Explanation of Some Phenomena Using Kinetic Theory – JSS2 Basic Science Lesson Note (Second Term)

The kinetic theory explains changes of state, diffusion, evaporation and boiling by the movement and energy of particles.

Behavioural objectives

By the end of the lesson, students should be able to:

1. explain melting, condensation and freezing using the kinetic theory;
2. define diffusion and say why it is fastest in gases;
3. explain evaporation and boiling in terms of particles;
4. state the difference between evaporation and boiling;
5. explain how temperature and the nature of a liquid affect evaporation.

What is the kinetic theory of matter?

The kinetic theory states that all matter is made of tiny particles that are always moving.

• Heating: particles gain kinetic energy and move or vibrate faster.
• Cooling: particles lose kinetic energy and move or vibrate more slowly.
• Binding forces: these are the forces of attraction that hold particles together.
• Change of state: this happens when the energy of the particles changes enough to overcome or restore the binding forces.

How does the kinetic theory explain changes in the states of matter?

Changes of state happen when heating or cooling changes the kinetic energy of the particles.

What is melting?

Melting is the change of a solid to a liquid when it is heated.

1. The solid is heated and its particles gain kinetic energy.
2. The particles vibrate more vigorously about their fixed positions.
3. At the melting point, the vibrations overcome the binding forces.
4. The solid structure collapses and the particles are free to move.
5. The solid is now said to have melted or liquefied.

• Example: a block of ice melts to water when it is left in the sun.

What is condensation?

Condensation is the change of a vapour to a liquid when it is cooled.

• Energy: the vapour loses some of its kinetic energy.
• Particles: they slow down and move closer together.
• Result: the binding forces hold the particles together as a liquid.
• Example: water droplets form on the outside of a cold bottle of water.

What is freezing?

Freezing is the change of a liquid to a solid when it is cooled.

1. The liquid loses heat energy and its temperature drops.
2. Cooling continues and the temperature keeps dropping.
3. The temperature reaches the freezing point of the liquid.
4. The liquid changes into a solid.

• Key fact: the freezing point of a substance is the same as its melting point.
• Example: water becomes ice in a freezer.

What is diffusion?

Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration.

• Meaning: particles move from a more crowded region to a less crowded region.
• Medium: the particles move through a gas, a liquid or a solid.
• End point: when the particles are evenly spread, there is no net diffusion in any one direction.
• Example in gas: the smell of cooking spreads from the kitchen to other rooms.
• Example in liquid: a drop of ink spreads through a glass of water.

Why is diffusion fastest in gases?

Diffusion is fastest in gases because gas particles have more kinetic energy than particles in liquids and solids.

• Gases: particles are far apart and move freely, so diffusion is fastest.
• Liquids: particles are closer together and move more slowly, so diffusion is slower.
• Solids: particles only vibrate in fixed positions, so diffusion is very slow.

What is evaporation?

Evaporation is the escape of particles from the surface of a liquid to become vapour or gas.

1. Particles in a liquid are attracted by their neighbours in all directions.
2. Some particles near the surface have enough energy to break away from these attractive forces.
3. They escape into the space above the liquid.
4. They become vapour or gas.

• Temperature: evaporation occurs at any temperature.
• Where: it takes place only at the surface of the liquid.
• Example: wet clothes dry on a line.

What is boiling?

Boiling is the rapid change of a liquid to vapour throughout the liquid at its boiling point.

1. The liquid is heated and the rate of evaporation increases.
2. The vapour pressure of the liquid also increases.
3. A temperature is reached at which the vapour pressure equals the atmospheric pressure.
4. Bubbles of vapour form freely inside the liquid and rise to the surface.

• Boiling point: the temperature at which boiling takes place.
• Example: water bubbling in a pot on a stove.

What is the difference between evaporation and boiling?

Evaporation happens only at the surface at any temperature, while boiling happens throughout the liquid at the boiling point.

What factors affect evaporation?

The rate of evaporation depends on temperature, the nature of the liquid, surface area, wind and humidity.

1. What is the effect of the temperature of the liquid?

• Heating: particles move more rapidly and collide with one another, changing direction.
• More energy: some particles gain enough energy to escape from the surface as gas.
• Rate: evaporation occurs at all temperatures, but the rate increases as temperature rises.
• Cooling effect: the most energetic particles escape, so the average kinetic energy of the liquid falls.
• Result: evaporation causes a drop in the temperature of the liquid.

2. What is the effect of the nature of the liquid?

• Type of liquid: different liquids evaporate at different rates at the same temperature.
• Example: water evaporates more slowly than petrol.

3. What other factors increase evaporation?

• Wind: moving air carries vapour away, so more particles can escape.
• Larger surface area: more particles are at the surface, so more can escape.
• Lower relative humidity: drier air can take in more vapour.

Evaluation

1. Explain, using the kinetic theory, what happens to the particles of a solid at its melting point.
2. What is diffusion, and why is it fastest in gases?
3. State the difference between evaporation and boiling.
4. Why does evaporation cause a drop in the temperature of a liquid?
5. Which evaporates faster, water or petrol?

Assignment

(a) Define condensation and freezing, and state the relationship between the freezing point and the melting point of a substance. (b) Apart from temperature, list three factors that increase the rate of evaporation.