KSEEB Class 8 Science Important Questions Chapter 11 Chemical Reactions and their Types

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Karnataka State Syllabus Class 8 Science Important Questions Chapter 11 Chemical Reactions and their Types

Question 1.
How are changes around us broadly classified?
Answer:
The various changes around us are broadly classified as

• physical changes and
• chemical changes.

Question 2.
What is a physical change?
Answer:
A physical change is a temporary change in which no new substance is formed and the composition of the substance is not altered although certain specific physical properties may be changed.

Question 3.
What are the characteristics of physical change?
Answer:

1. It is temporary and is easily reversible.
2. No new substance is formed during the change.
3. No alteration in mass takes place.
4. There is usually no change or a small change in energy during a physical change.

Question 4.
What is a chemical change?
Answer:
A chemical change is a permanent change in which a new chemical substance is formed with different properties than the substances which undergo chemical change.

Question 5.
What are the characteristics of chemical change?
Answer:

1. It is permanent but sometimes reversible (may or may not be).
2. New substances are formed.
3. Weight of the substance is altered.
4. Energy is generally involved in a chemical change.

Question 6.
Give some examples of chemical changes commonly happening around us.
Answer:
Examples of chemical changes include burning of fuels, rusting of iron, curding of milk, rotting of leaves, respiration, etc.

Question 7.
Write any three differences between physical change and chemical change.
Answer:

Physical change	Chemical change
No new substance is formed during a physical change.	One or more new substances are formed during a chemical change.
Most physical changes are reversible.	Most chemical changes are irreversible
Composition of substances involved does not change.	Composition of substances involved will change during chemical changes.

Question 8.
Classify the following as chemical changes and physical changes

1. rusting of iron
2. ripening of fruits
3. magnetizing a steel plate
4. melting of iron
5. combustion of LPG
6. cooling effect produced by ice. Give justification for your answer.

Answer:

1. Rusting of iron: This is a chemical change because iron oxide, a new substance, is formed during this change.
2. Ripening of fruits: This is a chemical change. Many new substances are formed during the process. Raw fruit is not so sweet but ripened fruits become sweeter.
3. Magnetizing a steel plate: This is a physical change. During magnetization, no new substances are formed. Simply the atoms are realigned during magnetization.
4. Melting of iron: This is a physical change. During melting, iron changes from solid to liquid state. This change does not involve the formation of any new substance.
5. Combustion of LPG: This is a chemical change. During combustion of LPG, carbon dioxide is formed.
6. Cooling effect of ice: This is a physical change. No new substance is formed when an object is cooled by placing ice over it.

Question 9.
Group the following into physical change and chemical change:
Answer:
Dissolving salt in H₂O, burning of a fuel, rusting of iron, melting of ice cube.

• Physical changes: Melting of ice cube, dissolving of salt in water.
• Chemical changes: Burning of a fuel, rusting of iron.

Question 10.
Why do chemical reactions occur?
Answer:
Chemical reactions occur so that atoms gain stability. Atoms and molecules have a tendency to attain stable electronic arrangements. This makes them to participate in chemical reactions. Chemical reactions encompass changes that strictly involve the rearrangement of atoms in the breaking and forming of chemical bonds.

Question 11.
What are valence electrons? Why are they important in the study and understanding of chemical reactions?
Answer:
Electrons present in the penultimate and outermost shells of an atom are called valence electrons. Chemical reactions are due to the activity of valence electrons. Therefore, valence electrons help us to understand the chemical reactions.

Question 12.
What is a chemical equation?
Answer:
A chemical equation is a representation of a chemical reaction using symbols and formula to indicate the amount of substance of each reactant and product.

Question 13.
Explain the terms ‘reactants’ and ‘products’ with reference to a chemical reaction.
Answer:
Chemical substances which take part in a chemical reaction are called reactants. Chemical substances which are produced during a chemical reaction are called products.

Question 14.
Where do we write the reactants and products in a chemical equation?
Answer:
While writing a chemical equation, the reactants of the chemical reaction are written on the left hand side and the products are show n on the right hand side with an arrow in between the reactants and products.

Question 15.
Explain with an example, the reactants and products of a chemical reaction.
Answer:
Consider a chemical reaction in which dilute hydrochloric acid reacts with zinc granules to form zinc chloride and hydrogen gas. In this reaction, the participating chemicals are zinc and hydrochloric acid. These are the reactants of the reaction.

