Put the Kettle On

Bring water to a boil in a teakettle with a spout. Let it cook! Now watch the mouth of the spout carefully. What do you see? Can you see the water vapor come out?

Answer

No, you cannot see water vapor, that is, water molecules in their gaseous state. If you look closely at the orifice of the spout, there is a clear region perhaps up to one inch long. That’s where the water vapor is before it condenses into the steam you can see. The temperature of the vapor in the clear region is still too high for droplets of steam to form that is, collisions of water molecules are too violent to allow them to bind together to form droplets.

In the clear region at the orifice of the kettle, the water molecules are moving so rapidly that when they do collide, the van der Waals attractive force an induced dipole-dipole electromagnetic interaction at close range cannot keep them together. As the water vapor cools farther away from the end of the spout, these same collisions produce droplets that grow in size.