RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions

Rajasthan Board RBSE Class 8 Science Chapter 4 Chemical Reactions

RBSE Solutions for Class 8 Science

RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions

RBSE Class 8 Science Chemical Reactions Intext Questions and Answers

Page No 38.

Question 1.
Why the bubbling of soda wafer on squeezing lemon juice.
Answer:
This is because sodawaler contains base which reacts with acids lo produce gas which evolves in the form of hubbies.

Question 2.
Why the colour of peeled apple becomes brownish after some time ?
Answer:
Apple contains iron which reacts with oxygen [O2] of air to form a brownish colour substance iron oxide [EeO].

Question 3.
Why the bubbles are formed on adding water in lime at the time of white washing?
Answer:
On adding water to lime carbon dioxide [CO2] gas evolves out which comes out from water in the form of bubbles.

Page No 39.

Question 1.
What will happen when we iron roti tawa in open and humid environment ?
Answer:
The iron articles absorbs oxygen and water from air and reacts and forms a new substance named iron oxide (FeO) i.e. rust. It is a chemical reaction also.

RBSE Class 8 Science Chemical Reactions Text Book Questions and Answers

Choose the correct options:

Question 1.
The following is an example of which type of reaction.
NH3(g) + HCl(g) → NH4 Cl(s)
(a) Addition
(b) Displacement
(c) Decomposition
(d) Neutralisation
Answer:
(a) Addition

Question 2.
in oxidation reaction.
(a) oxygen is added
(b) hydrogen is added
(c) oxygen is removed
(d) Hydrogen is removed
Answer:
(a) oxygen is added

Question 3.
Acid change blue litmus to.
(a) White
(b) Red
(c) Black
(d) Purple
Answer:
(b) Red

RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions

Question 4.
Copper (Cu) + oxygen (O2) →
(a) H2O
(b) O2
(c) CuO
(d) CuSO4
Answer:
(c) CuO

Fill in the blanks:

  1. Dissociation reaction is opposite of ……………. reaction.
  2. On adding fixed volume of acid and bases ……………. and are obtained.
  3. Addition of oxygen is known as …………….
  4. The reaction in which energy is liberated ……………. is called .

Answer:

  1. Addition
  2. Salt, water
  3. Oxidation
  4. Exothermic

Match the following columns:
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-1
Answer:

  1. (b)
  2. (c)
  3. (d)
  4. (a)

Short answer type questions:

Question 1.
Write an example of displacement reaction.
Answer:
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-2

Question 2.
Why lime water turns milky when carbon dioxide gas pass through it ?
Answer:
On passing carbon dioxide into lime water it turns milky due to the fòrmation of calcium carbonate (CaCO3).
Ca (OH)2 + CO2 → CaCO3 + H2O

Question 3.
Explain addition reaction with example.
Answer:
When two or more than two elements or compounds combine together to form a new products then such type of reaction is called addition or combination reaction. Example – Take some amount of iron powder in a deflagrating spoon. Heat it up to red hot. Being this spoon into a oxygen jar. Iron powder burned with sparkling fire and form iron oxide.
Iron (Fe) + Oxygen (O2) → Iron Oxide (FeO)

RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions

Question 4.
Explain displacement reaction with example.
Answer:
A reaction in which a single reactant decomposes or break down to give simpler products is called dissociation reaction. Example – On heating calcium carbonate (CaCO3) it gives calcium oxide and carbon dioxide gas is evolved out.
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-3

Question 5.
Write properties of chemical Reaction.
Answer:
Properties of Chemical reaction
The properties of chemical reaction are:

  1. Evolution of a gas.
  2. Change in colour.
  3. Change in temperature.
  4. Change in slate.

Long answer type Questions:

Question 1.
How many types of chemical reactions are there ? Explain any four with example.
Answer:
Kinds of Chemical Reactions
1. Addition or Combination Reaction:
When two or more reactants combine to form a single product is known as addition reaction.
3Fe + 2O2 → Fe3O4

