Chemical Reactions ICSE Class-8th Concise Selina Chemistry Solutions

Chemical Reactions ICSE Class-8th Concise Selina Chemistry Solutions Chapter-6. We Provide Step by Step Answer of Objective, True False , Fill in the blanks, Match the following , Short / Long Answer Type of Exercise-6 Chemical Reactions. Visit official Website CISCE for detail information about ICSE Board Class-8.


Chemical Reactions ICSE Class-8th Concise Selina Chemistry Solutions


Exercise – I

Question 1.

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(a) Define a chemical reaction.
(b) What happens during a chemical reaction ?
(c) What do you understand by a chemical bond ?
Answer:
(a) Any chemical change in matter which involves transformation into one or more substances with entirely different properties is called a chemical reaction.
(b) A chemical reaction involves breaking of chemical bonds between the atoms or groups of atoms of reacting substances and rearrangement of atoms making new bonds to form new substances.

(c) A chemical bond is the attractive force that holds the atoms of a molecule together, in a compound.

Question 2.

Give one example each of which illustrates the following characteristics of a chemical reaction:
(a) evolution of a gas
(b) change of colour
(c) change in state
Answer:
(a) When Zinc reacts with dil. sulphuric acid. Hydrogen gas is evolved, with an effervescence

Ans 2 Chemical Reactions ICSE

(b) When blue coloure d copper sulphate reacts with hydrogen sulphide gas, a black coloured substance copper sulphide is formed.

(c) The reaction between hydrogen sulphide and chlorine (both gases) produces sulphur (solid) and hydrogen chloride (gas).

Ans 2 Chemical Reactions ICSE

Question 3.

How do the following help in bringing about a chemical change?
(a) pressure (b) light
(c) catalyst (d) heat.
Answer:
(a) Some chemical reactions take place when reactants are subjected to high pressure.
e.g: Nitrogen and hydrogen when subjected to high pressure produce ammonia gas.

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(b) Some chemical reactions can take place in the presence of light. Ex. Photosynthesis.

 

(c) A catalyst can either increases or decreases the rate of chemical reaction and some chemical reactions need a catalyst to change the rate of the reaction, in case it is too slow or too fast.

  1. Positive catalyst: When a catalyst increase the rate of reaction finely divided iron is used as a positive catalyst in the manufacturing of ammonia from hydrogen and nitrogen.

 

  1. Negative Catalyst: When a catalyst decreases the rate of reaction.
    Ex. Phosphoric acid act as a negative catalyst to decrease the rate of the decomposition of hydrogen peroxide.

(d) Some chemical reactions take place only in the presence of heat.
e.g. When lead nitrate is heated, it breaks into lead monoxide, nitrogen dioxide and oxygen.

Ans 3 Chemical Reactions ICSE

Question 4.

(a) Define catalyst.
(b) What are (i) positive catalysts and (ii) negative catalysts? Support your answer with one example for each of them.
(c) Name three biochemical catalysts found in the human body.
Answer:
(a) Catalyst: A catalyst is a substance that either increases or decreases the rate of a chemical reaction without itself undergoing any chemical change during the reaction.

(b) (i) Positive catalyst: When a catalyst increases the rate of chemical reaction, it is called positive catalyst.
e.g. when potassium chlorate heated to 700°C decomposes to evolve oxygen gas, when MnO2 is added the decomposition takes place at 300°C

Ans 4 Chemical Reactions ICSE

(ii) Negative catalyst: When a catalyst decreases the rate of chemical reaction it is called negative catalyst.
Example. Phosphoric acid acts as a negative catalyst to decrease the rate of the decomposition of hydrogen peroxide. Alcohol too acts as a negative catalyst in certain chemical reactions.

(c) Biochemical catalysts found in human body:

  1. Pepsin
  2. Tryspin
  3. lipase.

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Question 5.

What do you observe when
(a) dilute sulphuric acid is added to granulated zinc?
(b) a few pieces of iron are dropped in a blue solution of copper sulphate?
(c) silver nitrate is added to a solution of sodium chloride?

(d) ferrous sulphate solution is added to an aqueous solution of sodium hydroxide.
(e) solid lead nitrate is heated?
(f) when dilute sulphuric acid is added to barium chloride solution ?
Answer:
(a) When Zinc reacts with dilute sulphuric acid, hydrogen gas is evolved with an effervesence.
Zn + dil. H2SO4 → Zn SO4 + H2.

(b) When a few pieces of iron are dropped into a blue coloured copper sulphate solution, the blue colour of the solution fades and eventually turns into green.

(c) When a solution of silver nitrate is added to a solution of sodium chloride, white insoluble ppt. of silver chloride is formed.
AgNO3 (aq) + NaCl (aq) → AgCl (ppt) + NaNO3 (aq)

(d) When ferrous sulphate solution is added to sodium hydroxide solution, a dirty green ppt. of ferrous hydroxide is formed.
FeSO4 (aq) + 2NaOH (aq) → Fe(OH)2 ↓ + Na2SO4(aq)

(e) When solid lead nitrate is heated, it decomposes to produce light yellow solid lead monoxide, reddish brown nitrogen dioxide gas and colourless oxygen gas.

Ans 5 Chemical Reactions ICSE

Question 6.

Complete and balance the following chemical equations:

………….

Answer:

Ans 6 Chemical Reactions ICSE


Exercise – II

Question 1.

1. Fill in the blanks.
(a) A reaction in which two or more substances combine to form a single substance is called a combination reaction.
(b) A catalyst is a substance which changes the rate of a chemical reaction without undergoing a chemical change.
(c) The formation of gas bubbles in a liquid during a reaction is called effervesence
(d) The reaction between an acid and a base is called neutralization reaction.
(e) Soluble bases are called alkalis.
(f) The chemical change involving iron and hydrochloric acid illustrates a displacement reaction.
(g) In the type of reaction called double decomposition reaction, ions two compounds exchange their positive and negative radicals ions respectively.
(h) A catalyst either increases or decreases the rate of a chemical change

but itself remains unchanged at the end of the reaction.
(i) The chemical reaction between hydrogen and chlorine is a combinaton reaction
(j) When a piece of copper is added to silver nitrate solution, it turns blue in colour.

Question 2.

Classify the following reactions as combination, decomposition, displacement, precipitation and neutralization. Also balance the equations.

Ans 2 Chemical Reactions ICSE

Question 3.

Define:
(a) precipitation (b) neutralization (c) catalyst
Answer:
(a) Precipitation: A chemical reaction in which two compounds in their aqueous state react to form an insoluble salt as one of the product.
Acid + Base → Salt + Water
Example.

Ans 3 Chemical Reactions ICSE

 

here iron act as a catalyst and increases the rate of chemical reaction.

Question 4.

Explain the following types of chemical reactions giving two examples for each of them.
(a) combination reaction
(b) decomposition reaction
(c) displacement reaction
(d) double decomposition reaction
Answer:
(a) Combination reaction: A reaction in which two or more substances combine to form a single substance is called combination reaction.
A + B → AB
e.g (i) When iron and sulphur are heated together, they combine to form iron sulphide.

Ans 4 Chemical Reactions ICSE

c) Displacement reaction: A reaction in which a more active element displaces a less active element from a compound is called displacement reaction.
AB + C → CB + A
e.g. (i) Zinc, displaces copper from copper sulphate solution.
Zn + CuSO4 (aq) → ZnSO4 (aq) + Cu
(ii) Iron piece when added to copper sulphate solution, copper is displaced.
Fe + CuSO4 → FeSO4 + Cu.

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