During the chemical change two new substances namely zinc chloride and hydrogen are formed. These are the products of the reaction. We may write this chemical reaction in a simple way as follows:

Question 16.
What is a word equation of a chemical reaction? Explain with an example.
Answer:
Representation of a chemical reaction in the form of an equation by indicating the reactants and products in words is called a word equation. It is the simplest way of representing a chemical reaction. Consider a chemical reaction in which hydrogen combines with oxygen to form water.

In this chemical reaction, hydrogen and oxygen are the reactants and water is the product of the reaction. The word equation for this reaction can be written as:
Hydrogen + Oxygen → Water.

Question 17.
Write the advantages of writing a chemical equation using formulae and symbols.
Answer:
The advantages of writing the word equation are:

1. Qualitative: Helps in knowing the reactants and products only.
2. Quantitative: Helps in knowing the relation between the quantities of reactants used and product formed.

Question 18.
What is a symbol equation? Explain with an example.
Answer:
The representation of a chemical reaction in the form of an equation using symbols and formulae of the reactants and products is called a symbol equation. For example, carbon burns in oxygen forming carbon dioxide. The word equation and the symbol equation for this chemical reaction are as follows

Question 19.
Why should we write the molecular formula of reactants and products in a chemical equation?
Answer:
Generally atoms are highly unstable. They generally exist in the form of molecules. Therefore, it is correct to consider the number of molecules of each substance that enters a chemical reaction and the number of molecules of each substance formed in a chemical reaction. This is why we write the molecular formula of reactants and products in a chemical equation.

Question 20.
What is meant by balancing of a chemical equation?
Answer:
The practice of equalizing the number of atoms of elements on the left hand side to the corresponding atoms on the right hand side is called balancing of equation.

Question 21.
In a chemical reaction, if either oxygen or chlorine is a reactant, they are represented as O₂ and Cl₂ and not as ‘O’ or ‘Cl’. Give reason.
Answer:
Oxygen atom (O) and chlorine atom (Cl) are highly unstable and they rarely exist in nature as single atoms. However, the molecules of chlorine and oxygen are stable forms and hence we use their molecular formula while writing equations for chemical reactions involving oxygen and chlorine.

Question 22.
Balance the following chemical equations:

• Al + Cl₂ → AlCl₃
• KClO₃ → KCl+O₂
• Pb₃O₄ → PbO + O₂
• Zn + HCl → ZnCl₂ + H₂

Answer:
The balanced equations are as follows

• 2 Al + 3 Cl₂ → 2 AlCl₃
• 2 KClO₃ → 2 KCl + 3 O₂
• 2 Pb₃O₄ → 6 PbO + O₂
• Zn + 2 HCl → ZnCI₂ + H₂

Question 23.
Which are the four major types of chemical reactions?
Answer:
The four major types of chemical reactions based on the nature of reactants and products are

1. Chemical combination
2. Chemical decomposition
3. Chemical displacement
4. Chemical double decomposition.

Question 24.
What is chemical combination? Give an example.
Answer:
A chemical reaction in which two or more elements or compounds combine to form a single product is called chemical combination. For example, sulphur burns in air with a blue flame to form sulphur dioxide. In this reaction, two reactants have combined to give a single product.
S + O₂ → SO₂

Question 25.
Ammonia reacts with hydrochloric acid to form ammonium chloride. What type of reaction is this? Why? Write a balanced chemical equation to represent this reaction.
Answer:
In this reaction two reactants have combined chemically to form a single product namely ammonium chloride. Therefore, this is an example of chemical combination.
NH₃ + HCl → NH₄Cl.

Question 26.
Give one industrial application of chemical combination. Write a balanced chemical equation to represent the reaction.
Answer:
One of the industrial applications of chemical combination is the manufacture of ammonia by Haber’s process. In this process, nitrogen combines with hydrogen to form ammonia. The chemical equation of this reaction is:
N₂ + 3 H₂ → 2 NH₃.

