Karnataka Board Class 8 Science Chapter 11 Chemical Reactions and their Types

KSEEB Class 8 Science Chapter 11 Textual Questions & Answers

I. Four alternatives are given to each of the following incomplete statement/ question. Choose the right answer.

Question 1.
Balancing of equations is based on
a) Avogadro’s number
b) condition for a chemical reaction to take place
c) law of conservation of mass or matter.
Answer:
c) law of conservation of mass or matter.

Question 2.
Identify the balanced equation in the following.
SiO2 + Mg → Si + MgO
SiO2 + 2Mg → Si + 2MgO
SiO + Mg → Si + MgO2
SiO2 + Mg → SiO + 2MgO
Answer:
b) SiO2 + 2Mg → Si + 2MgO

Question 3.
Correct statement about chemical displacement is.
a) less reactive element displaces a more reactive element
b) a non-metal displaces more reactive non-metal
c)  more reactive element displaces the less reactive element
d) evolution of heat when carbon is burnt in air
Answer:
c) more reactive element displaces the less reactive element

Question 4.
Dissolving of sugar in water is not an example of a 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 5.
Which of the following represents a combination of two compounds forming another compound?
a) 2SO2 + O3 → 2SO3
b) CaCO2 → CaCO +CO2
c) H2O+ SO2→ H2SO3
d) 2Na + 2H2O → 2NaOH + H2 
Answer:
c) H2O + SO2 → H2SO3

II. Balance the following reactions.

1. Cu + H2SO4  → CuSO4 + SO2 + H2O ↑
2. Fe + H2O → Fe3O4 + H2
3. CH4 + O2 → CO2+ H2O
Answer:
1. Cu + 2H2SO4 → CuSO4 + SO2+ 2H2O
2. 3Fe + 4H2O → Fe3O4 + 4H2
3. CH4 + 2O2 →  CO2 + 2H2O

III. Answer the following.

1) Write any three differences between a physical change and chemical change.
Answer:

Physical change Chemical change
1) Temporary and easily reversible Permanent and sometimes reversible
2) No new substance is formed. New substance is formed.
3) No change in mass Weight of the substance is altered.

2) Group the following into physical change and chemical change. Dissolving salt in H2O, Burning of a fuel, Rusting or iron, melting of an ice cube.
Answer:

Physical change Chemical change
1) Dissolving salt in H2O Burning of a fuel
2) Melting of ice cube Rusting of iron

Question 3.
Mention one example each for the different types of chemical reactions.
Answer:
The different types of chemical reactions are
1. Chemical combination
Ex : S + O2 → SO2
2. Chemical decomposition
Ex: 2KMnO4 → K2MnO4+ MnO2 + O2
3. Chemical displacement
Ex : Zn + H2SO4 →  ZnSO2 + H2 ↑
4. Chemical double decomposition
Ex : MgSO4  + Na2CO3  →  MgCO3 + Na2CO4

Question 4.
Classify the following into chemical combination, decomposition, displacement and double displacement reaction.
1. CaCO3  → CaO + CO2 ↑
2. 2AgNO3+ Cu → Cu(NO3)2 + 2Ag ↓
3. 2SO2+ O2 → 2SO3 ↑
4. NaCl + AgNO3 →  AgCl +NaNO3
Answer:

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

Question 5.
Write the advantages of writing a chemical equation using formulas and symbols.
Answer:
The advantages for writing a chemical equation using formula and symbols are

  1. Qualitative: Helps in knowing the reactants and products.
  2. Qualitative: Helps in knowing the relation between the quantities of reactants consumed and -products formed
  3. can represent in a short scientific way and saves time and space.
  4. Molecular formula helps to find out the exact number of atoms of different elements present in a molecule of the compound.

Question 6.
During winter so many plant leaves fall on earth But it will not cause pollution plastic papers which arc strew on the earth cause pollution. Name the type of chemical reaction associated with this observation.
Answer:
The type of reaction observed here is chemical decomposition the leaves that fall on the earth during the winter season get decomposed the carbohydrates present in the leaves are decomposed into simple sugar which can be absorbed by the plants whereas plastic is not decomposed so it adds to pollution.

