RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions

RBSE Solutions for Class 8 Science Chapter 4 Chemical Reactions are part of RBSE Solutions for Class 8 Science. Here we have given RBSE Rajasthan Board Solutions for Class 8 Science Chapter 4 Chemical Reactions.

Rajasthan Board Solutions for Class 8 Science Chapter 4 Chemical Reactions

Board	RBSE
Class	Class 8
Subject	Science
Chapter	Chapter 4
Chapter Name	Chemical Reactions
Number of Questions Solved	57
Category	RBSE Solutions

Chemical Reactions Textbook Questions Solved

I. Multiple Choice Questions

Question 1.

1. The following is the example of which type of reaction?
   NH_s (g) + HCl (g) → NH₄Cl (s)
   (a) addition
   (b) displacement
   (c) decomposition
   (d) neutralizaion
2. In oxidation reaction
   (a) oxygen is added
   (b) hydrogen is added
   (c) oxygen is removed
   (d) hydrogen is removed
3. Acid change blue litmus to-
   (a) White
   (b) Red
   (c) Black
   (d) Purple
4. Copper (Cu) + Oxygen (O₂)
   (a) H₂O
   (b) O₂
   (c) CuO
   (d) CuSO₄

Answers:

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

Question 2.
Fill in the blanks:

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

Answers:

1. addition reaction
2. salt and water
3. oxidation
4. an exothermic reaction

Question 3.
Match the column A with column B

Column 1	Column 2
1. Salt and water are formed from acid and base 2.  Liberation of oxygen 3.  Liberation of energy 4.  Removal of hydrogen	(a) Oxidation (b) Neutralization (c) Reduction (d) Exothermic

Answer:

Column 1	Column 2
1. Salt and water are formed from acid and base 2. Liberation of oxygen 3. Liberation of energy 4. Removal of hydrogen	(a) Neutralization (b) Reduction (c) Exothermic (d) Oxidation

Chemical Reactions Short Answer Type Questions

Question 1.
Give one example of displacement reaction?
Answer:
Those reaction in which less reactive element of compound is displaced by more reactive element is called displacement reaction.

Question 2.
Why did lime water turns milky when CO₂ gas is flow?
Answer:
Powder of lime n h t
Calcium carbonate CaCO₃ \(\underrightarrow { On\quad heating } \) Calcium Oxide CaO → Carbon dioxide CO₂
When lime powder is heated a gas is produced when it is allowed to blow in limewater, it turns milky. This gas is CO₂.

Question 3.
Explain addition reaction with example?
Answer:
When two or more than two elements or compounds combines produces a new product (compound) is called an addition reaction.
Iron (Fe) + Oxygen (O₂ from air) → Iron oxide (Fe₃O₄) 3Fe + 2O₂ → Fe3O₄

Question 4.
Explain dissociation reaction with example.
Answer:
Dissociation reaction- Powder of lime
Calcium carbonate \(\underrightarrow { On\quad heating } \) Calcium oxide (CaO) → Carbon dioxide (CO₂ ) When lime powder is heated, gas is produced. When it is allowed to blow in lime water, it turns milky. This gas is CO₂ . A reaction in which a reactant decomposes and forms two or more product is called decomposition reaction.

Question 5.
Write the properties of chemical reaction.
Answer:
Properties of chemical reaction

• Iron reacts with oxygen and moisture present in the air, forms the iron oxide (rust).
• Production of gas: When fuel like petrol, kerosene etc are burnt then carbon present in them reacts with air and forms (CO₂ ) carbon dioxide in the same manner CO₂ is produced when coal is burnt. C + O → CO₂ (gas)
• When sulphuric acid is poured slowly in a test tube in granulated zinc, hydrogen gas is produced. If a burning matchstick is taken at the mouth of the test tube it burns with an explosion.
• Change in colour: Colour of cut apple is changed because iron present in apple reacts with oxygen present in the air and form iron oxide.
• Change of heat: When ammonium chloride is put in water it becomes cold.
• Dissolution: In hard water, soap does not form foams but produces white undissolved substance.

