St. Thomas’ Day School | Class 8 Half Yearly Examination | 19 Sept 2023

St. Thomas’s Day School

Class 8 | Unit 2 Examination

Section A

A. Choose the correct answer from the choices given: [10]

1. The change from a gaseous state to a liquid state is called:

1. condensation
2. evaporation
3. sublimation
4. fusion

2. The substance which does not sublime on heating is:

1. iodine
2. sodium chloride
3. camphor
4. ammonium chloride

3. The gaseous state of which substance is not called a vapour;

1. water
2. iodine
3. carbon dioxide
4. bromine

4. A mixture of sawdust and iron dust can be separated by:

1. winnowing
2. sedimentation
3. filtration
4. magnetisation

5. The chemical name for lime water is:

1. calcium hydroxide
2. calcium oxide
3. calcium chloride
4. calcium carbonate

6. A gas that turns lime water milky is:

1. a.oxygen
2. carbon dioxide
3. nitrogen dioxide
4. nitrogen

7. The pie-pudding model of an atom was given by:

1. J.J.Thompson
2. Mendeleev
3. Rutherford
4. Newton

8. Which of the following is not a subatomic particle:

1. electron
2. proton
3. neutron
4. nucleus

9. K_₂Cr_₂O_₇ is the formula of the compound;

1. potassium chromate
2. potassium dichromate
3. potassium chromium oxide
4. potassium carbonate

10. Which of the following is not a chemical change?

1. growing of a tree
2. melting of wax
3. photosynthesis
4. respiration

B. Draw the structure of the following atoms mentioning the Number of electrons, protons and neutrons. [10]

1. boron
2. magnesium
3. potassium
4. chlorine
5. neon

C. Complete the following table: [10]

D. Name the method used to separate the following mixtures: [10]

1. sugar and sand
2. petrol and water
3. salt and water
4. mud from muddy water
5. dyes in ink
6. sugar and salt
7. husk from grains
8. pebbles from cereals
9. alcohol and water
10. Sand and water

Section B

Answer Any 4 Questions

Question 1

1. Write four postulates of Dalton’s atomic theory.
2. Write the three observations and three conclusions of Rutherford’s alpha ray scattering experiment.

[4 + 6]

Question 2

1. Write any three postulates of Bohr’s Atomic Theory.
2. What is the significance of the 2n² formula?
3. Define isotopes and isobars with one example each.
4. What is radiocarbon dating?

[3+2+4+1]

Question 3

(a) Fill in the blanks:

1. ____ is considered as a Universal solvent.
2. The freezing point of pure water is _____.
3. Water shows abnormal behaviour between _____ degrees Celsius.
4. ____ water bodies cover the percent of the earth.
5. On adding salt to the water, its boiling point ____.

(b) Define the atomic number and mass number of an atom.

(c) State three differences between an atom and an ion.

[5+2+3]

Question 4

(a) Three beakers contain three types of solution at 25°C. Identify them.

• Beaker A – Can dissolve more solute at that temperature.
• Beaker B – Cannot dissolve any more solute at that temperature.
• Beaker C- Holds more solute than that present in beaker B at that very temperature.

(b) How does the solubility of a gas depend on temperature and pressure?

(c) What are homogenous and Heterogenous mixtures? Give examples.

(d) Is water a mixture or a compound? Give two points in support of your answer.

[3+2+3+2]

Question 5

1. Write four differences between metals and non-metals.
2. What is soft water and hard water?
3. What causes temporary and permanent hardness in water?
4. How can you remove temporary hardness from water? Write a chemical equation.

[4+2+2+2]

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