The melting point of any pure substance is the temperature at which, under common atmospheric pressure, that substance changes its state from solid to liquid. If the substance is a liquid at 25 degrees Celsius, this temperature is usually called the ''freezing point''. Some substances, all of them compounds, will simply decompose before they ever melt. | The melting point of any pure substance is the temperature at which, under common atmospheric pressure, that substance changes its state from solid to liquid. If the substance is a liquid at 25 degrees Celsius, this temperature is usually called the ''freezing point''. Some substances, all of them compounds, will simply decompose before they ever melt. |