The Gaseous State
In gases, the forces between particles are very weak, and the spaces between them are very large. This allows gas particles to move freely and randomly throughout any available space.
1 Shape and Volume
A gas has no fixed shape or volume. It takes the shape and size of the container it is kept in, spreading out to fill every corner.
2 Density
Gases have a much lower density compared to solids and liquids.
3 Melting and Boiling Point
The melting and boiling points of gases are lower than those of solids and liquids. At normal atmospheric pressure, gases generally have melting and boiling points that are below room temperature.
4 Compressibility
Gases can be highly compressed due to the large spaces between their particles. For example:
- When you push the plunger of a syringe containing air, you can compress the air and reduce its volume.
- In contrast, if you try the same with water, you will notice that the plunger doesn’t move much because liquids are nearly incompressible.
This is why gases can be stored in small spaces, like in LPG (liquefied petroleum gas) cylinders, CNG (compressed natural gas) tanks used in vehicles, or medical oxygen cylinders for hospitals.
5 Effect of Heating and Cooling
Gases expand when heated because the gas particles gain more energy, move faster, and spread out. When cooled, gas particles lose energy, slow down, and move closer together, making the gas contract.
6 Diffusion of Gases
Gas particles move quickly and have lots of space between them, allowing gases to mix easily with one another. Examples of diffusion are:
- The smell of perfume spreading across a room soon after the bottle is opened.
- The aroma of cooking reaching your nose from the kitchen.
- The spreading of viruses when a person sneezes.
- The mixing of gases like oxygen and carbon dioxide in the air and oceans.
7 Condensation of Gases
Gas can be turned into a liquid by cooling it down or compressing it. Reducing the temperature or increasing the pressure brings gas particles closer, allowing intermolecular forces to dominate and the gas to condense into a liquid. Examples include:
- Dry ice (frozen CO₂) turning directly from a solid into gas, skipping the liquid state when exposed to air (a process called sublimation).
- Water droplets forming on the outside of a cold glass due to condensation of water vapour present in the air.
8 Pressure Exerted by Gases
Gas particles move constantly in all directions, colliding with each other and with the walls of their container. These collisions create pressure. The pressure exerted by gas particles on the walls of the container is called gas pressure. The pressure of air around us is called atmospheric pressure, and its average value at sea level is 1 atmosphere (1 atm). In SI units, pressure is measured in Pascals (Pa).
📋 Summary Table: Properties of Gases
| Property | Description |
|---|---|
| Shape and Volume | No fixed shape or volume. Gases fill the entire container. |
| Density | Very low compared to solids and liquids. |
| Melting and Boiling Point | Below room temperature at atmospheric pressure. |
| Compressibility | Highly compressible due to large spaces between particles. |
| Effect of Heating/Cooling | Expands when heated, contracts when cooled. |
| Diffusion | Mixes quickly with other gases due to high particle speeds and spaces between them. |
| Condensation | Gas can be turned into liquid by cooling or compressing it. |
| Pressure | Gas particles exert pressure by colliding with the walls of the container (measured in Pascals). |
Quick Recap
- Gases have no fixed shape or volume — they fill any container completely.
- Density of gases is low compared to liquids and solids.
- Gases can be highly compressed due to large spaces between particles.
- Gases expand when heated and contract when cooled.
- Gases diffuse quickly and mix with other gases easily.
- Gas pressure is caused by collisions of gas particles with the walls of a container.
- Gas can be condensed into liquid by cooling or compressing it (e.g., LPG, CNG).