States of matter
The freezing point and the melting point are said to be the same, because any increase in temperature from that point will cause the substance to melt, while any drop in temperature will cause it to freeze. Likewise, the vaporization and condensation points (or the sublimation and desublimation points) always match. In most substances, as the pressure increases, the vaporization/condensation point also increases, or vice versa. For example, the boiling point of water decreases as you go up a mountain, because the air pressure is lower. The relationship for freezing and melting can go in different directions depending on the substance.