Question 27.
During lightning, the atmospheric nitrogen combines with oxygen. Write the reactants and products of this reaction. Write the balanced equation for this reaction.
Answer:
During lightning, nitrogen in the atmosphere combines with oxygen to form nitric oxide and nitrogen dioxide. In the vicinity of lightning where the temperature is higher, nitrogen reacts with oxygen to form nitric oxide.
N₂ + O₂ → 2 NO
The nitric oxide so formed quickly reacts with more oxygen to form nitrogen dioxide.
NO + O₂ → NO₂

Question 28.
What happens when a glass rod dipped in ammonia is held over the mouth of a bottle containing hydrochloric acid? Give the balanced equation to represent the reaction.
Answer:
When a glass rod dipped in ammonia is brought near the mouth of a bottle containing hydrochloric acid, thick white cloud of hydrogen chloride is formed.
NH₃ + HCl → NH₄Cl

Question 29.
What is chemical decomposition? Give two examples. Write balanced chemical equation for each of the reactions.
Answer:
A chemical reaction in which a single reactant yields two or more new substances is called chemical decomposition.
Example 1: Potassium permanganate on heating decomposes to give a mixture of potassium manganate and manganese dioxide along with the release of oxygen.
2 KMnO₄ → K₂MnO₄ + MnO₂ + O₂↑
In the above reaction a single compound has yielded three new substances. This is a case of chemical decomposition.

Example 2: When mercuric oxide is strongly heated in a hard glass tube, it decomposes to give mercury and oxygen gas.
2 HgO → 2 Hg + O₂ ↑

Question 30.
Describe the experiment of Joseph Priestley which led to the discovery of oxygen.
Answer:
Joseph Priestley, in 1774, prepared oxygen by focussing sunlight on a lump of reddish mercuric oxide taken in a glass container. He used a hand lens (convex lens) to focus sunlight.
2 HgO → 2 Hg + O₂ ↑

Question 31.
Describe the experiment which established that water is a chemical compound.
Answer:
When an electric current is passed through acidified water, it decomposes to give hydrogen and oxygen. This decomposition reaction showed that water is a compound.
2 H2O → 2 H₂ + O₂ ↑

Question 32.
What is chemical displacement? Explain with an example.
Answer:
When the more reactive elements displace the less reactive elements from their compounds, it is called chemical displacement. Consider an example. When an iron nail is dipped in a solution of copper sulphate, the iron atoms displace copper from copper sulphate. This reaction produces iron sulphate and copper.

In the above reaction, iron is the more reactive metal than copper. Therefore, iron displaces copper from copper sulphate solution.

Question 33.
Give an example for a displacement reaction in which a more reactive non-metal replaces another non-metal atom.
Answer:
When potassium iodide is added to a solution of bleaching powder, the solution turns pink due to the release of iodine. In this reaction, the more reactive chlorine displaces the less reactive iodine from potassium iodide.
Cl₂ +2 KI → 2 KCl + I₂ ↑

Question 34.
Write copper, iron and silver in the increasing order of their reactivity. Which of these can displace which others from the solution of their salts?
Answer:
Reactivity of iron > Reactivity of copper > Reactivity of silver
These metals in the increasing order of reactivity are silver, copper, iron. Of the three given metals, Iron can displace both copper and silver from solution of their salts. Copper can displace silver from a solution of its salt. Silver cannot displace iron and copper from solution of their salts.

Question 35.
Describe an activity to show that copper can displace silver from a solution of its salt.
Answer:
Take silver nitrate solution in a beaker. Dip a copper plate in the solution. Leave it fora few minutes. After some time, a fine deposit of silver appears on the copper plate. This is because copper is chemically more reactive than silver. Therefore, copper displaces silver from a solution of silver nitrate.

Question 36.
Dilute sulphuric acid reacts with zinc to form zinc sulphate and hydrogen. Write the balanced chemical equation to represent this reaction. What type of reaction is this? Explain.
Answer:
The balanced equation for the above reaction is
Zn + H₂SO₄ → Zn S0₄ + H₂ ↑
This is a displacement reaction. Here, zinc has displaced hydrogen from sulphuric acid.

Question 37.
What is double decomposition? Explain.
Answer:
A chemical reaction between two compounds in which parts of each are interchanged to form two new compounds is called double decomposition. Such reactions are also known as double displacement. Sometimes people use ‘chemical exchange’ for double decomposition reactions.

Consider a general reaction of the type
AB + CD → AD + BC
In the above reaction, the two reactants have interchanged one part of each to form two different compounds.

