When air is compressed, its temperature:

When air is compressed, its temperature increases due to the principles of thermodynamics. This phenomenon occurs because compressing air forces the molecules closer together, which in turn increases their kinetic energy. As the kinetic energy of the molecules rises, the temperature of the air also rises. This is an application of the ideal gas law and the adiabatic process, where, in the absence of heat exchange with the environment, work done on the gas raises its temperature.

To understand this further, consider that during the compression process (such as when using a piston), the work done on the air results in energy transfer that manifests as increased molecular motion, thus elevating the temperature. This principle applies across various scenarios, such as in compressors and engines where air is frequently compressed.

The other options do not accurately reflect this relationship. For instance, while one might assume that temperature could remain constant or decrease during compression, this contradicts the basic physical laws governing gas behavior. Hence, the increase in temperature during compression is a well-established concept in physics and engineering.