Molecular Nature of Matter - UNSOLVED PRACTICE SET

Q7. What is meant by the molecular nature of matter? Name the three common states of matter and give one example of each from everyday life.

Q8. Explain why solids have a definite shape and volume, while gases have neither.

Q9. What is diffusion? Give one example of diffusion that you can observe in your kitchen at home.

Q10. Describe Brownian motion. What does it tell us about the nature of molecules in a liquid or gas?

Q11. Why does a smell of perfume spread quickly in a room but much more slowly in a closed bottle? Explain using the concept of molecular motion.

Q12. Explain the difference between cohesive forces and adhesive forces with one example of each.

Q13. Describe the molecular picture of the three states of matter — solid, liquid, and gas. For each state, explain:

(i) The arrangement of molecules

(ii) The intermolecular forces

(iii) The motion of molecules

(iv) The shape and volume characteristics

Use a simple diagram to illustrate the arrangement in each state.

Q14. Explain the phenomenon of Brownian motion in detail. Discuss how Robert Brown's observation of pollen grains in water led to important conclusions about the molecular nature of matter. Why is Brownian motion more vigorous at higher temperatures?

Q15. A student places a crystal of potassium permanganate at the bottom of a beaker of cold water and another crystal in a beaker of hot water.

(i) In which beaker will the colour spread faster? Why?

(ii) What does this experiment demonstrate about molecular motion?

(iii) If the student stirs the water, what happens to the rate of spreading? Is this still diffusion? Explain.

Q16. The average distance between molecules in a gas at STP is approximately 3.3 × 10⁻⁹ m, while the size of a molecule is about 3 × 10⁻¹⁰ m.

(i) Calculate the ratio of the intermolecular distance to the molecular size.

(ii) What does this ratio tell you about the space occupied by molecules compared to the total volume of the gas?

(iii) Explain why gases are highly compressible based on this ratio.

Q17. In a science experiment, a student observes Brownian motion of spherical particles of radius 1 μm suspended in water at 300 K. The viscosity of water is 10⁻³ Pa·s.

(i) Calculate the root mean square speed of these particles using the equipartition theorem. (Hint: use ½mv² = (3/2)kT)

(ii) Why is this speed much smaller than the speed of gas molecules at the same temperature?

(iii) If the temperature is increased to 350 K, by what factor does the rms speed change?

(Given: k = 1.38 × 10⁻²³ J/K, density of particle material = 2000 kg/m³)

Q18. A room has dimensions 5 m × 4 m × 3 m and contains air at 27°C and 1 atm pressure. The average mass of an air molecule is approximately 4.8 × 10⁻²⁶ kg.

(i) Calculate the number of air molecules in the room.

(ii) Calculate the total mass of air in the room.

(iii) If all the air molecules were packed tightly together with no gaps, what volume would they occupy? (Assume each molecule occupies a cube of side 3 × 10⁻¹⁰ m)

(iv) What fraction of the room's volume is actually occupied by molecules? What does this tell you about gases?

(Given: R = 8.31 J mol⁻¹ K⁻¹, N_A = 6.022 × 10²³ mol⁻¹)