ii) Reduction
Answer: (a) (i) Oxidation is a chemical reaction in which a substance gains oxygen or loses hydrogen. Examples include:
(i) The reaction between magnesium and oxygen to form magnesium oxide: 2Mg (s) + O2 (g) –(Heat)→ 2MgO (s)
(ii) The reaction between copper and oxygen to form copper oxide: 2Cu (s) + O2 (g) –(Heat)→ 2CuO (s)
(a) (ii) Reduction is a chemical reaction in which a substance loses oxygen or gains hydrogen. Examples include:
(i) The reaction between copper oxide and hydrogen to form copper and water: CuO + H2–(Heat)→ Cu + H2O
(ii) The reaction between zinc oxide and carbon to form zinc and carbon monoxide: ZnO + C –(Heat)→ Zn + CO
22. A shiny brown-coloured element ‘X’ on heating in the air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
Answer: ‘X’ is copper (Cu) and the black-coloured compound formed is copper oxide (CuO). The equation of the reaction involved in heating copper is given below.
2Cu +O2 –(Heat)→ 2CuO
(Shiny Brown in Colour) (Black in Colour)
23. Why do we apply paint on iron articles?
Answer: Iron articles are painted to prevent them from rusting. After painting iron surface does not come in contact with moisture and air which cause rusting.
24. Oil and fat containing food items are flushed with nitrogen. Why?
Answer: In the presence of oxygen in the air, fats present in fatty food are oxidized to compounds and cause a bad smell. That is why oil and fat containing food items are flushed with nitrogen to cut off the contact of food with oxygen and protect the food from rancidity.
25. Explain the following terms with one example each.
(a) Corrosion
(b) Rancidity
Answer: (a) Corrosion is the deterioration of a material, typically a metal, due to its reaction with its environment. An example of corrosion is the rusting of iron when it is exposed to moisture.
(b) Rancidity is the process of fats or oils becoming spoiled or spoiled-tasting due to oxidation or hydrolysis. An example of rancidity is the change in taste and smell of cooking oil after it has been used and exposed to air for a long period of time.
