Why can hand sanitizer feel cool as it dries?

A squirt of hand gel feels icy the second it lands — but a drop of water from the same warm room just feels… wet. They came from the same room, so how can one feel so much colder? Let's catch the cold being made.

1What "warm" and "drying" really are

Heat is jiggle — and the fastest jigglers fly away

You only need two ideas. Watch each one move:

Warm = fast jiggling

A liquid is a crowd of tiny molecules, always jiggling. The faster they jiggle, the warmer it is. Heat isn't a thing inside — heat is the jiggle.

The fastest ones escape

Now and then a super-fast jiggler at the top leaps into the air. That's a drop drying. And the fastest one leaving means the crowd left behind is a little slower — a little cooler.

2Two kinds of puddle

The clingy puddle vs the runaway puddle

Not every liquid lets its molecules leave at the same speed. Here are the two extremes, side by side:

Water

The clingy puddle

Molecules hold on tight. Only a few escape, slowly — so it dries slowly and barely cools.

Alcohol

The runaway puddle

Molecules barely hold on. They leap off in a rush — so it dries fast and carries away lots of heat.

3Your turn — turn up the warmth

Make the molecules jiggle and watch some fly off

Here's one puddle, up close. Slide the warmth up and watch the molecules jiggle faster — and more of the speedy ones escape off the top. Keep an eye on the puddle's own temperature as they leave.

How fast they jiggle
warm
Escaping each second
a few
Turn up the warmthwarm
COOLHOT

4Now the real test

Can a cover change the cooling?

Imagine two equal alcohol drops at the same starting temperature. Leave the right drop open. Cover the left with a small clear lid that keeps its vapor nearby. Will covering one change how much cooling it causes? Use the model to find out.

Same liquid and starting temperature; change only the coversame start

Guess before you dry them

Which spot will cool more while the uncovered alcohol dries?