Heating curves, melting and boiling points

At a given pressure, every pure substance melts and boils at its own set temperatures, called its melting point and boiling point. Water boils at 100 °C only at sea level. Higher pressure raises the boiling point: in a pressure cooker, steam is trapped, the water boils above 100 °C and food cooks faster (adults only use it). Lower pressure lowers the boiling point: on a mountain, water boils at a lower temperature, about 1 °C lower for every 300 m, so food takes longer to cook. Impurities also change the points: a salty solution boils a little higher and freezes lower than pure water, which is why salt is spread on icy roads. Because pure substances have fixed points, we can check purity by measuring a melting or boiling point and comparing it with the known value. To tell the state from a table: below the melting point a substance is solid, between the two points it is liquid, above the boiling point it is a gas.