Why can hand sanitizer feel cool as it dries?
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After you watchWhy can hand sanitizer feel cool as it dries?
The short answer
Evaporating liquid carries energy away. Alcohol in sanitizer can evaporate quickly, so it can draw energy from skin and feel cool. The surroundings affect evaporation as well as the liquid itself.
Try this next
- How might moving air change evaporation? Make a prediction and explain what you would keep the same in a fair comparison.
The whole story
How it works
Molecules escape from a liquid surface into vapor. In an open room, vapor can disperse. Under a cover it builds up and more molecules return, reducing net evaporation and its cooling effect. The animation uses relative meters rather than predicted skin temperatures.
What people get wrong
A liquid need not start colder to cool its surroundings as it evaporates. Two equal alcohol drops can cause different cooling if one is covered and vapor is trapped nearby.
The catch
Rapid evaporation cools, but alcohol products can dry or irritate skin and are flammable. Use the on-screen model; do not heat or deliberately trap sanitizer on skin to copy it.
Questions kids ask
Why can hand sanitizer feel cool as it dries?
Evaporating liquid carries energy away. Alcohol in sanitizer can evaporate quickly, so it can draw energy from skin and feel cool. The surroundings affect evaporation as well as the liquid itself.
What does the experiment compare?
Predict how a small vapor-trapping cover changes cooling from an alcohol drop, then compare covered and uncovered equal drops using qualitative cooling meters.
What is this model leaving out?
Rapid evaporation cools, but alcohol products can dry or irritate skin and are flammable. Use the on-screen model; do not heat or deliberately trap sanitizer on skin to copy it.
Talk about it
- Two equal alcohol drops at the same temperature: will a covered spot or an open spot cool more as the open drop dries?
- What changed in the pictures that supports your idea?
- How might moving air change evaporation?
For grown-ups
Evaporation requires energy, and vapor pressure and mass transport help determine the net rate. As a closed headspace approaches equilibrium, returning molecules increasingly offset evaporation. Real skin cooling also depends on conduction, airflow and composition; the meters are qualitative.
Keep going
What else makes you wonder?
- How might moving air change evaporation?
- What would you change in the experiment next?
