Heat must be supplied to a substance for it to melt, evaporate or boil. For example, you need to heat ice to melt it, and you need to heat water to make steam.
Heat must be removed from a substance to condense or freeze it. In other words, the substance must be cooled down.

Under certain conditions, some solids turn straight into a gas when heated. This process is called sublimation. A good example is solid carbon dioxide, also called ‘dry ice’. At atmospheric pressure, it turns straight into gaseous carbon dioxide.

Liquid carbon dioxide can only exist under high pressure, such as in fire extinguishers. Iodine also sublimes - it turns directly from shiny purple-black crystals to a purple vapour when warmed up.

	Created by Hisham Mahmoud over 2 years ago

IGCSE IGCSE CHEMISTRY 0620 Note on Changes in State of Matter, created by Hisham Mahmoud on 08/05/2022.