2. Dissociation or Decomposition Reaction:
The reactions in which a single reactant breaks down to give simpler products is called a decomposition reaction.
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-4

3. Displacement Reaction:
The reaction in which less reactive elements of compound are displaced by highly reactive elements is known as the Displacement Reaction.
Fe + CuSO4 → FeSO4 + CO

4. Redox Reactions
(a) Oxidation Reaction:
The reaction in which the substance gains oxygen is called oxidation reaction.
2Mg + O2 → 2MgO

(b) Reduction Reaction:
The reaction in which substance loses oxygen is called reduction reaction.
CuO + H2 → Cu + H2O

(d) Redox Reaction :
The reaction in which a reactant gets oxidised while the other gets reduced is called red-ox or oxidation reduction reactions.
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-5

Question 2.
Explain with experiment any two properties of chemical reaction.
Answer:
1. Evolution of Gas:
When zinc granules are treated with dilute solution of sulfuric acid, hydrogen gas is evolved in a test tube. It can be detected by taking burnt match stick to the mouth of the test tube. It burns with blue lames.

2. Change in Colour:
When apple is cut by a knife, its colour changes after sometime to brown because iron present in apple combines with oxygen in atmosphere and forms ferrous oxide.

Question 3.
Explain with examples.

  1. Neutralization Reaction
  2. Dissociation Reaction
  3. Exothermic Reaction
  4. Endothermic Reaction.

Answer:
1. Neutralization Reaction:
The reaction in which acid or acidic oxide reacts with base or basic oxides to form salt and water is called neutralization reactions.
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-6

2. Dissociation or Decomposition Reaction:
The reaction in which a single reactant breaks into two or more simpler products is called decomposition reaction. Example : When powdered lime is heated, a gas is evolved. This gas is passed through lime water, lime water turns milky. This gas is carbon-dioxide (CO2) gas. When calcium carbonate is heated. it forms carbon dioxide gas.
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-7

3. Exothermic Reaction:
The reactions in which heat releases with the formation of products is known as exothermic reactions.

Example:
Methane gas burns with atmospheric oxygen to form carbon dioxide and water vapors with release of heat energy.
CH2 + 2O2 → CO2 + 2H2O + Heat energy

4. Endothermic Reaction:
The reactions in which heat is absorbed with the formation of products is called endothermic reaction.

Example:
Hydrogen and iodine combines to form hydrogen iodine with the absorption of heat energy.
H2 + I2 → 2HI + Heat energy

RBSE Class 8 Science Chemical Reactions Important Questions and Answers

Objective Questions:

Question 1.
The gas evolved out when zinc reacts with dil H2SO4 is …………….
(a) O2 (Oxygen)
(b) CO2 (Carbon dioxide)
(c) H2 (Hydrogen)
(d) SO2 (Sulphur dioxide)
Answer:
(c) H2 (Hydrogen)

Question 2.
A base changes red litmus to ………….
(a) Black
(b) White
(c) Blue
(d) Purple
Answer:
(c) Blue

Question 3.
The reaction of NH4CI with water is –
(a) Exothermic
(b) Endothermic
(c) Neutralization
(d) Decomposition
Answer:
(c) Neutralization

RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions

Question 4.
The gas evolved out when lime stone (CaCO3) is heated is ……………
(a) CO2
(b) O2
(c) H2
(d) SO2
Answer:
(a) CO2

Quetion 5.
The colour of a peeled apple turns brown due to The presence of in apple.
(a) Calcium
(b) Zinc
(c) Iron
(d) Iodine
Answer:
(c) Iron

Question 6.
What is the direction of the arrow of the reaction?
(a) Towards reactants
(b) Towards products
(c) Towards gas
(d) None of these
Answer:
(b) Towards products

Question 7.
CaO + H2O → Ca(OH)2 is an example of …………..
(a) Decomposition reaction
(b) Combination reaction
(c) Endothermic reaction
(d) None of these
Answer:
(b) Combination reaction

