Final answer:
The refrigerant that enters the compressor is a gas, which is later condensed to a liquid. Gases are easier to compress than liquids and solids because of the larger spaces between their molecules and weaker intermolecular forces. Gases and liquids can both be considered fluids, but they differ in their volume and ability to hold shape.
Step-by-step explanation:
The refrigerant that enters the compressor under normal operating conditions is a gas. This gas is compressed and its temperature and pressure are increased. When it is later moved to the condenser, the gas condenses into a liquid. This liquid is then cooled as it goes through an expansion valve to the outdoor evaporator coils, where it evaporates back into a gas, absorbing heat and thus providing the cooling effect.
Considering other substances at room temperature, air and mercury are both fluids, as air is a gas and mercury is a liquid at room temperature. Water is also a fluid in its liquid state. In contrast, glass remains a solid under standard room temperature conditions.
Gases are much easier to compress than liquids and solids due to the larger spaces between their molecules. Solids have strong intermolecular forces and a fixed shape, making them almost incompressible. Liquids are slightly compressible because they have weaker intermolecular forces than solids, but still a definite volume. Gases have very weak intermolecular forces, no definite shape, nor a definite volume, thus they can be compressed more easily.
The key difference between gases and liquids is that gases can expand to fill their container and have highly variable shapes and volumes, whereas liquids have fixed volumes but adapt to the shape of their containers. These differences are due to varying strengths of intermolecular forces and the amount of kinetic energy in the particles.