Carbon dioxide (CO2), like water and most other pure substances, exists in solid, liquid, and gaseous states and can undergo changes from one state to another. Solid CO2, however, has an interesting property: at normal pressures, it passes directly to the gaseous state without first melting to the liquid state. This property, together with the fact that this change occurs at -78°C, makes solid CO2 useful for keeping materials very cold. Because solid CO2 cools other objects and does not leave a liquid residue, it is called “dry ice”. As for liquid CO2, it is obtained by putting carbon dioxide gas under pressure. When liquid CO2 evaporates, it absorbs large quantities of heat, cooling as low as -57°C. Because of this property, it is often used as a refrigerant. If the compressed gas from the evaporating CO2 liquid is allowed to expand through a valve, the rapidly cooled vapour forms solid carbon dioxide “snow”. This CO2 snow is compacted into blocks and is the source of dry ice.