Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)
8. Chemistry in the World
Why does snow melt?
It’s a matter of balance. In chemistry, the word for balance is equilibrium. If something is in equilibrium, the amounts of things in it stay unchanged.
A snowflake is in equilibrium between solid water and liquid water. The bulk of the snowflake is solid, but at the surface there is a layer of liquid water, and some of that surface water escapes to become the gas water vapor. Molecules of water can also escape directly from the solid ice into the air to become water vapor without becoming a liquid first.
What keeps the snowflake intact is equilibrium. For each water molecule that leaves the ice to become water or water vapor, there is a molecule of water or water vapor that crystallizes onto the surface of the ice.
If the balance is disturbed, more atoms will solidify, forming more ice, or more atoms will liquefy, forming more water. The balance (or shift in the equilibrium) can be changed by changing the temperature or changing the pressure.
Heat is the motion of molecules. If heat is added to something, it raises the temperature, which is the speed of the molecules. In ice, the molecules are bouncing around against one another, vibrating back and forth. If the speed at which they bounce is increased, they can jostle loose and become liquid water.
If the pressure is increased, the water molecules on the outside of the ice are pushed back toward the ice, where they have a better chance of sticking to the other molecules there. So increasing the pressure makes more ice, and decreasing the pressure makes more water or water vapor.