How do you boil ice water? (I think it has something to do with a vacuum.) – MW
You’re right, it does have to do with a vacuum. While water molecules can evaporate from the surface of liquid water at almost any temperature, boiling can only occur when the evaporation rate is high enough to support the appearance of evaporation bubbles inside the body of the liquid water. Normally, atmospheric pressure pushes inward on cold water so hard that any evaporation bubble that appears inside the water is immediately crushed out of existence. But in water that’s at 100° C, evaporation is so rapid that the evaporation bubbles in the liquid water can survive and grow, despite the crushing inward forces of atmospheric pressure. The hot water boils.
Water boils not because it’s hot but because any evaporation bubble that forms inside it is able to survive and grow despite the surrounding atmospheric pressure. At normal atmospheric pressure, the water does have to be hot for this to happen. But if you remove the atmospheric pressure, the water can boil at much lower temperatures. In fact, at sufficiently low pressures, even ice water will boil! It’s funny to see ice cubes floating in a container of boiling water, but it happens when you remove the air from around the ice water.