SCIE 4001
Unit 1
About Matter
Nature of Matter
Properties of Matter
States of Matter
Kinetic Theory of Gases
Brownian Motion

Unit 1-1.e-g Kinetic Theory of Gases


Matter is made up of tiny building blocks (molecules) which may be single atoms or pairs or other groups of atoms.

Molecules in gases

— are in constant random motion (rotation, vibration and free translation).

— collide with each other as they move about.

— collide with the walls of the container.

— have negligable forces between them.

Relationship between temperature of the gas and the average kinetic energy of the molecules

Investigations beyond the scope of this course have created knowledge called the Maxwell-Boltzman distribution which can be sumarized as follows:

at a particular temperature, molecules display a wide range of speeds and therefore wide range of kinetic energies.

— as temperature increases the average kinetic energy of the molecules increase and vice versa.

Phenomena which supports this theory:


Fig 1.1.43

1. A gas exerts pressure on the walls of its container.

— the gas molecules are constantly collisiding with the walls of the container.

— each collision produce a force.

— the force per unit area exerted on the walls is the pressure of the gas.

— e.g. pressure in a car tyre.

kinetic theory

Fig 1.1.44

2. Liquids evaporate

— liquid molecules are confined to the bulk of the liquid by intermolecular forces.

— a molecules on the surface experience intermolecular forces pulling them back into the liquid bulk.

— a molecule on the surface may have acquired enough kinetic energy through collisions to break the intermolecular forces.

— e.g. water or other liquids left uncovered evaporate into the air.


3. Energy is required to evaporate a liquid.

— liquid molecules have intermolecular forces but gases do not.

— energy is required to break the attractive forces so only molecules with enough kinetic energy can overcome the intermolecular forces and become gaseous.

e.g a warm liquid evaporates faster.

3. Cooling occur when a liquid evaporate.

— temperature is directly proportional to the average kinetic energy of the liquid molecules.

— if the fastest molecules leave, then the average kinetic energy drops, therefore temperature falls, and cooling occurs.

— e.g. alcohol feels cool as it evaporates or water evaporating off the skin at a windy beach feels cold..

4. Liquids evaporate more quickly in a draught.

the liquid molecules that leave the surface of the liquid will be blown away and have less chance of returning.

— more empty space for liquid molecules when molecules blown away.

— e.g. clothes dry faster on a windy day.

5. Larger surface area of a liquid increases evaporation.

— evaporation only occurs at the surface of a liquid.

— larger the surface areaprovides more opportunity for liquid molecules to escape through the surface.

5. Increasing temperature increases evaporation.

— higher temperature means higher kinetic energy as temperature is directly proportional to average kinetic energy of the molecules

— more energetic molecules means greater chance of escaping through the surface.

— e.g. clothes dry faster on a hot day.

6. Gas Laws

— Temperature & pressure - fixed volume.

— heating increases average kinetic energy.

— more kinetic energy means collisions with walls occur more frequently, hence more pressure.

— e,g. heating an aerosol can may increase pressure to cause explosion.

Temperature & volume - fixed pressure.

— When temperature is increased and the molecules move faster there are more frequent collisions with the walls.

— increasing the area of the walls (increase volume) the average force per unit area (pressure) can remain the same.

e.g. a blown -up baloon is cooled it contracts and when heated the volume increases as the pressure remains constant..

— Pressure & volume - fixed temperature.

— Pressure increases as volume decreases.

— More collisions per unit area happen as temperature decreases.

— e.g. using a baloon or bicycle pump slowly.

7. Diffusion.

Since intermolecular forces in gases are negligable, molecules will randomly move away from regions of high concentration to areas of lower concentration.





The following is a simulation from PhET Interactive Simulations
University of Colorado

If you have difficulty running it click here to my Q & A Page



Concept by Kishore Lal. Programmed by Kishore Lal... Copyright © 2015 Kishore Lal. All rights reserved.