Why does a cold glass get wet on the outside?
Water vapor already in the air can turn into liquid on the cold surface. The droplets usually come from the surrounding air, rather than passing through the glass.
- The scientific idea
- Condensation
- Fields of science
- Physics
How it works
The droplets outside a glass of iced water have usually arrived from the room. The glass cools the air immediately beside it. If that air is cooled to its dew point, liquid water can begin collecting on the surface. An intact glass can therefore become wet outside without leaking.
Look closer
The same process fogs a bathroom mirror or glasses brought into warm, humid air from somewhere cold. The moisture was already present as invisible water vapor; the newly formed droplets make it visible. Condensation also releases energy to its surroundings, reversing the energy change involved in evaporation. A fogged mirror is both a change of state and a small exchange of heat.