C3 Structure and Bonding: 3.1 States of Matter

Solids - Fixed shape and volume. Cannot be compressed. The particles are very close and remain in a fixed arrangement. They cannot move around, but they vibrate constantly.

Liquids - No fixed shape, but fixed volume. They can flow. Liquids occupy slighty more space than when solid. (Water and Ice are exceptions) They are slightly more far apart, they can move past each other. This results are in a constantly changing, random arrangement of particles. 

Gases - have no fixed shape and no fixed volume and can be compressed easily. Much more space between them, particles move around randomly at high speed. Can be affect by heat. The more frequent the energetic the collisions, the higher the pressure of the gas. 

To explain the properties of solids, liquids and gase, the particle theory is used. It is based on the fact that all matter is made up of tiny particles and describes:

- the movement of particles;

- the average distance between particles.

The hotter a solid is, the faster its particles vibrate. Eventually, the vibrations will be so strong, the faster that its particles begin to break free. At this point, the solid starts to melt and become a liquid.

A liquid turns into a gas at its boiling point. The gas condenses back into a liquid at the same temperature. The hotter a liquid is, the faster its particles move around. As the temperature rises, more and more energy is transferred from the surroundings to the particles escape from the surface of the liquid. Its rate of evaporation increases. Eventually, the liquid boils and bubbles of gas rise and escape from within the liquid.

Each change of state is reversible. They are examples of physical changes. No new substances are formed in changes of state. 

Substances with higer boiling and melting points have stronger bonds between their particles.

When you monitors the temperatures of solids as you heat it to beyond, the temperature stops rising at the solid's melting point. It remains constant untll all the solid has melted, only then it starts to increase.

At its melting point, enough energy is transferred from the surroundings to the solid for the forces between the particles in the solids begin to break. This enables the particles to break away from their fixed positions in the solids and move around. Once all the solid has meltyed the transfer of the energey from the surroundings ro the substance caused the temperature of the liquid to continue to rise as expected.

On the other hand, changes of state in which you cool down a substance involve particles becoming closer toether, such as condensing and freezing. The cooling ceases as the boiling point abd melting point is transferred from the substance to the surroundings as stronger forces form between particles.

This simple particle assumes that particles are made up of solid spheres with no forces operating between them. This is useful when comparing solids, liquids and gases. However, the particles that make up substances are atoms, molecules or ions. They can vary in size from the small He atoms to the polymer molecules in plastics, which can contain may thousand of atoms and are not spherical. 

The interactions between neighbouring atoms, molecules and ions can also disort their shapes. Atoms are mostly empty space, so real particles aren't solid spheres.