It sounds like this“Ice melts by itself once it is warm enough, and melting takes no energy of its own.”
Thinking melting is free once it is warm enough
Why it feels right
It feels as if temperature is the only thing that matters. Ice above 0 degrees melts, below 0 it doesn't, and the melting itself looks like nothing more than a change of shape. Nobody is aware of heat flowing in when a cube vanishes in a drink, so melting seems to cost nothing.
Why it’s wrong
Turning ice into water takes heat: 334 joules for every gram, with no change in temperature. That heat has to come from somewhere, usually the water, the glass and the air around it. Melting is a way of soaking up heat.
A worked example
Melting is a real transformation, and it takes real energy. To melt one gram of ice at 0 degrees into one gram of water at 0 degrees, you must supply 334 joules, which is the same heat that would warm a gram of water by about 80 degrees. The temperature doesn't change while it happens; the energy goes into loosening the bonds that hold the ice together.
Take 100 g of ice at 0 degrees. Melting it all takes 100 x 334 = 33,400 J, or 33.4 kJ. If that heat comes from 300 g of warm water, the water cools by 33,400 / (300 x 4.18), about 27 degrees. That is why a few ice cubes cool a drink so well: most of the cooling is the melting itself, not the cold of the ice.
So whenever ice melts, something nearby gets colder. The only question is what it is. In a glass of ice water, it is the water and the glass; on a road, it is the road and the air.
How to catch yourself
- You say ice melts 'because it is warm' and stop there.
- You expect melting to leave the temperature of its surroundings unchanged.
- You think a cube that melts has had no effect on the drink.
What to hold instead
Melting ice absorbs 334 joules per gram without changing temperature. The heat comes from the surroundings, so they cool as the ice melts. A lot of an ice cube's cooling power is in this melting, not in the ice's own coldness.
Where you’ll meet it
- Wondering why ice cools a drink much more than cold water would.
- Understanding why a glass of ice and water stays at 0 degrees while the ice lasts.
- Seeing why melting snow on a spring day keeps the air chilly.
Questions people ask
- How much heat does it take to melt ice?
- About 334 joules per gram, called the latent heat of fusion. Melting 100 g of ice takes 33.4 kJ, enough to cool 300 g of water by about 27 degrees.
- Why does ice water stay at 0 degrees?
- While ice and liquid water are both present, the heat that flows in goes into melting ice, not into warming the mixture. The temperature holds at 0 degrees until the last of the ice has gone.