Intext Questions Solved

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Question 1 : Why should magnesium ribbon be cleaned before burning in air?

Answer : It is because there is oxide layer on magnesium which must be removed, otherwise it will react very slowly.

Question 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(aq) + Al2(SO4)3(aq) ———-> 3BaSO4(s) + 2AlCl3(aq)

iii) 2Na(s) + 2H2O(l) ———> 2NaOH(aq) + H2(g)

Question 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)

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Question 1 : A solution of a substance ‘X’ is used for white-washing.

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) ‘X’ is calcium hydroxide. Its formula is Ca(OH)2.

ii) Ca(OH)2(aq) + CO2(g) ———-> CaCO3(s) + H2O(l)

A milky solution of slaked lime Ca(OH)2 is used for white washing the walls. Calcium hydroxide reacts slowly with CO2 in the air to form thin layer of calcium carbonate on the walls. The calcium carbonate formed after two to three days of white washing gives shines to the walls. The chemical formula of marble is also CaCO3.

Question 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 : The amount of H2 gas collected will be double than that of oxygen. In water, hydrogen and oxygen are present in molar ration of 2:1.

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Question 1 : What is colour of precipitate formed and name the precipitate when lead (II) nitrate reacts with potassium iodine?

Answer : The yellow ppt. is formed due to lead iodine.

Question 2 : Write the balanced chemical equation for the above reation.

Answer : Pb(NO3)2 (aq) + 2Ki(aq) ——–> PbI2(s) + 2KNO3(aq)

Yes, it is double displacement reaction.

Question 3 : Is this also a double displacement reaction.

Answer : Yes, it is double displacement reaction.

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

Answer : It is because iron displaces copper from copper sulphate solution to form FeSO4 and copper metal gets deposited as iron is more reactive than copper.

Fe(s) + CuSO4(aq) ——-> FeSO4(aq) + Cu(s)

Question 2 : Give an example of a double displacement reaction.

Answer : CuSO4(aq) + 2NaOH(aq) ——> Cu(OH)2(s) + Na2SO4(aq)

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

i) 4Ns(s) + O2(g) ————> 2Na2O(s)

ii) CuO(s) + H2(g) ————> Cu(s) + H2O(l)

Answe : i) Sodium is getting oxidised, O2 is getting reduced.

ii) Copper oxide is getting reduced and H2 is getting oxidized.

Textbook Questions Solved

Question 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

Answer : (i) (a) and (b) are incorrect statements.

Question 2 : Fe2O3 + 2Al ———-> Al2O3 + 2Fe

The above reaction is an example of a

a) combination rection
b) double displacement reaction
c) decomposition reaction
d) displacement reaction

Answer : (d) It is displacement reaction because ‘Al’ displace iron from ferric oxide.

Question 3 : What happens when dil. HCl is added to iron filings? Tick the correct answer.

a) Hydrogen gas and iron(II) chloride are produced.
b) Chlorine gas and iron hydroxide are produced.
c) No reaction takes place.
d) Iron salt and water are produced.

Answer : (d) is correct answer.
Fe(s) + 2HCl(dil) ——–> FeCl2(aq) + H2(g)

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

Answer : The equation in which atoms of different elements are equal on both sides is called balanced chemical equation.

Equations must be balanced so as to satisfy law of conservation of mass. The total mass of reactants must be equal to total mass of products.

Question 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.

Answer : 

a) 3H2 + N2(g) ———–> 2NH3(g)
b) 2H2S(g) + 3O2(g) ———-> 2H2O(g)  + 2SO2(g)
c) 3BaCl2(aq) + Al2(SO4)3(aq) ———> 3BaSO4(s) + 2AlCl3(aq)
d) 2K(s) + 2H2O(l) ——–> 2KOH(aq) + H2(g)

Question 6 : Balance the following chemical equations:

a) HNO3 + Ca(OH)2 ———-> Ca(NO3)2 + H2O
b) NaOH + H2SO4 ———> Na2SO4 + 2H2O
c) NaCl + AgNO3 ———-> AgCl + NaNO3
d) BaCl2 + H2SO4 ———–> BaSO4 + HCl

Answer : 

a) 2HNO3 + Ca(OH)2 ————-> Ca(NO3)2 + 2H2O
b) 2NaOH + H2SO4 ————-> Na2SO4 + 2H2O
c) NaCl + AgNO3 ————> AgCl + NaNO3
d) BaCl2 + H2SO4 ————-> BaSO4 + 2HCl

Question 7 : Write the balanced chemical equations for the following reactions:

a) Calcium hydroxide + Carbon dioxide ———-> Calcium carbonate + Water
b) Zinc + Silver nitrate ———> Zinc nitrate + Silver
c) Aluminium  + Copper chloride ———-> Aluminium chloride + Copper
d) Barium chloride + Potassium sulphate ———–> Barium sulphate + Potassium chloride

