- At standard pressure (the pressure exerted by the air at sea level), most materials have three phases: solid, liquid and gas. They can move from one to another in a process called phase change. As you heat up a solid, the molecules move more quickly and lose their rigid structure, becoming a liquid. If you heat up a liquid even more, the molecules will move fast enough to escape their hold on each other and become a gas, dispersing through the air. For example, at 32 degrees Fahrenheit, a block of ice begins to melt and turn into liquid water. At 212 degrees, that same water begins to boil and turn into a gas. Dry ice--the solid form of carbon dioxide--is different. It normally has no liquid phase. Instead it sublimates, turning directly from a solid to a gas.
- Part of what holds a material together is intermolecular forces, but part of it is air pressure. A block of ice sitting in a room, for example, is constantly bombarded by air molecules that push against it. This helps prevent individual molecules of water from flying off into the air (although some actually do). If there were too little air pressure, however, molecules would start popping off in large numbers as soon as the ice reached a certain temperature. It would go directly from a solid to a gas.
- Dry ice behaves differently than water. The boiling point is the freezing point for a substance that sublimates. Its boiling point is much lower than water. If dry ice is heated to -109 degrees Fahrenheit, it starts to boil. More importantly, it requires much more pressure to have a liquid phase than water does. At about 5 atmospheres (5 times the air pressure at sea level on earth), carbon dioxide has a solid, liquid and gas phase. At normal pressure, however, it doesn't.
