What is the temperature at which a substance changes from a solid to a liquid?
The melting point of substances depends of different factors and conditions.
The question is not appropriate, because different substance change from solid to liquid at different temperature. For e.g. Water changes from Ice to liquid water at 32 °F or 0 °C while Gold changes from solid to molten liquid at 1064.18 °C or 1947.52 °F.
Some substances changes from solid to gas directly under certain conditions for e.g. Napthalene in a process called Sublimation