An internal combustion engine provides a good example of the ease with which gases can be compressed. The ratio of the volume of the gas in the cylinder after the first stroke to its volume after the second stroke is the compression ratio of the engine. Finally, the piston is pushed back into the cylinder in the fourth stroke, clearing out the exhaust gases. Liquids are much harder to compress than gases. They are so hard to compress that the hydraulic brake systems used in most cars operate on the principle that there is essentially no change in the volume of the brake fluid when pressure is applied to this liquid.
Most solids are even harder to compress.
The only exceptions belong to a rare class of compounds that includes natural and synthetic rubber. Most rubber balls that seem easy to compress, such as a racquetball, are filled with air, which is compressed when the ball is squeezed. Expandability Anyone who has walked into a kitchen where bread was baking has experienced the fact that gases expand to fill their containers, as the air in the kitchen becomes filled with wonderful odors. Because gases expand to fill their containers, it is safe to assume that the volume of a gas is equal to the volume of its container. The difference between the volume of a gas and the volume of the liquid or solid from which it forms can be illustrated with the following examples.