Why does hot water make steam? First, one needs to understand what steam is. Steam is “very quick evaporation of water”. Evaporation is when water molecules at the surface of a container of water have enough kinetic energy to leave the container.

Water is made of up many water molecules. If you have an open glass of water. These molecules are all bouncing around the glass, fairly well packed. At the surface of the water, some of the molecules will have enough energy to leave the glass, entering the atmosphere. You can think about it like each molecule is running about, if it is running fast enough and at the surface it can leave. As you heat the glass the number of molecules with enough kinetic energy increases, more evaporation occurs to the point where it is visible, this is steam.

If you heat the water too much then it will start to boil. At this point, all the molecules have enough energy to be a gas and want to leave the glass as quickly as possible.