Why does hand sanitizer feel freezing cold but water doesn't?
After you watchWhy does hand sanitizer feel freezing cold but water doesn't?
The short answer
Hand sanitizer feels freezing cold because the alcohol in it evaporates very fast, and every molecule that leaps off your skin carries a little heat away with it. Water feels barely cool because it evaporates much more slowly, even though both start at the same temperature.
Try this next
- What if you blow air across the alcohol while it dries? In the lab, picture moving air over the drop. Predict first: does fresh air sweep escaping molecules away faster and chill your skin more, or does it slow things down?
- What if you used a much bigger drop of alcohol? Predict whether a bigger puddle makes the cold last longer or feel colder, then watch the skin-temperature line: more liquid means more molecules left to keep leaping off.
- What if the room were really humid? Try the real-world check: water dries slower on a sticky, humid day. Predict how that changes the chill before you test a wet hand indoors versus on a dry day.
The whole story
How it works
A liquid is a crowd of jiggling molecules, and the faster they jiggle the warmer it is. To escape into the air, a molecule has to grab energy and break free, and it pulls that energy as heat from the liquid and skin it leaves behind. Alcohol's molecules barely cling together, so they fly off in a rush and yank heat away quickly, cooling your skin fast. Water's molecules hold on tightly, so only a few leave at a time and your skin barely cools.
What people get wrong
Many people think a liquid feels cold because it is colder than the other one. But a thermometer shows water and alcohol from the same room start at the exact same temperature. The chill is made after the liquid touches you, by how fast it evaporates, not by the drop being colder to begin with.
The catch
Fast-evaporating alcohol cools you in a flash and dries with no towel, but the cold is gone in seconds once there is nothing left to evaporate, and it dries out and stings your skin. Slow-evaporating water clings for a long time, so sweat can keep cooling you all afternoon, but it is slow to cool and slow to dry.
Questions kids ask
If both drops start at the same temperature, why does alcohol feel colder?
Because alcohol evaporates much faster. Each molecule that escapes into the air carries a bit of heat away from your skin, so the fast rush of escaping alcohol molecules cools your skin quickly, while slow water barely cools it at all.
Does evaporation always make things colder?
Yes. Any time a liquid evaporates, the escaping molecules carry heat away, so the surface left behind gets cooler. That is exactly why sweating cools you down and why you feel cold stepping out of a pool into the wind.
Why does the cold feeling from sanitizer go away so fast?
Because the alcohol dries up quickly. Once almost all of it has evaporated, there is nothing left to carry heat away, so your skin warms back up to normal in a few seconds.
Why does water evaporate so much slower than alcohol?
Water molecules pull on each other strongly and hold together, so it takes more energy for one to break free and fly off. Alcohol molecules barely hold on, so they escape into the air far more easily and faster.
Talk about it
- Both drops start the same temperature on the thermometer — so where does the cold actually come from? Guess first.
- Why do you think we shiver getting out of a warm pool on a breezy day?
- If alcohol cools your skin by leaving, what does that tell us about what your sweat is doing all summer?
For grown-ups
This is evaporative cooling. To vaporize, a molecule must absorb its latent heat of vaporization, which is drawn from the remaining liquid and the skin, lowering their temperature. Alcohol (ethanol or isopropyl) has weaker intermolecular forces and a much higher vapor pressure than water, so it evaporates far faster and removes heat much more quickly. It's the same physics behind sweating, an unglazed clay water pot, and a swamp cooler.
Keep going
What else makes you wonder?
- Why does blowing on a wet hand make it feel even colder?
- Could a liquid that evaporates super fast freeze your skin?
- Why do dogs pant and people sweat instead of just evaporating water everywhere?