Chemical Reactions Long Answer Type Questions

Question 1.
What are the types of chemical reactions? Explain the four types of chemical reactions with example.
Answer:
The different types of chemical reaction are:

1. Combination reaction
2. Decomposition reaction
3. Displacement reaction
4. Neutralisation reaction
5. Double displacement reaction
6. Oxidation and reduction reaction

1. Combination reaction:
In a combination reaction, two or more elements combine to form a compound or an element and a compound can combine to form a new compound.
For example:
Magnesium and oxygen combine when heated to form magnesium oxide , . .
2Mg(s) + O₂(g) \(\underrightarrow { Combination } \) 2MgO(s)

2. Displacement reaction:
Those reaction in which one element takes the place of another element in a compound, are known as displacement reaction. In general a more reactive element displaces a less reactive element from its compound.
For example:
When a strip of zinc metal is placed in copper sulphate solution, then zinc sulphate solution and copper are obtained:
CuSO₄ (aq) + Zn (s) → ZnSO₄ (aq) + Cu (s)
In this reaction zinc displaces copper from copper sulphate solution.

3. Double displacement reaction:
Those reaction in which two compounds by the exchange of ions to form two new compound are called double displacement reactions. A double displacement reaction usually occurs in solution and one of the products, being insoluble, precipitates out.
For example:
When silver nitrate solution is added to sodium chloride solution, then a white precipitate of silver chloride is formed along with sodium nitrate solution:
AgNO₂ (aq) + NaCl (aq) → AgCl (s) + NaNO₃                                     .
In this double displacement reaction, two compounds, silver nitrate and sodium chloride react to form two new compounds silver chloride and sodium nitrate.

4. Oxidation and reduction reactions:
Oxidation:

1. The addition of oxygen to a substance is called oxidation.
2. The removal of hydrogen from a substance is also called oxidation.

Reduction:

1. The addition of hydrogen to a substance is called reduction.
2. The removal of oxygen from a substance is also called reduction.

From the above definitions it is clear that the process of reduction is just opposite oxidation. Moreover, oxidation and reduction reactions occurs together.

Question 2.
Explain any two characteristics of chemical reaction with experiment.
Answer:
Characteristics of Chemical Reactions:
When a chemical reaction takes place, it is accompained by one or more of the following characteristics:
Evolution of a gas:
A gas may evolve during a chemical reaction, if one of the reactants is a liquid or in solution form. Evolution of gas is confirmed by appearance of effervescence (bubbling) in the liquid. The gas can be identified by testing it. In a chemical equation, the evolution of a gas is shown by putting an arrow pointing upwards
(↑).
Example:
When zinc reacts with dilute sulphuric acid, hydrogen gas is given off. The gas can be tested with the help of burning matchstick when the burning matchstick is taken near the test tube containing hydrogen gas, hydrogen burns with a pop sound.
Zn + H₂SO₄ →  ZnSO₄ + H₂↑
Change of colour:
One of the product formed during a chemical reaction may have a different colour than that of the reactants. In such case, change of colour accompanies the chemical reaction.
Example:
When white crystal of lead nitrate are heated strongly, yellow-coloured lead monoxide is formed. A reddish – brown gas, nitrogen dioxide is also given off.
2Pb (NO₃)₂ → 2PbO + 4NO₂ ↑+ O₂ ↑
Lead nitrate (white) Lead monoxide (yellow) Nitrogen dioxide (reddish brown) oxygen.

Question 3.
Explain the following reactions with example.

1. Neutralization reaction
2. Decomposition reaction
3. Exothermic reaction
4. Endothermic reaction

1. Neutralization reaction:
The reaction in which acid or acidic oxide react with base or basic oxides to form salt and water is called neutralization reactions.

