## Properties of matter

We are going to use what we have learnt in chemical bonds, intermolecular forces, and kinectic theroy of matter to understand the macroscopic properties of matter.

## Melting point

Melting point is the temperature at which a solid changes its phase or state to become a liquid. The reverse process (change in phase from liquid to solid) is called freezing.

In order for a solid to melt, the kinetic energy of the particles must increase enough to break the bonds that are holding the particles together.

Solid which is held together by strong bonds will have a higher melting point than one where the bonds are weak, because it will require more energy  to break the bonds.

The intermolecular forces between molecular solids are weaker than those between ionic and metallic solids.

In the examples we have looked at, metals, ionic solids and some atomic lattices (e.g. diamond) have high melting points, whereas the melting points for molecular solids and other atomic lattices (e.g. graphite) are much lower.

## Boiling point

Boiling point is the temperature at which a liquid changes to a gas.

The average kinetic energy of the particles tends to increase the temperature of a liquid increases, therefore are able to overcome the bonding forces that are holding them in the liquid.

When boiling point is reached, evaporation takes place and some particles in the liquid become a gas. In other words, the energy of the particles is too great for them to be held in a liquid anymore.

Metal generally have higher boiling points than liquids because they have stronger bond force. The stronger the bond the higher the boiling point

## Density and viscosity

Density is the mass of a substance per unit volume. Solids usually have higher density than liquids because solid particles are much more closely together and therefore there are more particles packed together in a particular volume. In other words, there is a greater mass of the substance in a particular volume.

In general, density increases with increase in intermolecular force

Viscosity is a measure of how resistant a liquid is to changing its form. Viscosity is also sometimes described as the ’thickness’ of a ﬂuid. Think for example of syrup and how slowly it pours from one container into another. Now compare this to how easy it is to pour water. The viscosity of syrup is greater than the viscosity of water. Once again, the stronger the intermolecular forces in the liquid, the greater its viscosity.

It should be clear now that we can explain a lot of the macroscopic properties of matter (i.e. the characteristics we can see or observe) by understanding their microscopic structure and the way in which the atoms and molecules that make up matter are held together.