Why does the mirror fog?

Cause: When water vapor is cold, its temperature drops, and it liquefies into small water droplets and adheres to objects.

The process of changing a substance from a gas state to a liquid state will give off heat to the outside world. Liquefaction is the reverse process of vaporization. Gas molecules attract each other and condense into liquid. When a substance is liquefied, it gives off heat. Gases below the critical temperature can be liquefied.

There are two ways to realize liquefaction, one is to reduce the temperature, and the other is to compress the volume. The method of cooling is omnipotent, and it can be liquefied if it is low enough. However, when compressing the volume, if the gas temperature is higher than its critical temperature, it cannot be compressed to liquefy it.

Extended data:

Similar to the example of mirror fogging:

1, drink boiled water, and the lenses are blurred when eating.

2, winter nights, after returning home from self-study at night, the lenses are blurred.

3. When the air-conditioned room comes out in summer, the lens is blurred. In winter, the classroom glass is blurred during self-study at night (small drops of water are in the glass window).

4. In summer, the glass of air-conditioned room may be blurred (small water drops are outside the glass window).

5. When the doctor examines the oral cavity, he should bake the small mirror put in the oral cavity to avoid the water vapor in the oral cavity from liquefying and blurring the mirror.

Baidu encyclopedia-liquefaction