Question 38.
Explain chemical double decomposition with an example.
Answer:
Add sodium carbonate solution to magnesium sulphate solution in a test tube. We get a white precipitate of magnesium carbonate instantly.

In this reaction, SO₄ part of magnesium sulphate and CO₃ part of sodium carbonate are exchanged. The two new compounds formed are magnesium carbonate and sodium sulphate. This is an example of double decomposition which is also called double displacement.

Question 39.
In the chemical reaction given below, write the parts which are exchanged
MgCl₂ + Na₂CO₃ → MgCO₃l + 2 NaCl.
Answer:
In the double decomposition reaction above, Cl₂ part of magnesium chloride and CO₃ part of sodium carbonate are exchanged to form magnesium carbonate and sodium chloride.

Question 40.
Mention one example each for different types of chemical reactions.
Answer:
The four main types of chemical reactions and an example for each type are given below:

1. Chemical combination: N₂ + 3 H₂ → 2 NH₃
2. Chemical decomposition: 2 HgO → 2 Hg + O₂
3. Chemical displacement: Cl₂ + 2 KI → 2 KCl + I₂^–
4. Double decomposition: MgSO₄ + Na₂CO₃ → MgCO₃^– + NaSO₄

Question 41.
Classify the following into chemical combination, decomposition, displacement and double displacement reaction:

• \(\mathrm{CaCO}_{3} \stackrel{\text {heat}}{\longrightarrow} \mathrm{CaO}+\mathrm{CO}_{2}\)
• 2 AgNO₃ + Cu → Cu(NO₃)₂ + 2 Ag^–
• 2 SO₂ + O₂ → 2 SO₃
• NaCl + AgNO₃ → AgCl^– + NaNO₃

Answer:

• Decomposition,
• Displacement
• Combination
• Double displacement.

Question 42.
Balance the following reactions:

• Cu + H₂SO₄ → CuSO₄ + SO₂ + H₂O
• Fe + H₂O → Fe₃O₄ + H₂
• CH₄ + O₂ → CO₂ + H₂O.

Answer:

• Cu + 2 H₂SO₄ → CuSO₄ + SO₂ + 2 H₂O
• 3 Fe + 4 H₂O → Fe₃O₄ + 4 H₂
• CH₄ + 2 O₂ → CO₂ + 2 H₂O.

Question 43.
During winter, so many plants shed their leaves. These leaves do not cause pollution. However, plastic thrown around cause pollution. Name the type of chemical reaction associated with the first one.
Answer:
The litter leaves of plants do not cause pollution because they are broken down into smaller particles by insects and worms which are then broken down into simple molecules by microorganisms. Plastic is not degraded by organisms and hence causes pollution.

Question 44.
Mention one example each for the different types of chemical reactions.
Answer:
There are four major types of chemical reactions. They are
(i) Chemical combination:
eg: Hydrogen + Oxygen → Water.

(ii) Chemical decomposition:
eg: \(Mercuric oxide \stackrel{\text { Heat }}{\rightarrow} Mercury + Oxygen\)

(iii) Chemical displacement:
eg: Copper sulphate + Iron → Iron sulphate + Copper

(iv) Chemical double decomposition:
eg: Silver nitrate + Hydrochloric acid → Silver chloride + Nitric acid

Question 45.
What are the reactants of a neutralization reaction?
Answer:
The reactants of neutralization consist of an acid and a base.

Question 46.
What are the products of a neutralization reaction?
Answer:
The products of a neutralization reaction are a salt and water.

Question 47.
What is a neutralization reaction? Give an example.
Answer:
A chemical reaction between an acid and a base resulting in the formation of a salt and water is known as neutralization. During neutralization reaction, the acid loses its acidic properties and the base loses its basic properties and the resulting solution becomes neutral. eg: Sodium hydroxide is a base and Hydrochloric acid is an acid. When requisite amount of sodium hydroxide is made to react with a known quantity of hydrochloric acid, we get sodium chloride (a salt) and water.
2 NaOH + 2 HCl → 2 NaCl + H₂O.

Question 48.
A certain amount of a base has completely neutralized a given amount of an acid. Will the resulting solution be acidic or basic? How can we know it?
Answer:
The resulting solution would be neither acidic nor basic. The solution does not change the colour of either blue litmus or red litmus paper. From this we can know that the resulting solution is neutral.