Question 7.
LPG gas contains butane. Its molecular formula is C4H10. Name the possible products when it undergoes complete combustion.
Answer:
Combustion is an oxidation reaction 2C4H10 + 16O2  → 16O2  8CO2 + 10H2O
CO2 and H2O are the products formed when C4H10 undergoes complete combustion.

Question 8.
Suggest some measures to avoid rancidity of oils.
Answer:
Some of the measures to avoid rancidity of oils are as follows

  1. Keep away from sunlight
  2. Keep in a refrigerator
  3. To be flushed with nitrogen gas which prevents further oxidation of any food item.
  4. To add antioxidants like BHT and BHA
    • BHA- Butylated hydroxyanisole.
    • BHT – Butylated hydroxyl toluene

IV. Given the scientific reason for the following

Question 1.
Iron displaces copper from copper sulfate but copper cannot displace iron.
Answer:
Iron displaces copper from copper sulfate but copper cannot displace iron. Iron is more reactive than copper. Hence the more reactive element iron displaces the less reactive element copper.

Question 2.
Respiration is an exothermic reaction.
Answer:
During respiration, glucose combines with the oxygen in the cells of the body and releases energy. Hence respiration is an exothermic reaction.

Question 3.
Use of plastic must be avoided or minimized.
Answer:
Plastic must be avoided or minimized because plastic pollutes soil and water and kill thousands of marine mammals yearly. Conventional plastics like polyethylene, polypropylene, polystyrene, PVC, PET or non-biodegradable.

KSEEB Class 8 Science Chapter 11 Additional Questions & Answers

Question 1.
Give an example of a complex chemical reaction.
Answer:
‘Photosynthesis’ is a complex chemical reaction

Question 2.
Give an example of simple chemical reactions.
Answer:
Rusting of iron and combustion of fuels are simply chemical reactions.

Question 3.
Classify the’following into chemical 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 plate

Answer:

  1. chemical change
  2. Physical change
  3. physical change
  4. Physical change
  5. chemical change
  6. chemical change

Question 4.
What is the cause of the chemical reaction?
Answer:
A chemical reaction means a rearrangement of the configuration to acquire the electronic configuration of the nearest stable or noble gas by transferring or sharing the electrons.

Question 5.
What is a valence electron?
Answer:
The electrons which take part in chemical reactions are called valence electrons.

Question 6.
What is the balancing of the 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 the equation.

Question 7.
State the law of conservation of matter or mass.
Answer:
Law of conservation of matter or mass states that matter can neither be created nor destroyed.

Question 8.
Mention the types of chemical reactions.
Answer:
The chemical reactions are mainly of the following types based on the nature of reactants and products.

  1. chemical combination
  2. chemical decomposition
  3. chemical displacement
  4. chemical double decomposition or exchange.

Question 9.
What is meant by chemical combination? Give an example.
Answer:
Chemical combination is a reaction in which two or more elements or compounds combine to form a new product.
Ex:
1. N2 + H2 → 2NH3
2. S + O2 → SO2

Question 10.
What do you mean by Haber’s process
Answer:
Haber designed the technology of synthesizing ammonia. He won a Nobel prize in chemistry for this contribution. In this process, Nitrogen and Hydrogen are subjected to high pressure at a temperature of about 500°C in the presence of a catalyst.

Question 11.
What is meant by chemical decomposition? Give an example.
Answer:
Chemical decomposition is a reaction in which a compound breaks up into two or more new substances.
Ex :
1. 2HgO → 2Hg + O2 ↑
2. 2KMnO4 →  K2MnO4 + MnO2 + O2

Question 12.
What is chemical displacement? Give an example.
Answer:
Chemical displacement is a reaction in which more reactive elements displaces the less reactive elements from their compounds.
Ex:
(1) CuSO4 + Fe →  FeSO4+ Cu ↓
(2) Cl2+ 2Kl →  2KCl +l2  ↓

Question 13.
What is meant by chemical double decomposition? Give an example.
Answer:
Chemical double decomposition is a reaction in which there is a mutual exchange of metallic and non-metal lie parts. Such reactions are also called a double chemical exchange.
CaCl2 + Na2CO3 →  CaCO3 ↓ + 2NaCl
MgSO4 + Na2CO3 →  MgCO3 ↓  + Na2SO4

KSEEB Solutions for Class 8 Science