# Lesson Notes By Weeks and Term - Junior Secondary School 2

Explanation of some phenomena using Kinetic THEORY

TERM: 2nd TERM

SUBJECT: BASIC SCIENCE

CLASS:  JSS 2

REFRENCE

• Precious seed BASIC SCIENCE FOR JUNIOR SECONDARY SCHOOLS BOOK 2

WEEK SEVEN                                                                                            DATE:………………..

TOPIC: Explanation of some phenomena using Kinetic THEORY

How does a gas exerts pressure on the walls of its container?

The gas particles are in constant and continual collisions with the walls. The collisions give rise to the force. The force per unit area exerted on the walls is the pressure of the gas.

Why is energy required to evaporate a liquid?

There are intermolecular forces between liquid molecules.

Only the fastest-moving molecules will have large enough energy to overcome the intermolecular forces. Hence energy is required to break the attractive forces.

Why does evaporation occur?

In liquids, there are intermolecular forces holding the liquid molecules together. Due to the constant collisions between molecules, some molecules at the surface may attain greater velocities.

Such molecules, if moving in the upward direction, may have large enough kinetic energy to overcome the downward attractive forces exerted by the molecules in the liquid and break away from the liquid surface and exist independently as vapour molecules. Hence evaporation occurs, and can occur at any temperature.

Why does cooling occur when a liquid evaporate?

Since only the fastest-moving molecules leave the liquid, it follows that the average kinetic energy of the molecules remaining in the liquid is decreased.

Since temperature is directly proportional to the average kinetic energy of the liquid molecules, therefore temperature falls, and cooling occurs.

Why does a liquid evaporate more quickly in a draught?

In a draught, the liquid molecules that leave the surface of the liquid will be blown away. More liquid molecules are able to take up the space left empty by the previous molecules.

At the same time, fewer molecules are able to return to the liquid. Hence liquid evaporate more quickly in a draught.

How does the surface area of a liquid affect evaporation?

Evaporation only takes place at the surface of a liquid.

The larger the surface area, there are more number of liquid molecules that can escape from the surface. Hence evaporation will be rapid. This in turns lead to rapid cooling of the liquid.

How does temperature affect evaporation?

The rate of evaporation is greater at higher temperature.

This is because of the greater average kinetic energy of the molecules, leading to greater velocities. Hence the molecules readily overcome the attractive forces and contribute to the process of evaporation.

EVALUATION

1. Why does evaporation occur?
2. How does the kinetic THEORY explain diffusion?

What happens when a fixed mass of gas is heated in a container of fixed volume?

When the container is heated, the moving molecules gain internal energy. Since temperature is directly proportional to the average kinetic energy of the molecules, their average speed and hence their kinetic energy are increased.

As the molecules are moving faster than before, they will make more collisions per second with the walls. At the same time, each collision now results in a greater force imparted. Hence pressure is increased.

(i.e. Pressure increases as Temperature increases) (and vice versa)

What happens when a fixed mass of gas is heated in a container of fixed pressure?

For pressure to remain constant, the number of collisions per unit time with the container can be reduced. This can be done by increasing the volume of the container.

(i.e. Volume increases as Temperature increases) (and vice versa)

What happens when a gas is heated in a container of fixed temperature? (a.k.a. Boyle's Law)

At constant temperature, the average speed of the molecules remains constant.

When the volume of the container is halved, the number of molecules per unit volume in the container will be doubled. The number of molecules hitting the wall per second will also be doubled. Hence pressure will be doubled.

(i.e. Pressure increases as Volume decreases) (and vice versa)

How does the kinetic THEORY explain diffusion?

All molecules are in constant motion. Because of the rapid, constant and chaotic motion of molecules, especially in a gas, molecules spread out from a region of higher concentration to a region of lower concentration.

In a gas, the spreading out is faster because of the larger intermolecular distance and negligible force among molecules.

Evaluation

1. Why is energy required to evaporate a liquid?
2. How does the surface area of a liquid affect evaporation?

GENERAL EVALUATION

1. How does temperature affect evaporation?
2. How does the kinetic THEORY explain diffusion?
3. How does the kinetic THEORY explain diffusion?
4. Why is energy required to evaporate a liquid?
5. How does the surface area of liquid affect evaporation?

WEEKEND ASSIGNMENT

1. The gas particles are in constant and continual collisions with the walls. The collisions give rise to the--------A. force B. energy C. work D. power
2. Only the fastest-moving molecules will have large enough --------to overcome the intermolecular forces. A. energy B. power C. work D. sufficiency
3. ---------occurs, and can occur at any temperature. A. Evaporation B. Pressure
1. Boiling D. Melting
1.  At the same time, each collision now results in a greater force imparted. Hence pressure is---------A.  increased B. decreased C. suspended D. affected
2. When the volume of the container is halved, the number of molecules per unit volume in the container will be doubled. The number of molecules hitting the wall per second will also be doubled. Hence pressure will be--------- A.  doubled.
1. reduced C. suppressed D. affect

THEORY

1. How does the kinetic THEORY explain diffusion?
2. Why is energy required to evaporate a liquid?