Under these conditions the deuterium becomes ionized and is no longer a gas. It becomes a plasma. Change of Phase An in-depth discussion of the changes in phase of matter will be covered in Lesson 12, since these changes quite often involve heat, temperature changes, and changes in energy. At this point we will discuss just the basic definitions involved in changes of phase. A liquid may change into a gas or into a solid.
Gases usually change into the liquid state.
Melting is the change of phase from a solid to a liquid. Solidification , or freezing, occurs when a liquid turns into a solid. We call the change freezing when the temperature at which the liquid turns into a solid is fairly low. We normally don't imagine metals as freezing when they turn solid at temperatures of several hundred degrees on the Celsius scale. In this case we would say they solidify.