NCERT Exemplar Solution for CBSE Class 10 Science Chemical Reactions and Equations (Part-II)

Here you get the CBSE Class 10 Science chapter 1- Chemical Reactions and Equations: NCERT Exemplar Solution (Part-II). This part of the chapter includes solutions for Question No. 22 to 44 from the NCERT Exemplar Problems for Class 10 Science Chapter-Chemical Reactions and Equations.

NCERT Exemplar Solution for CBSE Class 10 Science, Chemical Reactions and Equations (Part-I)

NCERT Exemplar problems are a very good resource for preparing the critical questions like Higher Order Thinking Skill (HOTS) questions. All these questions are very important to prepare for CBSE Class 10 Science Board Examination 2017-2018 as well as other competitive exams.

Find below the NCERT Exemplar problems and their solutions for Class 10 Science Chapter-Chemical Reactions and Equations:

Short Answer Type Questions

Question 19. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.

(a) Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773 K to form ammonia gas.

(b) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water.

(c) Ethanol is warmed with ethanoic acid to form ethyl acetate in the presence of concentrated H₂SO₄.

(d) Ethane is burnt in the presence of oxygen to form carbon dioxide, water and releases heat and light.

Answer.                           

This is an example of combination reaction.

(b) NaOH (aq)  +  CH₃COOH (l) → CH₃COONa (aq)  +  H₂O (l)

This is an example of neutralization reaction as well as double displacement reaction.

This is an example of double displacement reaction and also called as esterification.

(d) 2C₂H₆(g)  +  7O₂(g) → 4CO₂(g)  +  6H₂O(g)

This is an example of combustion reaction.

CBSE Class 10 Science Syllabus 2017-2018

Question 20. Write the balanced chemical equation for the following reactions and identify the type of reaction in each case.

(a) In Thermite reaction, iron (III) oxide reacts with aluminium and gives molten iron and aluminium oxide.

(b) Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium nitride.

(c) Chlorine gas is passed in an aqueous potassium iodide solution to form potassium chloride solution and solid iodine.

(d) Ethanol is burnt in air to form carbon dioxide, water and releases heat.

Answer.

Question 21. Complete the missing components/variables given as X and Y in the following.

 

Question 22. Which among the following changes are exothermic or endothermic in nature?

(a) Decomposition of ferrous sulphate

(b) Dilution of sulphuric acid

(c) Dissolution of sodium hydroxide in water

(d) Dissolution of ammonium chloride in water

Answer.

(a) Decomposition of ferrous sulphate absorbs heat during reaction therefore it is an example of endothermic reaction.

(b) When we dissolve sulphuric acid in water, large amount of heat is liberated so it is an exothermic reaction.

(c) When we dissolve sodium hydroxide in water, large amount of heat is liberated so it is an exothermic reaction.’

(d) Mixing of ammonium chloride in water absorbs heat from reaction mixture therefore it is an example of endothermic reaction.

Question. 23 Identify the reducing agent in the following reactions.

(a) 4NH₃ + 5O₂  →  4NO + 6H₂O

(b) 2H₂O + 2F₂  →  4HF + O₂

(c) Fe₂O₃ + 3CO  →  2Fe + 3CO₂

(d) 2H₂ + O₂  →  2H₂O

Answer.

(a) NH₃ is the reducing agent as it gives hydrogen to O_2.

(b) H₂O is the reducing agent.

(c) CO is a reducing agent.

(d) H₂ is the reducing agent.

Question. 24 Identify the oxidizing agent (oxidant) in the following reactions.

(a) Pb₃O₄  +  8HCl → 3PbCl₂  +  Cl₂  +  4H₂O

(b) 2Mg  +  O₂  →  2MgO

(c) CuSO₄   +  Zn   →  Cu  +  ZnSO₄

(d) V₂O₅  +  5Ca  →  2V   +  5CaO

(e) 3Fe  +  4H₂O  →  Fe₃O₄   +   4H₂

(f) CuO  +   H₂  →  Cu   +   H₂O

Answer.

(a) Since Pb₃O₄ gives oxygen so it is an oxidizing agent (oxidant).

(b) Since O₂ provides oxygen so it is oxidant.

(c) CuSO₄ is the oxidizing agent because it gets reduces to Cu.

(d) V₂O₅ has been reduced to V so it will be the oxidant.

(e) H₂O is the oxidant as it is reduced to H₂.

(f) CuO is the oxidant as it is reduced to Cu.

Question. 25 Write the balanced chemical equations for the following reactions.

(a) Sodium carbonate on reaction with hydrochloric acid in equal molar concentrations gives sodium chloride and sodium hydrogen carbonate.

(b) Sodium hydrogen carbonate on reaction with hydrochloric acid gives sodium chloride, water and liberates carbon dioxide.

(c) Copper sulphate on treatment with potassium iodide precipitates cuprous iodide (Cu₂I₂), liberates iodine gas and also forms potassium sulphate.

Answer.

(a) Na₂CO₃(s)  + HCl (aq) → NaCl (aq)  +  NaHCO₃ (aq)

(b) NaHCO₃ (s)  +  HCl (aq) →  NaCl (aq) + H₂O(l) + CO₂ (g)

(c) 2CuSO₄ (s) + 4KI (aq)  →  Cu₂I₂ (s)  +  I₂ (g) +  2K₂SO₄ (aq)

Question. 26 A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of the chemical reaction.

Answer.

The solution of KCl with AgNO3 form white precipitate of AgCl. This is a double displacement reaction.