Fill in the blanks:

  1. If we bring a burning matchstick near the mouth of a test tube filled with hydrogen. it burns with a ………………. flame. (blue/red)
  2. The solution of sodium hydroxide (NaOH) in water is an examples of ………………. reaction. (Exothermic/Endothermic)
  3. ………………. bonds are formed and braked between the substances during a chemical reaction. (Physical/Chemical)
  4. The lime water turns milky a passing ………………. gas in it. (Oxygen/Carbon dioxide)
  5. The substances which takes part in chemical reaction are called . (products/react ants)
  6. Iron reacts with oxygen to form ………………. (Ironoxide/Irondioxide)
  7. Colour of upper oxide is ………………. (red/black)
  8. Acids and bases reacts to form and ………………. (water, salt/acid, base)

Answer:

  1. Blue
  2. Enothermic
  3. Chemical
  4. Carbon dioxide
  5. reactants
  6. Iron oxide
  7. Black
  8. Water, Salt.

State whether the following are True or False:

  1. On burning titels CO2 gas is evolved out. ( )
  2. The digestion of food is an combination reaction. ( )
  3. CuO + H2 → Cu + H2O is an oxidation reaction. ( )
  4. The products formed in neutralisation reaction are salt and water. ( )

Answer:

  1. True
  2. False
  3. False
  4. True

Match Column a With Column B:

RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-8

Answer:

  1. (b)
  2. (a)
  3. (d)
  4. (c)
  5. (f)
  6. (e)

Very Short Answer Type Questions:

Question 1.
How can we represent a chemical reaction?
Answer:
A chemical reaction is represented in the form of chemical equation. In a chemical reaction reaclarns are written on the left hand side of the arrow (→) while products are written on the right hand side of the arrow.
i.e. A + B → C + D
Here, A & B are reactants and
C & D are products.

Question 2.
Write any two important properties of a chemical reaction?
Answer:

  1. Evolution of gas
  2. Change in colour.

Question 3.
What is Precipitation ?
Answer:
A reaction in which an insoluble substance is formed is called a precipitation reaction.

Question 4.
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-9
Which type of reaction is this ?
Answer:
This reaction is combination as well as oxidation reaction.

Question 5.
What is meant by chemical reaction?
Answer:
The making or breaking of an chemical band is called a chemical reaction.

Question 6.
Name the type of reaction in which the reactants breaks to form two or more than two products.
Answer:
Dissociation or Decomposition reaction.

RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions

Question 7.
Digestion of food is which type of reaction?
Answer:
It is a decomposition reaction in which the large molecules of food breaks down into simpler and smaller molecules, It is also an exothermic reaction.

Question 8.
Which type of reaction is photosynthesis?
Answer:
It is a combination reaction in which green plants prepare their food in the form of carbohydrates using carbon dioxide and water. It is also an endothermic reaction.

Question 9.
What is the colour of rusting of iron?
Answer:
Brown.

Question 10.
What happens when iron granules react with sulphur on heating ?
Answer:
Ferrous sulphyde is formed.

Question 11.
Which gas is formed by burning of coal?
Answer:
Carbon dioxide
C + O2 → CO2

Question 12.
What is the colour of apple after cutting with knife ?
Answer:
Brown.

Question 13.
The reaction Cl2 + 2KI → 2KCI + I2 is what type of reaction ?
Answer:
Displacement reaction.

Question 14.
What happens when iron granules is heated ?
Answer:
Iron granules forms ferrous oxide. (Fe3O4)

Question 15.
Which gas is evolved, when powdered lime is heated ?
Answer:
Carbon dioxide (CO2).

Question 16.
Write formula of calcium carbonate.
Answer:
CaCO3.

RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions

Question 17.
Name the reaction when copper reacts with atmospheric oxygen and forms copper oxide.
Answer:
Oxidation.

Question 18.
Give the name of the reaction – “When one substance is oxidised and another is reduced” ?
Answer:
Redox reaction.