Question 49.
What is an exothermic reaction? Give an example.
Answer:
A chemical reaction which releases heat is called an exothermic reaction. Burning of methane in air is an example of an exothermic reaction because the reaction releases heat.
CH₄ + 2 O₂ → CO₂ + 2 H₂O.

Question 50.
Why is respiration called an exothermic process?
Answer:
Respiration is a chemical reaction that involves liberation of heat. Hence it is considered an exothermic process.

Question 51.
What is an endothermic reaction? Give an example.
Answer:
A chemical reaction during which heat is absorbed is known as an endothermic reaction. eg: The decomposition of limestone into calcium oxide and carbon dioxide is an endothermic reaction.
\(\mathrm{CaCO}_{3} \stackrel{\text { Heat }}{\longrightarrow} \mathrm{CaO}+\mathrm{CO}_{2}\)

Question 52.
What are oxidation and reduction in terms of oxygen transfer? Explain with an example.
Answer:
The addition of oxygen is called oxidation and the removal of oxygen is reduction. Consider the reaction in which iron oxide combines with carbon monoxide to give iron and carbon dioxide. In this reaction, oxygen is added to carbon monoxide.

Therefore, carbon monoxide is oxidized to carbon dioxide. At the same time, oxygen present in iron oxide is removed from iron oxide. Therefore, iron oxide is reduced to iron. We say that, in the above reaction, iron oxide has undergone reduction and carbon monoxide has undergone oxidation.

Question 53.
What are oxidizing agents? Give two examples.
Answer:
Any chemical which adds oxygen to a compound or an element during a chemical reaction is called an oxidizing agent. Examples of oxidizing agents include hydrogen peroxide, potassium nitrate, etc.

Question 54.
On what basis are the oxidation and reduction reactions explained in modern science?
Answer:
Modern science explains oxidation and reduction on the basis of electron transfer.

Question 55.
Define oxidation and reduction in terms of electron transfer. Explain with an example.
Answer:
The loss of electrons is called oxidation and the gain of electrons is called reduction. An atom or a group of atoms undergo oxidation when they lose electrons. Similarly, an atom or a group of atoms undergo reduction when they gain electrons.
Cu → Cu⁺⁺ + 2 e^-1
Cl + 2 e^-1 → Cl^-2
In the above examples, copper atom has lost two electrons to become a positively charged particle. We say that copper atom is oxidized. In the other instance, an atom of chlorine has gained two electrons to become a negatively charged particle. We say that chlorine atom is reduced.

Question 56.
Classify the following as oxidizing agents and reducing agents: magnesium, potassium permanganate, sulphur dioxide, potassium chlorate, carbon, hydrogen peroxide, potassium nitrate, hydrogen sulphide.
Answer:
Oxidizing agents: Potassium permanganate, potassium chlorate, hydrogen peroxide, potassium nitrate
Reducing agents: Magnesium, sulphur dioxide, carbon, hydrogen sulphide.

Question 57.
Why do we consider sulphur dioxide as a reducing agent?
Answer:
Sulphur dioxide can remove oxygen atoms from compounds. This is why sulphur dioxide is called a reducing agent.

Question 58.
Why do we consider hydrogen peroxide as an oxidizing agent?
Answer:
Hydrogen peroxide can add oxygen atoms to a compound. This is why hydrogen peroxide considered an oxidizing agent.

Question 59.
Suggest some measures to avoid rancidity of oils.
Answer:
Oils become rancid due to oxidation caused by exposure to air. This causes change in smell and taste of the oil. Rancidity of oils can be prevented by storing it in an airtight container. It can also be prevented by storing in cans where air (if any) is replaced by nitrogen gas.

