Looking for a Gas

Gas is everywhere. There is something called the atmosphere. That's a big layer of gas that surrounds the Earth. Gases are random groups of atoms. In solids, atoms and molecules are compact and close together. Liquids have atoms that are spread out a little more. Gases are really spread out and the atoms and molecules are full of energy. They are bouncing around constantly.

Gases can fill a container of any size or shape. It doesn't even matter how big the container is. The molecules still spread out to fill the whole space equally. That is one of their physical characteristics. Think about a balloon. No matter what shape you make the balloon, it will be evenly filled with the gas molecules. The molecules are spread equally throughout the entire balloon. Liquids can only fill the bottom of the container, while gases can fill it entirely. The shape of liquids is really dependent on the force of gravity, while gases are light enough to have a little more freedom to move.

You might hear the term "vapor." Vapor and gas mean the same thing. The word vapor is used to describe gases that are usually found as liquids. Good examples are water (H2O) or mercury (Hg). Compounds like carbon dioxide (CO2) are usually gases at room temperature, so scientists will rarely talk about carbon dioxide vapor. Water and mercury are liquids at room temperature, so they get the vapor title when they are in a gaseous phase.
Compressing Gases

Gases hold huge amounts of energy, and their molecules are spread out as much as possible. With very little pressure, when compared to liquids and solids, those molecules can be compressed. It happens all of the time. Combinations of pressure and decreasing temperature force gases into tubes that we use every day. You might see compressed air in a spray bottle or feel the carbon dioxide rush out of a can of soda. Those are both examples of gas forced into a smaller space than it would want, and the gas escapes the first chance it gets. The gas molecules move from an area of high pressure to one of low pressure.