Question 19.
Name the product formed in Neutralization reaction.
Answer:
Salt and water.

Question 20.
Write the formula of Saltpetre (Potassium nitrate).
Answer:
KNO3.

Question 21.
What happens to iron tawa in humid atmosphere ?
Answer:
Iron tawa gets corroded.

Question 22.
What happen to copper granules after heating ?
Answer:
Copper granules become black.

Question 23.
Why do copper granules become black?
Answer:
Because outer surface of copper granules is covered with black copper oxide.

Question 24.
What happens when iron granules react with dilute hydrochloric acid ?
Answer:
Hydrogen gas and iron chloride is formed.
Fe + 2HCl → FeCI2 + H2

Question 25.
Reacting with oxygen carbon elements form carbon dioxide gas, write the equation.
Answer:
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-10

Short Answer Type Questions:

Question 1.
What is meant by chemical reaction? Explain.
Answer:
Chemical Reaction – The process in which the chemical composition and properties of substances changes are called chemical reaction. During chemical reactions the bonds between the substances breaks or formed.

Question 2.
Explain the type of chemical reaction in the following –
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-11
Answer:
(a) This is both oxidation and combination or addition reation. In this oxygen is combining and a new compound is formed.

(b) This is a decomposition or dissociation reaction. In this water molecule dissociates into oxygen and hydrogen.

(c) This is both combination and oxidation reaction.

(d) This is both combination and reduction reaction. Here hydrogen is combining.

RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions

Question 3.
Differentiate between combination and decomposition reaction with one example each.
Answer:
Difference between combination and decomposition reaction.
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-12

Question 4.
What happens when copper powder is heated ¡n a China dish ? Explain. Is this reaction oxidation or reduction.
Answer:
On heating copper powder in a china dish, we will see that a black layer is deposited over copper powder. This black compound is copper oxide, which is formed by the combination of copper and oxygen.
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-13
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-14
In this reaction copper is oxidised into copper oxide. Thus it is an oxidation reaction.

Question 5.
On what bases the occurance of chemical reaction can be identified ? Explain.
Answer:
We can identify the occurance of a chemical reaction on the ‘basis of following properties. Existence of these properties identify the going on of a chemical reaction.

  1. Evolution of a gas.
  2. Change in colour.
  3. Change in temperature.
  4. Change in state.

Question 6.
Why the colour of CuSO4 solu. change when an iron nail ¡s kept In It?
Answer:
When iron nail is dipped in copper sulphate solution then the reaction given below is happened and thus the colour of the solution changed. Iron is more reactive than copper.
Hence this reaction takes place :
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-15
In this reaction the colour of the iron nail becomes brown and the blue colour of the copper sulphate solution faded.

Question 7.
What do you meant by precipitation reaction? Explain with an example.
Answer:
The reactions in which two substances combine together to form an insoluble substance which settles down at the bottom of the solution called precipitate is known
as precipitation reaction. Example – When soap is disolved in hand water, then it does not form foam hut it forms a sticky white precipitate.

Question 8.
Explain the following terms on the basis of oxygen added or reduced with two examples each.

  1. Oxidation
  2. Reduction.

Answer:
1. Oxidation:
The process or reactions in which oxygen increases or ¡s added are called oxidation reactions.
Example:
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-16

2. Reduction:
The process or reactions in which oxygen decreases or is reduced are called reduction reactions.
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-17

Question 9.
What will happen on passing hydrogen gas over hot copper oxide kept in a China dish? Which type of reaction is this?
Answer:
On passing hydrogen gas over hot copper oxide the surface of copper dxidc becomes brown because copper oxide is reduced to copper.
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-18
Those reactions in which oxygen is removed and hydrogen is added to a substance are called reduction reaction. Thus it is a reduction reaction.

Question 10.
What is meant by reactants and products?
Answer:
The substances that participate and undergo chemical change in the reaction are called reactants and the new substance formed after the reaction is called product.

