Explain, in terms of molecules, why it is possible to compress a gas, but not a liquid.

The difference in compressibility between gases and liquids comes down to the behavior and arrangement of their molecules.

1. Molecule Arrangement: In a gas, the molecules are spread out and have a lot of space between them. They are in constant, random motion and frequently collide with each other and the walls of their container. In a liquid, the molecules are much closer together. They still move and collide, but there is less space for them to move into.

2. Compressing Gases: When you compress a gas, you're essentially forcing the gas molecules closer together. Because there was a lot of space between the molecules to begin with, it's relatively easy to decrease the volume of a gas by applying pressure.

3. Compressing Liquids: In a liquid, the molecules are already close together. There's not much extra space for them to move into, so it's much harder to decrease the volume of a liquid by applying pressure. In other words, liquids are essentially already compressed, which is why they are much less compressible than gases.

4. Intermolecular Forces: The forces of attraction between molecules in a liquid are stronger than in a gas, which also contributes to the incompressibility of liquids. These forces keep the molecules close together and resist attempts to compress them further.

In conclusion, the compressibility of a substance is determined by the arrangement and behavior of its molecules. Gases are more compressible than liquids because their molecules are further apart and move more freely.