Samacheer Kalvi 8th Science Unit 12 Book Back Answers:

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Samacheer Kalvi 8th Science Book Back Solutions PDF:

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Science Book Back Answers

Unit 12 – Atomic Structure

I. 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) the atomic and the mass number are same.

II. Fill in the blanks

1. …………….. is the smallest particle of an element.
2. An element is composed of …………….. atoms.
3. An atom is made up of …………….., …………….. and ……………..
4. A negatively charged ion is called …………….. while positively charged ion is called
5. …………….. is a negatively charged particle (Electron/Proton).
6. Proton is deflected towards the …………….. charged plate (positively, negatively).

Answer:

1. Atom
2. same kind of
3. proton, electron, neutron
4. anion, cation
5. Electron
6. negatively

III. Match the following

1. Law of Conservation of Mass  – Sir William Crookes
2. Law of Constant Proportion    – James Chadwick
3. Cathode rays                               – Joseph Proust
4. Anode rays                                  – Lavoisier
5. Neutrons                                     – Goldstein

Answer:

1 – 4; 2- 3; 3 – 1; 4 – 5; 5 – 2

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:

1. Anode rays travel in straight lines.
2. Anode rays are made up of material particles.
3. 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 formula.

7. Write the names of the following compounds.

1. CO, 2. N₂O,3. NO_{2 ,} 4. PCl₅

Answer:

1. Carbon monoxide.
2. Nitrous oxide
3. Nitrogen dioxide
4. Phosphorous pentachloride

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