Multiple Choice Questions

Question 1.
Which among the following is not a physical change?
(a) Melting of solids to liquids
(b) LPG changing from liquid to gaseous state
(c) Turning gases into liquids by applying pressure
(d) Decay of organic matter
Answer:
(d) Decay of organic matter

Question 2.
Balancing chemical equations is based on
(a) Avogadro’s number
(b) Law of conservation of mass or matter
(c) Principle of conservation of momentum
(d) Physical conditions for a chemical change to occur
Answer:
(b) Law of conservation of mass or matter

Question 3.
Identify the balanced chemical equation in the following
(a) SiO₂ + Mg → Si + MgO
(b) SiO + Mg → Si + MgO₂
(c) SiO₂ + 2 Mg → Si + 2 MgO
(d) SiO₂ + Mg → SiO + 2 MgO
Answer:
(c) SiO₂ + 2 Mg → Si + 2 MgO

Question 4.
Which of the following information is conveyed by a chemical reaction?
(a) Colour changes that occur during the reaction
(b) Molecular structure of the reactants and products
(c) Chemical properties of the products
(d) The masses of the reactants and products involved in the reaction
Answer:
(d) The masses of the reactants and products involved in the reaction

Question 5.
In the reaction represented by the following equation, what numbers replace a, b and c to balance the equation
aAl + bO₂ → cAl₂O₃?
(a) a = 4,b = 6,c = 4
(b) a = 4,b = 3,c = 2
(c) a = 1, b = 3, c = 2
(d) a = 2, b = 3, c = 2
Answer:
(b) a = 4,b = 3,c = 2

Question 6.
Hydrogen sulphide reacts with sulphur dioxide to give sulphur and water.
2 H₂S + SO₂ → 3 S + 2 H₂O
Which statement is TRUE about the reaction?
(a) Hydrogen sulphide oxidises sulphur dioxide to sulphur.
(b) Hydrogen sulphide oxidises sulphur dioxide to water.
(c) Sulphur dioxide oxidises hydrogen sulphide to sulphur
(d) Sulphur dioxide oxidises hydrogen sulphide to water
Answer:
(c) Sulphur dioxide oxidises hydrogen sulphide to sulphur

Question 7.
The correctly balanced equation is
(a) BaCl₂ + Na₂SO₄ → BaSO₄ + 2 NaCI
(b) 2 Na + 4 Cl₂ → 2 NaCl
(c) 2 Na + 2 H₂O → 2 NaOH + H₂
(d) 2HCl + Mg → 2 MgCl₂ + H₂
Answer:
(a) BaCl₂ + Na₂SO₄ → BaSO₄ + 2 NaCI

Question 8.
Which one of the following is usually considered a reducing agent?
(a) hydrogen peroxide
(b) potassium permanganate
(c) carbon
(d) hydrogen
Answer:
(c) carbon

Question 9.
An oxidizing agent in a chemical reaction is
(a) reduced as it loses electrons.
(b) reduced as it gains electrons.
(c) oxidized as it loses electrons.
(d) oxidized as it gains electrons.
Answer:
(b) reduced as it gains electrons.

Question 10.
In a chemical displacement reaction
(a) less reactive element displaces more reactive element
(b) a non-metal displaces more reactive non-metal
(c) more reactive element displaces less reactive element
(d) heat is evolved when carbon is burnt in air.
Answer:
(c) more reactive element displaces less reactive element

Question 11.
Magnesium + Oxygen → Magnesium oxide [2 Mg + O₂ → 2 MgO] is an example of
(a) double decomposition reaction
(b) simple displacement reaction
(c) decomposition reaction
(d) chemical combination
Answer:
(d) chemical combination

Question 12.
Potassium chlorate → Potassium chloride + Oxygen [2 KClO₃ → 2 KCl + 3 O₂] is an example of
(a) decomposition reaction
(b) double decomposition reaction
(c) synthesis or direct combination reaction
(d) simple displacement reaction
Answer:
(a) decomposition reaction

Question 13.
Dissolving of sugar in water is not an example for chemical reaction because,
(a) there is no change in energy
(b) sufficient heat is not produced
(c) no new substance is formed
(d) no gases are evolved
Answer:
(c) no new substance is formed

Question 14.
Which of the following represents combination of two compounds forming another compound?
(a) 2 SO₂ + O₂ → 2 SO₃
(b) CaCO₃ → CaO + CO₂
(c) H₂O + SO₂ → H₂SO₃
(d) 2 Na + 2 H₂O → 2 NaOH + H₂
Answer:
(c) H₂O + SO₂ → H₂SO₃

Fill In The Blanks

• Removal of oxygen is called reduction
• AB → A + B is a general form of a chemical reaction called decomposition
• The atom displaced by iron in CuSO₄ solution is copper
• The electrons present in the outermost orbit of an atom are called valence electrons
• Rusting of iron is an example for a chemical change,