What rule can you state about the relationship between phase changes and temperature?

Generally a higher temperature is needed to get from liquid to gas than to get from solid to liquid. i.e boiling point is higher than melting point

Think of water as an example substance. It starts as Ice. As it is warmed from a very low temperature (< 0 C) to a higher temp, it absorbs heat from its surroundings (approx 2.06 J/g °C)). When it reaches its melting point (0 C), it continues to absorb heat from its environment, but its temperature remains constant as it melts. At first you'll have a mixture of water and ice, eventually it will all turn to ice. The heat that's absorbed as it melts is called the latent heat of fusion (333.55 J/g) . As water is warmed further, it will absorb heat from its surroundings (approx 4.18 J/g °C)) until it reaches its boiling point (100 C). At its boiling point, it will continue to absorb heat from its surroundings but the temperature will remain constant as it boils and turns from water to steam. The heat that's absorbed as it evaporates (boils) is called the latent heat of vaporization (2257 J/g). Once all of the water has been converted to steam (gas), the temperature of the steam will increase as it absorbs heat from its surroundings (approximately 1.87 J/g °C).

Phase changes occur at the melting point (freezing point) or the boiling point (condensation point). The boiling point is higher than the melting point. The melting point and boiling point are dependent on pressure.