This page relates to the physical factors affecting chemical processes..

Matter exists mainly in three states gas, liquid and solid.  A material such as water can exist as a gas (steam) a liquid (water) or a solid (ice).   Its state results from its condition of temperature and pressure.

A gas has no bounding surface it fills all of the space it occupies.   Its density is very much related to the quantity of matter the temperature and the pressure.

A liquid has no shape. It takes the shape of the vessel it occupies.   It does however have a surface which limits the volume it occupies.   It is normally has a greater density than a gas. Its volume and related density is affected to some extent by temperature and to a lesser effect by pressure. Liquids are often assumed to be incompressible

A solid has a fixed shape and its volume changes by only a relative small amount on changes of pressure and temperature.

In normal conditions a solid can be converted to a liquid if its temperature is raised to its melting point temperature and a liquid can be converted to a gas if its temperature is raised to its evaporation tempature which is related to the local pressure..