2. 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.
                        ‘
Calcium carbonate Calcium oxide Carbon dioxide

3. Exothermic Reactions:
Reactions accompanied with release of heat are called exothermic reactions. Release of heat is shown by writing ‘+ heat’ on right- hand side of the equation. All burning reactions are exothermic. When carbon bums in air heat is released.
C+ O₂ → CO₂ + heat
4. Endothermic reactions: Reactions accompanied with absorption of heat are called endothermic reactions. Absorption of heat is shown by writing – heat on right- hand side or + heat on the left- hand side of the equation.
Example:
When carbon and sulphur are heated they react to form carbon disulphide. Heat is absorbed in the reaction
C+2S → CS₂ – heat

Chemical Reactions Additional Questions Solved

I. Multiple Choice Questions
Question 1.
A chemical reaction in which heat is given out is called
(a) exothermic reaction
(b) endothermic reaction
(c) decomposition reaction
(d) neutralization reaction

Question 2.
What is the direction of the arrow in the reaction?
(a) towards reactants
(b) towards precipitate
(c) towards gas
(d) towards product

Question 3.
In which type of change, we observe alteration only in specific property of matter but not composition?
(a) chemical change
(b) physical change operator
(c) both physical and chemical change
(d) none of the above

Question 4.
Identify the combination reaction in the following:
(a) NH₃ + HCI → NH₄ CI
(b) NaCI + KOH → NaOH + KCI
(c) 2KCIO₃ → KCI + 3O₂
(d) NH₃ + HCI → NH₄CI

Question 5.
CaO + H₂O → Ca (OH)₂ is
(a) decomposition reaction
(b) combination reaction
(c) dissociation reaction
(d) endothermic reaction

Question 6.
A neutralization reaction is a
(a) decomposition reaction.
(b) displacement reaction.
(c) combination reaction.
(d) double displacement reaction.

Question 7.
In reduction reaction
(a) oxygen combines with the substance
(b) hydrogen reduces from the substance
(c) oxygen reduces from the substance
(d) none of the above

Question 8.
Chemical combination involves:
(a) combination of two atoms.
(b) combination of only same type of elements.
(c) combination of only same type of compounds.
(d) combination of any two or more elements or compounds.

Question 9.
An oxidation reaction involves addition of
(a) oxygen only
(b) hydrogen only
(c) oxygen or removal of hydrogen
(d) hydrogen or removal of oxygen

Question 10 .
Chemical reaction does not involves:
(a) absorption (or) release of energy
(b) forming of new substance (or) products
(c) two (or) more reactant
(d) none of the above

Question 11 .
“Reaction of potassium iodide solution with lead nitrate solution” is the example of:
(a) chemical replacement reaction
(b) neutralisation reaction
(c) precipitation reaction
(d) chemical addition reaction

Question 12.
In a combination reaction how many products are formed?
(a) two only
(b) one or two only
(c) one only
(d) number cannot be specified

Question 13.
A + B – + C + D → A + D – + C + B-.
Identify the above generalised type of reaction.
(a) chemical replacement reaction
(b) chemical double decomposition reaction
(c) chemical displacement reaction
(d) chemical combination reaction

Question 14.
Example of decomposition reaction is
(a) 2SO₂ + O₂ → 2SO₃
(b) 2KCIO₃ → 2KCI + 3O₂
(c) Mg + 2HCI → MgCl2 + H₂
(d) HCI + NaOH → MgCl + H₂O

Question 15.
In the reaction between zinc and
iodine, zinc iodide is formed. What is being oxidized?
(a) zinc ions
(b) iodide ions
(c) zinc atom
(d) Iodine

Answers:

1. (a)
2. (a)
3. (b)
4. (a)
5. (b)
6. (d)
7. (c)
8. (d)
9. (c)
10. (d)
11. (c)
12. (c)
13. (b)
14. (b)
15. (c)

Chemical Reactions Very Short Answer Type Questions

Question 1.
Define reactants and products?
Answer:
Original stibstances participating in a chemical reaction are known as the reactant and the new substance formed are known as the products.

Question 2.
How does rusting of iron occurs?
Answer:
Rusting of iron occurs in the presence of oxygen and water.

Question 3.
What is oxidizing agents?
Answer:
Oxidizing Agents-Substance which oxidize other substances by providing oxygen to them or by removing hydrogen from them.