Answer : 

a) Ca(OH)2(aq) + CO2(g) ———> CaCO3(s) + H2O(l)
b) Zn(s) + 2AgNO3(aq) ———–> Zn(NO3)2(aq) + 2Ag(s)
c) 2Al(s) + 3CuCl2(aq) ————> 2AlCl3(aq) + 3Cu(s)
d) BaCl2(aq) + K2SO4(aq) ———–> BaSO4(s) + 2KCl(aq)

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

a) Potassium bromide (aq) + Barium iodine (aq) ———–> Barium bromide(s) + Potassium iodide(aq)

b) Zinc carbonate(s) ———> Zinc oxide(s) + Carbonate dioxide (g)

c) Hydrogen (g) + Chlorine (g) ———-> Hydrogen chloride (g)

d) Magnesium (s) + Hydrochloric acid (aq) ———–> Magnesium chloride (aq) + Hydrogen (g)

Answer : 

a) 2KBr(aq) + BaI2(aq) ——–> 2KI(aq) + BaBr2(aq) {Double displacement reaction}

b) ZnCO3(s) ———-> ZnO(s) + CO2(g) {Decomposition reaction}

c) H2 (g) + Cl2 (g) ———-> 2HCl (g) {Combination reaction}

d) Mg (s) + 2HCl (aq) ———> MgCl2 (aq) + H2 (g) {Displacement reaction}

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

Answer : Exothermic reactions are those in which heat is evolved e.g.,

C (s) + O2 (g) ———–> CO2 (g)

CH4 (g) + 2O2 (g) ———–> CO2 (g) + 2H2O (l)

Endothermic reactions: Those reactions in which heat is absorbed are called endothermic reactions. e.g.,

CaCO3 (s) —-heat—–> CaO(s) + CO2 (g)

N2 (g) + O2 (g) ——–heat——> 2NO (g)

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

Answer : It is because energy is released during respiration, therefore, respiration is regarded as exothermic reaction.

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

Answer : In decomposition reaction, a compound is broken down into simple compounds or elements e.g.,

CuCO3 (s) ——heat——> CuO (g) + CO2 (g)

Combination reaction is a reaction in which two or more elements or compounds combine to form new compound , e.g.,

N2 (g) + 3H2 (g) ———> 2NH3 (g)

Thus, decomposition and combination reactions are opposite to each other.

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

Answer : 

a) CaCO3 (s) ———heat——-> CaO(s) + CO2 (g)

b) 2AgCl (s) —–light——-> 2Ag (s) + Cl2 (g)

c) 2H2O (l) ——electricity——-> 2H2 (g) + O2 (g)

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

Answer : In displacement reaction, more reactive metal can displace less reactive metal from its salt soultion.

Zn (s) + CuCl2 (aq) ——-> ZnCl2(aq) + Cu (s)

In double displacement reaction, two compounds exchange their ions to form two new compounds , e.g,

NaOH (aq) + HCl (dil.) ———-> NaCl (aq) + H2O (l)

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

Answer : Cu (s) + 2AgNO3 (aq) ———> Cu(NO3)2 (aq) + 2Ag

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

Answer : Those reactions in which two compounds react to form insoluble compound which is also called precipitate and these reactions are called precipitation reactions, e.g.,

AgNO3(aq) + NaCl (aq) ———-> AgCl (s) + NaNO3 (aq)

Pb(NO3)2 (aq) + 2HCl (aq) ———-> PbCl2 (s) + 2HNO3 (aq)

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

a) Oxidation
b) Reduction

Answer : 

a) Oxidation : It is a process in which gain of oxygen takes place, e.g.,

2Mg (s) + O2 (g) ——burning——-> 2MgO (s)

2Cu (s) + O2 (g) ——-heat———-> 2CuO (s)

b) Reduction : It is a process in which removal of oxygen takes place, e.g., 

CuO (s)  + H2 (g) ——heat——> Cu (s) + H2O (l)

Fe2O3 (s) + 2Al (s) ———-> Al2O3 (s) + 2Fe (l)

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

Answer : ‘X’ is copper and gets oxidised to copper oxide which is black in colour.

2Cu (s) + O2 (g) —–heat—> 2CuO (s)

Question 18 : Why do we apply paint on iron articles?

Answer : It is done so as to prevent iron from rusting. When surface of iron is coated with paint, its surface does not come in contact with oxygen, therefore, rusting does not take place.

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

Answer : The food items are prevented in the atmosphere of N2 so as to prevent them from oxidation.

Food is kept in refrigerator so as to reduce the temperature which slows the rate of oxidation of food materials and preserve the food for sometime.

Question 20 : Explain the following terms with an example each:

a) Corrosion
b) Rancidity.

Answer : a) Corrosion : It is a process in which metal reacts with substance present in atmosphere to from surface compounds , e.g., iron reacts with oxygen in presence of moisture to form rust, Fe2O3.xH2O (hydrated ferric oxide).

b) Rancidity : It is a process in which food materials gets spoiled due to oxidation. It leads to change in taste and smell of food material, e.g., butter gets spoiled due to oxidation at room temperature if kept for long time. It becomes sour in taste and gives a foul smell also.