1.41 understand that substances with simple molecular structures are gases or liquids, or solids with low melting points

Simple molecular substances

Both elements and compounds can exist as simple molecular substances.

Simple molecular substances usually have low melting and boiling points.

This is because the forces of attraction between oppositely charged ions, (i.e. ionic bonds) and therefore very little energy is required to overcome them.

Although, when simple molecular substances change state, the covalent bonds between the atoms are not usually broken. Covalent bonds are strong compared to the forces of attraction between the molecules.

TRIPLE: 1.35 understand the relationship between ionic charge and the melting point and boiling point of an ionic compound

The relationship between the ionic charge and boiling/melting points

For ions of similar size, the strength of the forces of attraction between the ions will depend on the size of their charge. For this reason, magnesium oxide (Mg²+O²-) has a higher melting and boiling point than sodium chloride (Na+Cl-). The more electrons that have been transferred, the stronger the bond because of the stronger electrostatic forces.
 

States of Matter 1.2 & 1.3 understand how the interconversions of solids, liquids and gases are achieved and recall the names used for these interconversions

Solid to Liquid- (melting) 

The particles gain kinetic energy and vibrate faster and faster. This allows the particles to overcome forces off attraction that holds them in place in the state of a solid. The regular pattern is broken down and the particles are free to 'slide over' each other.

Liquid to Solid- (freezing)

The particles lose kinetic energy and this allows the forces of attraction between the particles to hold them together. The particles arrange themselves into a regular pattern and cannot slide over each other.

Liquid to Gas- (boiling)

The particles gain kinetic energy and move further apart. Eventually the forces of attraction between the particles are completely destroyed  and are then able to escape from the liquid.

Gas to Liquid- (condensing) 

The particles lose kinetic energy and this allows the forces of attraction to bring the particles closer together. The particles eventually clump together to form a liquid.

Solid to Gas- (sublimation)

The particles gain kinetic energy and vibrate faster and faster. Eventually the forces of attraction between particles are completely broken and they are able to escape from the solid.