Question 4.
Why do copper granules become black after heating?
Answer:
Copper granules become black after heating because outer surface of copper granules is covered with black copper oxide.
CuCO₃ → CuO + CO₂

Question 5.
What are catalyst?
Answer:
Catalyst is a substance which does not take part in chemical reaction, but change the rate of a reaction.

Question 6.
Define redox reaction?
Answer:
In a chemical reaction whenever one substance is oxidised, another substance is reduced. This means that oxidation and reduction reaction always occur simultaneously and known as redox reaction.

Chemical Reactions Short Answer Type Questions

Question 1.
Write five chemical changes that you observed in your day to day life.
Answer:
The chemical changes that we observe in our daily life are
(a) rusting of iron
(b) colour change in food items
(c) photosynthesis
(d) burning of paper

Question 2.
Below are given two chemical reactions:

1. 2KBr (aq) + CI₂ (aq) → 2KCI (aq) +Br₂ (aq)
2. Fe (s) + S (s) → FeS (s)

Which is combination reaction and which is displacement reaction?
Answer:

1. In the first reaction, potassium bromide solution reacts with the chlorine solution to form potassium chloride solution and bromine. So, in this reaction, chlorine is displacing bromine from potassium bromide to form potassium chloride and bromine is set free, thus, it is a displacement reaction.
2. In the second reaction, iron combines with sulphur to form iron sulphide. So, it is a combination reaction.

Question 3.
Define chemical reaction and write its characteristics.
Answer:
When one or more substances undergo a complete change with the release of absorption of energy to form one or more new substances called products. It is characterised by one or more characters like evolution of gas, change of colour, formation of precipitate, energy changes and change of state.

Question 4.
The equation below shows the reaction between nitrogen and oxygen.
N₂ (g) + 2O₂ (g) + 66KJ → 2NO₂ (g)
(a) Is this an exothermic or endothermic reaction? Explain your answer.
Answer:
(a) Endothermic reaction. 66 KJ of heat energy is observed in this reaction.

Question 5
A substance X undergo a chemical change involving the loss of hydrogen and this result in the formation of a byproduct. What type of agents are X and Y? (Reducing agent or oxidizing agent).
Answer:
We know that, oxidizing agent undergoes reduction and reducing agent undergoes oxidation. Hence, we need to know whether the substance has undergone oxidation or reduction. As X undergoes a chemical change by losing hydrogen atom, it undergoes oxidation. Hence, it is a reducing agent. To the product Y, hydrogen is added, it undergoes reduction. Hence, Y acts as an oxidising agent.

Question 6.
Differentiate between exothermic and endothermic reactions?
Answer:
Difference between exothermic and endothermic reactions: .
Exothermic reaction

1. Heat is given out during the reaction.
2. Heat liberated is indicated by a positive sign on the product side.
3. An exothermic reaction is represented
   reactants → product + heat
4. Energy of the reactant is greater than the product.

Endothermic reaction

1. Heat is absorbed during the reaction,
2. Heat observed is indicated by negative sign on the product side.
3. An endothermic reaction is represented as
   reactants → product – heat
4. Energy of the reactants is lesser than the products.

Activity and questions based on it.

Activity 1
Take sulphur powder in a China dish. Mix iron granules in it and heat

Question 1.
What do you see?
Answer:
Iron granules and sulphur powder after reaction make a new colour substance iron sulphide.
Iron + sulphur → iron sulphide

Activity 2
Take zinc granules in a test tube and add dilute sulphuric acid in it slowly.

Question 1.
What do you see?
Answer:
Hydrogen gas evolved from test tube. If we place a burning match stick near the opening of test tube, then it burns with a blue flame.

Activity 3
Take a small amount of powdered iron in our deflagrating spoon and heat it. Now transfer it in a jar of oxygen.

Question 1.
What do you see?
Answer:
Powdered iron burns like fireworks and form oxide of iron.
3Fe + 2O₂ → Fe3O₄

Activity 4
Heat the powdered lime. You will see an emerging gas.

