1. Free and Random Motion of Gas Molecules
Gas molecules are in constant, rapid, and completely random motion. They move in straight lines until they collide with other molecules or with the container walls. After each collision, they change direction and continue moving.
This random motion explains why gases spread out and fill any container.
1.1. Example
When a room freshener is sprayed in one corner, its smell spreads throughout the room. The molecules move randomly and travel in all directions, slowly filling the entire space.
2. Gases Fill All Available Space
Because gas molecules move freely and are far apart, they do not stay in one place. They continuously spread out until they occupy the entire available volume.
2.1. Reason
The weak intermolecular forces in gases allow molecules to move away from each other without resistance. This is why gases do not have a fixed volume.
3. Compressibility of Gases
Gases can be compressed easily because the molecules have large empty spaces between them. Pushing the molecules closer reduces the overall volume of the gas.
3.1. Example
In a bicycle pump, when the piston is pushed, the trapped air gets compressed because the molecules are forced into a smaller space.
4. Effect of Temperature on Gas Behaviour
Heating a gas increases the speed of its molecules. Faster-moving molecules strike the walls more frequently and with greater force, leading to an increase in pressure.
4.1. Observation
A sealed container becomes more pressurised when heated because the molecular motion increases. This is why pressure cookers have safety valves.
5. Effect of Volume on Pressure
When the volume available to the gas decreases, the molecules have less space to move. This increases the number of collisions with the container walls, raising the pressure.
5.1. Simple Example
If air is squeezed into a smaller balloon, it becomes harder because the pressure inside increases.
6. Effect of Number of Molecules
Adding more gas molecules to a container increases the pressure because there are more collisions with the walls.
6.1. Everyday Example
Pumping more air into a football increases its pressure because more molecules crowd inside the same space and collide more often.
7. Diffusion in Gases
Diffusion is the process by which gas molecules mix with other gases without any stirring. Due to their fast motion and large intermolecular spaces, gases diffuse very quickly.
7.1. Example
Smoke from incense sticks spreads throughout a room because smoke particles diffuse through air.
8. Pressure Due to Molecular Collisions
The pressure of a gas is produced by the molecules hitting the container walls. Faster motion or more frequent collisions increases pressure.
8.1. Key Idea
Any factor that increases molecular speed (like heating) or collision frequency (like compressing) increases the pressure of the gas.