Intext Questions 

                                                                        (page no. 6)

 

1. Why should a magnesium ribbon be cleaned before burning in air? 

Ans. The Magnesium ribbon generally has a coating of Magnesium oxide on to its surface.

— It is formed by the slow combustion of moist air.

— This Magnesium oxide is an obstacle for the process of burning of Magnesium.

— Hence, it is cleaned before burning.

2. Write the balanced equation for the following chemical reactions.

(i) Hydrogen + Chlorine —-→ Hydrogen chloride

      H2 + Cl2 ——–> 2HCl

(ii) Barium chloride + Aluminium sulphate —-→ Barium sulphate + Aluminium chloride

Ans. BaCl2 + Al2(SO4)3 ————> 3BaSO4 + 2AlCl3

 

(iii) Sodium + Water → Sodium hydroxide + Hydrogen

Ans. Na + H2O ——-> 2NaOH + H2 

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.

Ans. BaCl2 + Na2SO4 ——> 2NaCl + BaSO4

(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Ans. NaOH + HCl ——-> NaCl + H2O

                                                                       Page no. 10

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.

Ans. (i) Substance ‘X’ is Calcium oxide. Its formula CaO.

 (ii) CaO + H2O ——-> Ca(OH)2 + Heat

2. Why is the amount of gas collected in one of the test tubes double of the amount collected in the other in electrolysis of water experiment? Name this gas.

Ans. The gas which is collected in double the amount in the electrolysis of water experiment is Hydrogen.

— This is because water (H₂O) contains two parts of hydrogen element as compared to one part of Oxygen element by volume.

— Hydrogen : Oxygen = 2 : 1

                                                                           Page no. 13

1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it? 

Ans. Iron is more reactive than copper.

— It displaces copper from copper sulphate solution.

 Fe + CuSO4 ——–> FeSO4 + Cu

— Thus, as copper sulphate reacts to form iron (II) sulphate, the blue colour of copper sulphate solution fades and the solution turns green due to iron (II) sulphate.

2. Give one example of a double displacement reaction other than the one between barium chloride and sodium sulphate solution.

Ans. AgNO3 + NaCl —–→ AgCl + NaNO3 

3. Identify the substances that are oxidised and the substances that are reduced in the following reactions.

 (i) 4Na + O2 —–> 2Na2O

(ii) CuO + H2 ——> Cu + H2O

Ans. (i) In this reactions, sodium (Na) is changed into sodium oxide (Na₂O) by the addition of oxygen to sodium. Since oxygen is added it is called oxidation, the substance sodium (Na) is oxidized.

Here, copper oxide is reduced to copper whereas hydrogen is oxidized to water.

                                                                            Exercise 

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. (i) (a) and (b)

2. Fe2O3 + 2Al ——> Al2O3 + 2Fe

The above reaction is an example of

(A) combination reaction

(B) double displacement reaction

(C) decomposition reaction

(D) displacement reaction

Ans. (D) displacement reaction 

3. What happens when dilute hydrochloric acid is added to iron fillings ? Tick the correct answer.

(A) Hydrogen gas and iron chloride are produced

(B) Chlorine gas and iron hydroxide are produced

(C) No reaction takes place.

(D) Iron salt and water are produced.

Ans. (A) Hydrogen gas and iron chloride are produced

4. What is a balanced chemical equation ? Why should chemical equations be balanced ?

Ans. In chemical equation, reactants and products both side, when number of different elements is equal, such equation are called balanced chemical equations.

— As per law of conservation of mass, mass can neither be created nor destroyed in chemical reaction. That is total mass of the elements present in the products of a chemical reaction has to be equal to the total number of atoms of each element remains the same.

5. Translate the following statements into chemical equations and then balance them.

(a) Hydrogen gas combines with nitrogen to form ammonia.

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Ans. (a) N2 + 3H2 ——–> 2NH3

2H2S + 3O2 ——-> 2SO2 + 2H2O

3BaCl2 + Al2(SO4)3 ——–> 3BaSO4 + 2AlCl3

2K + 2H2O ——–> 2KOH + H2

6. Balance the following equations.

(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H₂O

Ans. 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H₂O

(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O

Ans. 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

(c) NaCl + AgNO3 → AgCl + NaNO3 

Ans. NaCl + AgNO3 → AgCl + NaNO3

(d) BaCl₂ + H₂SO₄ → BaSO4 + HCI

Ans. BaCl₂ + H₂SO₄ → BaSO4 + 2HCI

7. Write the balanced chemical equations for the following reactions.

(a) Calcium Hydroxide + Carbon Dioxide —–→ Calcium Carbonate + Water

Ans. Ca(OH)2 + CO2 ———-→ CaCO3 + H₂O

(b) Zinc + Silver Nitrate ——–> Zinc Nitrate + Silver

Ans. Zn + 2AgNO3 ———> Zn(NO3) + 2Ag

(c) Aluminium + Copper Chloride ———> Aluminium Chloride + Copper

Ans. 2Al + 3CuCl2 ——- > 2AlCl3 + 3Cu

(d) Barium Chloride + Potassium Chloride —-> Barium Sulphate + Potassium Chloride 