Question 11.
Explain the combination reaction with experiment.
Answer:

  1. Take small amount of iron granules is a spoon.
  2. Heat till it becomes red.
  3. Take this in a jar full of oxygen.
  4. We observe that iron granules burnt as sprinkle and form ferrous oxide (Fe3O4).

RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-19
When two or more elements or reactants combine to form a new single product, these reactions are known as combination reactions.

Question 12.
Give some examples of chemical reactions.
Answer:

  1. Cooking of food.
  2. Respiration.
  3. Digestion of food by body.
  4. In summer, milk turns sour at room temperature.

Question 13.
What is neutralization reaction ? Explain with example.
Answer:
The reaction in which acid or acidic oxide react with base or basic oxides in definite quantity and volume to form salt and water with release of heat is called neutralization reaction.

Example:
Hydrochloric acid reacts with sodium hydroxide and forms sodium chloride, water and releases heat.
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-20

Long Answer Type Questions:

Question 1.
What do you understand with chemical reaction? Differentiate between oxidation reaction and reduction reaction. Wnte one – one example of each reaction. Draw a labelled diagram of oxidation reaction.
Answer:
1. Chemical reaction – The process in which substances changes their chemical composit ion and properties are Known as chemical reaction.

The substances which are reacting together are called reactants while which are formed through their reaction is called products. During chemical reactions bonds between substances are formed or break.

2. Difference between oxidation and reduction reactions

Oxidation:

  1. Those reactions in which oxygen is added and hydro gen is removed from a substance are called oxidation reaction.
  2. In this reaction electrons are released.

Reduction:

  1. Those reactions in which oxygen is removed and hydrogen is added in a substance are called reduction reactions.
  2. In this reaction electrons are absorbed.

3. Example:
Oxidation reaction –
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-21

Example:
Reduction reaction –
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-22

4. Diagram – See in the as of next question.

Question 2.
Draw a diagram to explain the oxidation and reduction reactions. What type of a reaction takes place in rusting of iron?
Answer:
1. In oxidation reaction oxygen is added to any substance and hydrogen is removed. While in reduction reaction oxygen is removed and hydrogen is added in any substance.

2. Activity – Take copper power in a china dish and heat. After some time we will observe a blackish layer on the surface of copper powder. This layer is of copper oxide, which is formed by the reaction of copper and oxygen. In this reaction copper is oxidized to copper oxide. In the following reaction hydrogen is liberated out
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-23
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-24

3. Rusting of iron is an oxidation reaction.

Question 3.
Differentiate the endothermic and exothermic reactions. Explain wit h experiment. What type of reaction takes place when we drop sodium hydroxide into water?
Answer:
1. Endothermic reactions – The reaction in which energy is absorbed is endothermic reaction. Exothermic reactions – The reaction in which energy is liberated is called exothermic reaction.

2. Endothermic reaction – Activity – Take a glass beaker and add little water into it. Note the temperature with the help of thermometer. Now add powder of potassium nitrate to it. Stir and again note the temperature.
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-25
We will notice a change in temperature. The reason for decrease in temperature is absorption of heat during reaction. The reaction in which heat is absorbed are called endothermic react ion.

3. When we drop sodium hydroxide into water, then the temperature of water increases. Thus ii is a exothermic reaction.

Question 4.
What are redox reactions? Draw a diagram to show that during a chemical reaction the chemical bonding between the substances is either formed or break. Why green mehandi becomes red after drying.
Answer:
1. Redox reaction – The reactions in which oxidation and reduction take place together are called redox reaction or oxy – reduction reaction.

2. (a) Formation of bond
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-26

(b) Breaking of bond
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-27

3. Mehandi contains a substance named Lasone which reacts with air and sunlight to form a red colourd compound. Therefore green mehandi becomes red after drying.

Question 5.
What are decomposition reactions? Explain decomposition reaction taking an example diagrammatically. Write the chemical equation of this chemical reaction also.
(OR)
What is dissociation or decomposition reaction? Why lime water becomes milky white when exposed by the gas which is liberated by heating calcium carbonate? dissociation reaction.
Answer:
1. Decomposition reaction – The reaction in which reactant breaks in two or more products are called decomposition or dissociation reactions.

