Predicting a physical state
The state of a substance at a given temperature can be predicted if its melting point and boiling point are known.
Question
The melting point of oxygen is -218掳C and its boiling point is -183掳C. Predict the state of oxygen at -200掳C.
Oxygen will be in the liquid state at -200掳C (because this is between its melting and boiling points).
Explaining melting points and boiling points
Melting
Particles in a substance have forces of attraction between them. When a substance in the solid state is heated:
- energy is transferred to the particles
- the particles move faster and have more Energy which an object possesses by being in motion.
- some of the bonds are overcome and particles can slide round each other
Boiling
- energy is transferred to the particles
- particles eventually gain enough kinetic energy to overcome the forces of attraction
- the particles can then move apart
The strength of the forces of attraction between particles varies between substances.
The stronger the forces of attraction between particles, the more energy is needed to overcome them, and so the higher the boiling point and the higher the melting point.
Condensing and freezing
Condensation is a change of state in which gas becomes liquid by cooling. and A change of state in which liquid becomes solid by cooling. are the opposite of boiling and melting. During these processes energy is transferred to the surroundings.