Explanation of some phenomena using kinetic theory. Changes in the states of matter

Table of Contents

Subject :

Basic Science

TERM 

SECOND TERM

 

Week

Week 7

CLASS 

JSS 2 / Basic 8

 

Topic :

Explanation of some phenomena using kinetic theory. Changes in the states of matter 

 

 

Instructional Materials :

  • Gas cylinder
  • Bunsen Burner
  • Kettle for boiling water
  • Pictures of objects in solid, liquid and gaseous state of matter

 

Reference Materials

  • Scheme of Work
  • Online Information
  • Textbooks
  • Workbooks
  • 9 Year Basic Education Curriculum

Previous Knowledge :

 

The pupils have been taught

 

 KINETIC THEORY

 

in their previous lesson

 

Behavioural Objectives :  At the end of the lesson, the learners will be able to

  • Explain how temperature affects evaporation
  • Explain diffusion in respect kinetic theory
  • Define evaporation
  • State how gas exert pressure on walls of its containers

 

 

Entry Behaviour 

Learners are familiar with  Meaning Of Energy (Kinetic and Potential Energy)

 

Content :

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.

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.

 

Change of State and Kinetic Theory

 

The kinetic theory of matter gives a clear explanation of the internal processes involved at the particle level when matter undergoes a change of state.

 

Gas

or

Vapour

evaporates

Condenses

Liquid

 

melts

 

freezes

 

Solid

 

A given substance can exist as a solid, liquid or gas. Change of state is brought about by a change in temperature. When a substance is heated, its particles acquire more kinetic energy and when cooled they become less energetic.

Process of Heating

 

Theoretically, heating a solid to higher and higher temperatures changes its phase to a

 

liquid, and finally to a gas.

 

• Fusion (solid to liquid)

 

A solid consists of low kinetic energy vibrating particles locked into position by interparticle attractive forces. When heat is applied, energy is absorbed and the particles start vibrating more vigorously. Finally, the vibrations become energetic enough to overcome the attractive forces, and the particles start sliding out of their positions to flow about. The solid is now melting into a liquid.

 

• Vaporisation (liquid to gas)

 

On further application of heat to the liquid, the particles move around more energetically within the volume of the liquid. Finally, they become energetic enough to start escaping from the surface of the liquid, overcoming the backward pull by their neighbours in the volume of the liquid. The process of boiling has begun, wherein the liquid converts to gas as particles escape to move around independently without any constraints

Evaporation (liquid to gas)

 

According to the kinetic theory, the temperature is a measure of the average kinetic energy possessed by the particles of matter. This means that in any sample of matter, there will be particles with higher kinetic energy than average, balanced by those with lower energy than average. So, even in a liquid whose temperature is not high enough for boiling to occur, there will be some particles with sufficient kinetic energy to break through the surface of the liquid overcoming the backward pull of others. They slowly escape as gas particles, and the process is called evaporation.

 

Process of Cooling

 

Generally, cooling a gas changes its phase to a liquid, and finally to a solid.

 

.Condensation (gas to liquid)

 

When a gas is cooled (i.e. heat is removed) progressively, the free moving particles start losing kinetic energy and slowing down. Finally, the forces of attraction between the lower energy particles colliding with each other are strong enough to hold them together, and the gas begins to condense into liquid.

 

Solidification (liquid to solid)

 

The particles still have energy enough to slide about within the volume of the liquid, but further cooling lowers this energy further.

 

 Presentation

The topic is presented step by step

 

Step 1:

The class teacher revises the previous topics

 

Step 2.

He introduces the new topic

 

Step 3:

The class teacher allows the pupils to give their own examples and he corrects them when the needs arise

 

 

EVALUATION 

OBJECTIVES 

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

C. Boiling D. Melting

4. At the same time, each collision now results in a greater force imparted. Hence pressure is———A. increased B. decreased C. suspended D. affected

5. 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.

B. reduced C. suppressed D. affect

 

THEORY

1. Why does evaporation occur?

2. How does the kinetic THEORY explain diffusion?

3. Why is energy required to evaporate a liquid?

4. How does the surface area of a liquid affect evaporation?

5. How does temperature affect evaporation?

6. How does the kinetic THEORY explain diffusion?

7. How does the kinetic THEORY explain diffusion?

8. Why is energy required to evaporate a liquid?

9. How does the surface area of liquid affect evaporation?

 

 

 

Conclusion :

 

The class teacher wraps up or conclude the lesson by giving out short note to summarize the topic that he or she has just taught.

The class teacher also goes round to make sure that the notes are well copied or well written by the learners.

He or she does the necessary corrections when and where  the needs arise.

 

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