2. Activity – Thermal decomposition of calcium carbonate. Heat calcium carbonate. When it gets heated a gas starts liberating from it lime water. which
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-28
get milky while when exposed lo this gas. This gas is carbon dioxide. When we heat calcium carbonate, calcium oxide and carbon dioxide are formed.

3. Chemical equation of reaction:
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-29
It is called thermal decomposition reaction as heat energy is used in it. Reason for lime water becomes milky – [Note – For this see Answer 2 of Short Answers of Text Book questions.]

RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions

Question 6.
What is addition reaction? Explain addition reaction taking an example diagrammatically. Also write chemical equation of this chemical reaction.
Answer:
1. Addition reaction – When two or more elements or compounds reacts to form new compounds (products). this reaction is called Addition Reaction.

2. Activity – Take iron dust in an ignition spoon. Heat it till red hot. Now take this spoon in a gas jar filled with oxygen. Iron dust burn with sparkling flames and form ferric oxide.
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-30
3. Chemical equation of the reaction –
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-31

Question 7.
Define oxidation and reduction. Explain with an example that oxidation and reduction reactions occur simultaneously.
Answer:
Oxidation:
The reaction in which addition of oxygen and removal of hydrogen lakes place is called oxidation reaction.

Reduction: The reaction in which addition of hydrogen and removal of oxygen takes place is called reduction reaction.
Occurance of oxidation and reduction reactions simultaneously. This fact can he understood using following
example:
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-32
Here, hydrogen is reducing agent which reduces cupric oxide to copper and itself oxidised to water. Thus oxidation and reduction, both reactions occur simultaneously and are opposite to each other. Such reactions in which oxidation and reduction occur simultaneously are called ‘redox’ reactions.

Question 8.
CuO + H2 → Cu + H2O Identify with reason.

  1. Reducing agent
  2. Oxidising agent
  3. Reduced substance
  4. Oxidised substance.

Answer:

  1. Copper is the reducing agent because oxygen gets reduced.
  2. H, is oxidising agcnt because it gains oxygen.
  3. CuO is reduced substance because it breaks and reduces oxygen.
  4. H2O is oxidised substance because oxygen is added.

Question 9.
Identify oxidising agent and reducing agent in the following reaction.

  1. 4Na (s) + O2(g) → 2Na2O(s)
  2. CuO(s) + H2 (g) → Cu(s) + H2O(I)

Answer:
1. 4Na (s) + O2(g) → 2Na2O(s)
In this reaction, sodium is oxidised. This means tha oxidation reaction occurs.

2. CuO(s) + H2 (g) → Cu(s) + H2O(I)
In this reaction, copper ¡s reduced by oxygen and hydrogen is oxidised. Therefore, reduction reaction occurs.

Question 10.
Why decomposition reaction is opposite to combination reaction ? Explain with chemical equations.
Answer:
In combination reaction. two or more elements combine Lo form new single product.
Example:
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-33
In decomposition reaction, a single compound breaks to form two or more simpler products. Example : Calcium carbonate on heating gives calcium oxide and carbon dioxide.
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-34
Therefore, we can say that combination reactions are opposite to decomposition reactions.

Question 11.
Explain displacement reactions with the help of an experiment.
Answer:
Displacement Reactions:
Those reactions in which a more reactive element displaces a less reactive element from a compound are called displacement reactions.
Experiment:
1. Take 5 ml copper sulphate solution in a test – tube.

2. Put fresh iron nail in this solution and leave it for some time.

3. After sometime, we observe that a brown layer deposits on the surface of the iron nail. This layer is copper. The colour of solution changes blue to green. In this solution, copper is replaced by iron, as iron goes to the solution and copper deposits on the iron nail. Therefore, more reactive element displaces less reactive element. Iron is more reactive than copper, so iron displaces copper.
RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions img-35

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