Explain why fog is often seen on the river.

A layer of fog is often seen on the river because water passes through: the process of evaporation first and then liquefaction;

Analysis: This is a common natural phenomenon, which is caused by more water vapor evaporated from the river. Once the temperature drops, it will liquefy into small droplets when it is cold, which looks like white fog. Therefore, the river first experienced the evaporation process, and then the liquefaction process.

Explanation: We often encounter some statements such as white fog, water mist and white gas in physics problems. Students should be careful not to confuse similar words with water vapor when they see them. Water vapor is a colorless gas that we can't observe at all. What is mentioned in the above statement is not real gas, but small liquid beads liquefied by water vapor, which is a liquefaction phenomenon.