Question 1.
If we pass this gas in lime water then what happens?
Answer:
Lime water turns milky.

Question 2.
What is the name of this gas?
Answer:
Carbon dioxide (CO₂).

Question 3.
Write its equation.
Answer:
CaCO₃ → CaO + COa

Activity 5
Take a test tube and add 5 ml solution of CuSO₄. Now put an iron nail (without corrosion) in test tube. Wait for some time.

Question 1.
After observing the test tube, what do you see?
Answer:
After sometime there is deposit of brown layer on Iron nail.

Question 2.
Name the metal of this layer formed.
Answer:
Copper.

Question 3.
How does the colour of the solution changes of test tube in which nail was placed? <
Answer:
Solution turns green from blue.

Question 4.
Write the equation of the reaction.
Answer:
Fe + CuSO₄ → FeSO₄ + Cu

Question 5.
What is the name of this reaction.
Answer:
Displacement reaction.

Activity 6
Question 1.
What do you see?
Answer:
There is black colour layer on the copper powder.

Question 2.
Name the substance of black layer.
Answer:
This black layer is of copper oxide.

Question 3.
Name the type of this reaction.
Answer:
Oxidation reaction.

Activity 7
Take a glass beaker. Add some water in it. Note the temperature of water with the help of thermometer. Now add KNO₃ powder in it. Shake well the beaker and note down the temperature with thermometer.

Question 1.
What is the change in temperature?
Answer:
Temperature lowers down.

Question 2.
What happens on reduction in temperature?
Answer:
Heat absorbs.

Question 3.
Name the type of this reaction.
Answer:
Endothermic reaction.

Chemical Reactions Long Answer Type Questions

Question 1.
Identify the type of chemical reactions.
(a) H₂ + Cl₂ → 2HCl
(b) CaO + H₂O → Ca (OH)₂
(c) CaCO₃ → CaO + CO₂
(d) FeSO₄ + Mg → MgSO₄ + Fe
(e) 2KI + Pb (NO₃)2 → 2KNO₃ + Pbl₂
Answer:
Reaction:
(a) H₂ + Cl₂ → 2HCl
Explanation:
Two elements H₂ and Cl₂ combine to form a single compound, HCl. Hence, it is a combination reaction. As two elements combine to form a compound, it is an element – element combination.
(b) CaO + H₂O → Ca(OH)₂
Explanation:
Two compound CaO and HaO combine to form a single compound, Ca (OH)₂. Hence, it is a combination reaction. As two compounds combine to form a new compound, it is a compound – compound combination.
(c) CaCO₂ → CaO + CO₂
Explanation:
The compound CaCO₃ splits (decompose) into two compounds, CaO and
COO. Hence, it is a compound-com¬pound combination.
(d) FeSO₄ + Mg → MgSO₄ + Fe
Explanation:
Here magnesium (Mg) is displacing, iron (Fe) from its salt solution (FeSO₄). Hence, it is a displacement reaction.
(e) 2KI + Pb (NO₃)₂ → 2KNO₃ + Pbl₂
Explanation:
The reactants potassium iodide and lead nitrate react to form potassium nitrate and lead iodide. From this it is clear that, radicals are intercharged by the decomposition of reactants. Hence, it is a double decomposition reaction.

Question 2.
What is reduction also explain reducing agent?
Answer:
Reduction is a chemical reaction that involves addition of hydrogen or removal of oxygen from a substance. When hydrogen is passed over heated black copper oxide, the latter changes to reddish-brown copper metal. Here copper oxide is said to be reduced to copper by loss of oxygen.

Chlorine reacts with hydrogen in the presence of diffused light to form hydrogen chloride. Chlorine is said to be reduced to hydrogen chloride by gain of hydrogen.

A reducing agent is one that can reduce other substances by removing oxygen from them or by supplying hydrogen to them. In the reaction between copper oxide an$i hydrogen, hydrogen is the reducing agent as it removes oxygen from copper oxide. In the reaction between chlorine and hydrogen, hydrogen is the reducing agent as it supplies hydrogen to chlorine.

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