KCl (aq)  +  AgNO₃ (aq)  → AgCl (s)  +  KNO₃ (aq)

Question. 27 Ferrous sulphate decomposes with the evolution of a gas having a characteristic odour of burning sulphur. Write the chemical reaction involved and identify the type of reaction.

Answer.

2FeSO₄ (s)  →  Fe₂O₃ (s)   +   SO₂ (g)  + SO₃ (g)

The heating of ferrous sulphate gives ferric oxide and mixture of sulphur dioxide and sulphur trioxide gas. This is an example of thermal decomposition reaction and the odour of burning sulphur is due to SO₂ gas.

Question. 28 Why do fire flies glow at night?

Answer.

Fire flies have a luminous bag which contains a protein; luciferin. The oxidation of luciferin occurs in the presence of luciferare enzyme with emission of light which occurs due to oxidation of magnesium. Hence they glow at night.

Question. 29 Grapes hanging on the plant do not ferment but after being plucked from the plant can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change?

Answer.

After plucking grapes from plants, fermentation of sugar is carried out in the presence of yeast which changes sugar to ethanol and carbon dioxide. This process occurs in the absence of oxygen means in anaerobic conditions.

Whereas grapes attach to plant involve in aerobic respiration and no fermentation can be possible under aerobic conditions.

Here fermentation is a chemical change as it results the formation of new substances; alcohol and carbon dioxide.

Question. 30 Which among the following are physical or chemical changes?

(a) Evaporation of petrol

(b) Burning of Liquefied Petroleum Gas (LPG)

(c) Heating of an iron rod to red hot

(d) Curdling of milk

(e) Sublimation of solid ammonium chloride

Answer.

(a) Physical change

(b) Chemical change

(c) Physical change

(d) Chemical change

(e) Physical change

Question. 31 During the reaction of some metals with dilute hydrochloric acid, following observations were made.

(a) Silver metal does not show any change.

(b) The temperature of the reaction mixture rises when aluminium (Al) is added.

(c) The reaction of sodium metal is found to be highly explosive.

(d) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid Explain these observations giving suitable reasons.

Answer.

(a) The reactivity of silver is very low as it is placed below hydrogen in reactivity series so it does not react with dilute hydrochloric acid.

(b) Aluminium reacts with dilute HCl to form Aluminium chloride with hydrogen gas. It is an exothermic reaction so temperature of reaction mixture rises.

(c) Sodium is an alkali metal which is one of the most reactive metals and readily reacts with dilute HCl to form NaCl and hydrogen gas. The evolution of hydrogen gas cause explosion.

(d) Reaction of lead metal with dilute HCl forms lead (II) chloride and releases hydrogen gas in the form of bubbles. Since reaction is quite slow due to less reactivity of lead, only bubbles of H₂ are seen to evolve.

Question. 32 A substance X, which is an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and also write the chemical reactions involved.

Answer.

The substance X is calcium oxide (CaO) which is also known as quick lime. Reaction of quick lime with water forms calcium hydroxide. It is an alkaline solution and easily turns red litmus to blue.

Question. 33 Write a balanced chemical equation for each of the following reactions and also classify them.

(a) Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution.

(b) A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas.

(c) Iron (III) oxide on heating with carbon monoxide gas reacts to form solid iron and liberates carbon dioxide gas.

(d) Hydrogen sulphide gas reacts with oxygen gas to form solid sulphur and liquid water.

Answer.

(a) Pb(CH3COO)₂ (aq)  +  2HCl (dil)  →  PbCl₂ (s) ↓  +  2CH₃COOH (aq)

This is a double displacement and a precipitation reaction.

(b) 2C₂H₅OH (l)  +  2Na (s)  →  2C₂H₅O⁻Na⁺  +  H₂↑

This is a displacement reaction.

(c) Fe₂O₃ (s)  +  3CO(g)  →  2Fe (s)  +  3CO₂ (g)↑

This is an example of Redox reaction.

(d) 2H₂S (g)  +  O₂ (g)  →  2S (s)  +  2H₂O (l)

 This is an example of Redox reaction.

Question. 34 Why do we store silver chloride in dark coloured bottles?

Answer.

Silver chloride can decompose in the presence of light to form silver metal therefore it must store in dark coloured bottles which can interrupt the path of light and prevent the decomposition of silver chloride.

The photochemical decomposition of silver chloride is shown below;

Question. 35 Balance the following chemical equation and identify chemical reaction the type of chemical reaction.

 

Question. 36 A magnesium ribbon is burnt in oxygen to give a white compound X accompanied by emission of light. If the burning ribbon is now placed in an atmosphere of nitrogen, it continues to burn and forms a compound Y.

(a) Write the chemical formulae of X and Y.

(b) Write a balanced chemical equation, when X is dissolved in water.

Question. 37 Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain, why?

Answer.

Zinc is placed above hydrogen in reactivity series therefore it can replace hydrogen from HCl and form hydrogen gas.

Whereas copper is placed below hydrogen in reactivity series therefore it cannot replace hydrogen from dilute acid and does not release hydrogen gas from dilute hydrochloric acid.

Cu  +  HCl  →  No reaction

Question. 38 A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.

(a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.

(b) Name the black substance formed and gives its chemical formula.

Answer.

(a)    The metal surface of silver article reacts with atmospheric sulphur compounds like hydrogen sulphide and form black layer of sulphides. This phenomenon is called as corrosion or tarnishing of silver.

(b)   The black substance that is formed by the reaction of hydrogen sulphide and atmospheric oxygen is silver sulphide (Ag₂S).

4Ag  +  O₂ + 2H₂S → 2Ag₂S  +  2H₂O

 

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