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Chapter: 8th Science : Chapter 12 : Atomic Structure

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8th Science : Chapter 12 : Atomic Structure : Text Book Back Questions Answers, Solution



1. Choose the best answer.


1. The same proportion of carbon and oxygen in the carbon dioxide obtained from different sources proves the law of__________

a) reciprocal proportion

b) definite proportion

c) multiple proportion

d) conservation of mass

[Answer: (b) definite proportion]


2. Cathode rays are made up of

a) neutral particles

b) positively charged particles

 c) negatively charged particles

d) None of the above

[Answer: (c) negatively charged particles]


3. In water, hydrogen and oxygen are combined in the ratio of _______by mass.

a) 1:8 

b) 8:1 

c) 2:3 

d) 1:3

[Answer: (a) 1:8]


4. Which of the following statements made by Dalton has not undergone any change?

a) Atoms cannot be broken.

b) Atoms combine in small, whole numbers to form compounds.

c) Elements are made up of atoms.

d) All atoms of an elements are alike

[Answer: (d) All atoms of an elements are alike]


5. In all atoms of an element

a) the atomic and the mass number are same.

b) the mass number is same and the atomic number is different.

c) the atomic number is same and the mass number is different

d) both atomic and mass numbers may vary.

[Answer: (a) Valency]


II. Fill in the blanks.


1. Atom is the smallest particle of an element.

2. An element is composed of same kind of atoms.

3. An atom is made up of proton, electron and neutron.

4. A negatively charged ion is called anion, while positively charged ion is called cation.

5. Electron is a negatively charged particle (Electron/Proton).

6. Proton is deflected towards the negatively charged plate (positively, negatively).


III. Match the following.


Law of conservation of mass - Sir William Crookes

Law of constant proportion James - Chadwick

Cathode rays - Joseph Proust

Anode rays - Lavoisier

Neutrons - Goldstein


1. Law of Conservation of Mass - Lavoisier

2. Law of Constant Proportion - Joseph Proust

3. Cathode rays - Sir William Crookes

4. Anode rays - Goldstein

5. Neutrons - James Chadwick


IV. Answer briefly.


1. State the law of conservation of mass.

Answer: The law states that during any chemical change, the total mass of the products is equal to the total mass of the reactants.

2. State the law of constant proportions.

Answer: Law of constant proportions states that in a pure chemical compound the elements are always present in definite proportions by mass.

3. Write the properties of anode rays.

Answer: Properties of Anode rays:

(i) Anode rays travel in straight lines.

(ii) Anode rays are made up of material particles.

(iii) Anode rays are deflected by electric and magnetic fields. Since, they are deflected towards the negatively charged plate, they consist of positively charged particles.

4. Define valency of an element with respect to hydrogen.

Answer: Valency of an element is defined as the number of hydrogen atoms which combine with one atom of it. 

5. Define the term ions or radicals.

Answer: An atom or a group of atoms when they either lose or gain electrons, get converted into ions or radicals.

6. What is a chemical equation?

Answer: A chemical equation is a short hand representation of a chemical reaction with the help of chemical symbols and formulae.

7. Write the names of the following compounds.

a) CO

b) N2O

c) NO2

d) PCl5


a. Carbon monoxide.

b. Nitrous oxide

c. Nitrogen dioxide

d. Phosphorous pentachloride


V.  Answer the following.


1. Find the valency of the element which is underlined in the following formula.

a) Na Cl 

b) C O2

c) Al (PO4)

d) Ba (NO3)2

e) CaCl2


a) NaCI = 1

b) CO2 = 4

c) Al(PO4)= 3

d) Ba(NO3)2 = 2

e) CaCl2 = 2

2. Write the chemical formula for the following compounds

a) Aluminium sulphate

b) Silver nitrate

c) Magnesium oxide

d) Barium chloride


a. Aluminium sulphate = A12(SO4)3

b. Silver nitrate = AgNO3

c. Magnesium oxide = MgO

d. Barium chloride = BaCl2

3. Write the skeleton equation for the following word equation and then balance them.

a. Carbon  + Oxygen  →  Carbon dioxide

b. Phosphorus + Chlorine  → Phosphorus pentachloride.

c. Sulphur + Oxygen  → Sulphur dioxide

d. Magnesium  + hydrogen → Magnesium  + Hydrogen chloride  chloride

Balanced equation :

a) C + O2 → CO2

b) P4 + 10Cl2 → 4PCl5

c) S + O2 → SO2

d) Mg + 2HCI → MgCl2 + H2

Skeleton equation :

a) C + O2 → CO2

b) P4 + Cl2 → PCl2

c) S + O2 → SO2

d) Mg + HCI → MgCl2 + H2

4. Balance the following chemical equation.

a. Na + O2 → Na2O

b. Ca + N2 → Ca3N2

c. N2 + H2 →NH3

d. CaCO3 +HCl → CaCl2 + CO2 +H2O

 e. Pb(NO3)2 → PbO + NO2 + O2

a) 4Na + O2 →  2Na2O

b) 3Ca + N2 →  Ca3N2

c) N2 + 3H2 → 2NH3

d) CaCO3 + 2HCI →  CaCl2 + H2O + CO2

e) 2Pb (NO3)2 → 2PbO + 4N02 + O2


VI. Higher Order Thinking Questions.


1. Why does a light paddle wheel placed in the path of cathode rays begin to rotate, when cathode rays fall on it?

Answer: It is because the small particles of the cathode rays (electrons) have mass and energy. This energy is used in rotating the paddle wheels.

2. How can we prove that the electrons carry negative charge?

Answer: J.J. Thomson found that cathode rays were attracted by the positively charged plate and repelled by the negatively charged plate. This led him to the conclusion that the cathode rays (electrons) were made of negatively charged particles.

3. Ruthresh, Hari, Kanishka and Thahera collected different samples of water from a well, a pond, a river and underground water. All these samples were sent to a testing laboratory. The test result showed the ratio of hydrogen to oxygen as 1:8.

a) What conclusion would you draw fromthe above experiment?

b) Which law of chemical combination does it obey?

Answer: a) Water obtained from different sources like a well, a pond, a river and underground water will always consist of the same two elements hydrogen and oxygen in the ratio 1 : 8 by mass.

b) It obeys the law of constant proportion.

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