What are the key distinguishing properties of a gas?

Gases are distinguished by their ability to fill any container, their low density, and their compressibility.

Gases have unique properties that set them apart from solids and liquids. One of the most noticeable characteristics of gases is their ability to fill any container, regardless of its shape or size. This is due to the fact that gas particles are in constant, random motion and spread out to fill the available space. This property is often referred to as the 'expansibility' of gases.

Another key property of gases is their low density. Density is defined as mass per unit volume. In gases, the particles are far apart from each other, resulting in a large volume for a small amount of mass. This is why gases are lighter than solids and liquids of the same substance. For example, air, which is a mixture of gases, is much lighter than water or rock.

Gases are also highly compressible. This means that under pressure, the volume of a gas decreases significantly. This is again due to the large spaces between gas particles. When pressure is applied, these spaces are reduced, causing the gas to occupy less volume. This property is utilised in many practical applications, such as filling air into tyres or gas cylinders.

Lastly, gases have the property of diffusibility. This means that they can mix with other gases without requiring any mechanical agitation. This is why, for instance, when you spray perfume in one corner of a room, it eventually spreads throughout the entire room. This property is due to the high kinetic energy and rapid, random motion of gas particles.

In summary, the key distinguishing properties of gases are their ability to fill any container, their low density, their compressibility, and their ability to diffuse and mix with other gases. These properties are a result of the large spaces between gas particles and their constant, random motion.