 Ans. BaCl2 + K2SO4 ———> BaSO4 + 2KCl

8. Write a balanced chemical equation for the following and identify the type of reaction in each case.

(a) Potassium bromide + Barium Iodide –→ Potassium lodide + Barium Bromide

Ans. 2KBr + BaI2 ——–> 2KI + BaBr2

Type of reaction : Displacement reaction 

(b) Zinc Carbonate ——–> Zinc oxide + Carbon dioxide

Ans. ZnCO3 ——-> ZnO3 + CO2

Type of reaction : Decomposition reaction 

(c) Hydrogen + Chloride ——> Hydrogen Chloride

Ans. H2 + Cl2 ——–> 2HCl

Type of reaction : Combination reaction

(d) Magnesium + Hydrochloric acid ———> Magnesium chloride + Hydrogen 

Ans. Mg + 2HCl ———-> MgCl2 + H2

Type of reaction : Displacement reaction 

9. What does one mean by exothermic and endothermic reactions? Give examples.

Ans. Exothermic Reactions : The reactions in which heat is released alongwith the formation of products are called exothermic chemical reactions.

 CH4 + 2O2 ——-> CO2 + 2H2O

Endothermic Reactions : The reactions in which heat is supplied or absorbad are called endothermic reactions.

CaCO3 + Heat ———> CaO + CO2 

10. Why is respiration considered an exothermic reaction? Explain. 

Ans. During respiration, the glucose combines with oxygen in the cells of our body and provides energy. Thus, respiration is an exothermic reaction because energy is produced during this process.

C6H12O6 + 6O2 + 6H2O ——–> 6CO2 + 12H2O + Energy

11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions ?

Ans. In a combination reaction, two or more substances combine to form a single product. Also a large amount of heat is evolved.

— The decomposition reactions require energy either in the form of heat, light or electricity for breaking down one substance into two or more subtances.

N₂ + 3H2 ——> 2NH3 + Heat (Combination)

2NH3 + Heat ——> N₂ + 3H2 (Decomposition)

12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or energy.

Ans. (a) Heat is supplied [Thermal decomposition]

CaCO3 + Heat ——-> CaO + CO2

(b) Light is absorbed [Photolytic decomposition]

2AgCl (s) + sunlight ——-> 2Ag(s) + Cl2

(c) Electricity is supplied [Electrolytic decomposition]

2H2O + Electricity ——> 2H₂O

13. What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Ans. In a displacement reaction, a more reactive elements displaces or removes another element from its compound

Zn + CuSO4 ——-> ZnSO4 + Cu

— In a double displacement reactions, two compounds react by exchanging their ions and form two new products.

BaCl2 + Na2SO4 ——> 2NaCl + BaSO4 

14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Ans. Cu + 2AgNO3 ——> Cu(NO3)2 + 2Ag

15. What do you mean by precipitation reaction? Explain giving examples.

Ans. On mixing the clear solutions of two ionic compounds, a substance which is insoluble in water, is formed.

This insoluble substance formed is known as precipitate.

Any reaction that produces a precipitate is called a precipitation reaction.

BaCl2 + Na2SO4 ——> 2NaCl + BaSO4

16. Explain the following in terms of gain or loss of oxygen with two examples each:

 (a) Oxidation (b) Reduction

Ans. (a) In a chemical reaction Oxygen are gain or Hydrogen are loss is known as Oxidation reaction.

2Mg + O2 ——> 2MgO

In a chemical reaction Oxygen are loss or Hydrogen are gain is known as Reduction reaction.

ZnO + C ——-> Zn + CO

17. A shiny brown coloured element ‘x’ on heating in air becomes black in colour. Name the element ‘x’ and the black coloured compound.

Ans. Element ‘x’ is copper. The black coloured compound formed is copper (ii) oxide.

2Cu + O2 ——-> 2CuO

18. Why do we apply paint on iron articles ?

Ans. Ferrous reacts with oxygen in the air and form iron oxide.          

— This reaction is called corrosion. It spoils the iron articles by rusting. 

— Corrosion of iron articles can be prevented or minimized by shielding the metal surface from oxygen and moisture. 

— It can be prevented by applying paint on the articles.

19. Oil and fat containing food items are flushed with nitrogen. Why? 

Ans. In the presence of oxygen in the air, the fats present in the fatty food are getting oxidised to certain compounds which have a bad odour, that is, the food becomes rancid.

— Flushing with nitrogen cuts off the contact of food with oxygen and protects the food from rancidity.

 

20. Explain the following terms with one example each. 

(a) Corrosion.

Ans. When a metal is attacked by substances around it such as moisture, acids etc. it is said to corode and this process is called corrosion.

— Iron articles are shiny when new, but get coated with a reddish brown powder when left for some time. This process is known as rusting of Iron.

— The black coating on silver and the green coating on copper are other examples of corrosion.

— Corrosion causes damage to car-bodies, bridges, Iton railing, ships and to all objects made of metals.

(b) Rancidity

— When fats and oils are oxidised, they become rancid and their smell and taste changes.

— Usually substances which prevent oxidation (antioxidants) are added to foods containing fats and oils.

— Keeping food in air tight containers helps to slow down oxidation.

— Chips manufacturer usually flush bags of chips with gas such as nitrogen to prevent the chips from getting oxidised.