O Level Notes : Chemistry - The Particulate Nature of Matter

A matter is any substance that has mass and occupies space. There are three states of matter: solid, liquid and gas. Most of the matters can exist in all three states.

I). States of Matter:

Matter can change from one state to another due to changes in temperature or pressure. A common example is water: on room temperature it is liquid, on freezing it becomes solid (ice), and on boiling it becomes gas (water vapour).

 

Property

Solid

Liquid

Gas

Shape

Fixed

Not fixed

Not fixed

Volume

Fixed

Fixed

Not fixed

Compressibility

Cannot be compressed

Cannot be compressed

Can be compressed

Table 2.1. Properties of solids, liquids and gases.

 

II). Kinetic Particle Theory:

The kinetic particle theory states that all matter is made up of tiny particles, and that these particles are in constant, random motion. However, the movement of these particles varies according to the state of matter.

 

The Solid State:

The forces of attraction between the particles of solid are very strong, so they are held very close together and cannot move about freely. They have very less kinetic energy, enough to enable them to vibrate about their fixed positions. This is why a solid has a fixed shape.

As these particles are already very closely packed, a solid cannot be further compressed, which is why it has a fixed volume.

 

The Liquid State:

The forces of attraction between the particles of a liquid are weaker than those in a solid. These particles are not held in fixed positions, but instead arranged in a disorderly manner, and, due to higher kinetic energy as compared to solid, can move freely by sliding over each other. This is why a liquid does not have a fixed shape.

The particles of a liquid are further away from each other as compared to those in a solid, but are still packed quite closely together. Thus a liquid cannot be compressed and has a fixed volume.

 

The Gaseous State:

The particles of a gas have high kinetic energy and very low forces of attraction, therefore they are not held in fixed positions. They can move about rapidly in any direction, therefore a gas has no fixed shape.

A gas takes up as much free space as provided to it, which is why there are large distances between particles. These spaces make it possible for the gas to be compressed, or forced into coming closer. Therefore, a gas has no fixed volume.

 

 

III). Changes in State:

 

Solid to Liquid: Melting

Liquid to Solid: Freezing

 

Liquid to Gas: Evaporation or Boiling

Gas to Liquid: Condensation

 

Solid to Gas: Sublimation

Gas to Solid: Solidification

 

Melting and boiling points are important in identifying a substance and testing its purity. Pure substances have definite melting and boiling points. The presence of impurities will cause boiling points to rise and melting points to fall.

  • The constant temperature at which a pure solid changes into a liquid is called its melting point.
  • The constant temperature at which a pure liquid changes into a gas is called its boiling point.

 

 

 

Heating curve of a pure substance:

 

 

 

 

 

 

 

Cooling curve of a pure substance:

 

 

Note: the parts in both graphs that do not show any temperature change are the parts in which the state changes occur. In them, both states occur together.

 

 

IV). Diffusion:

Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration. As it involves a movement of particles, it is only possible in liquids and gases. Gases diffuse faster than liquids as they are generally lighter.

 

Gases take up whatever space is available to them. This is because of diffusion. This phenomenon is shown in the figure below.