What happens to steam when it cools down?

When steam cools down, it undergoes the process of condensation. This occurs because as the temperature of the steam decreases, the molecules lose energy and slow down, leading them to come together and transition from the gaseous phase to the liquid phase.

In practical terms, when steam comes into contact with a cooler surface or a cooler environment, it releases heat energy, which causes the formation of water droplets as the vapor condenses. This process is essential in various applications, such as in heat exchangers and in the operation of steam boilers, where controlled cooling and condensation of steam is necessary to maintain efficiency and safety.

The other options do not apply to the behavior of steam cooling. For instance, steam cannot expand when it cools since cooling reduces molecular movement and energy, leading to contraction rather than expansion. The concept of ignition is irrelevant in this context, as it pertains to combustion rather than the physical states of steam or water. Lastly, the idea of steam remaining unchanged contradicts the fundamental principles of thermodynamics, where changes in temperature and pressure will invariably affect the state of the steam.