Ice doesn't pour cold into your glass, because cold isn't a thing you can pour. What ice does is the opposite: it drinks up the heat from your drink. Melting takes a lot of energy, and the ice steals that energy from the warm liquid around it. The drink loses its heat to the ice, so the drink gets colder. And here's the neat part — while the ice is busy melting it stays stuck at 0 °C, no matter how much heat pours in. Press play in the simulator and watch the heat flow into the ice and the drink's temperature fall, then flatten out at zero.
Most people think a cold cube pours cold into the glass. In fact cold is not a thing that flows; melting ice soaks heat out of the warmer drink, absorbing about 334 joules per gram, and the mix holds at 0 °C until the last gram has melted.
What's actually happening
Almost everyone pictures it backwards. We imagine the ice cube as a little reservoir of cold that leaks out into the drink, the way a drop of dye spreads through water. But cold isn't a substance and it doesn't flow. There is only heat, and heat only ever moves from warmer to cooler. So the real story runs the other way: the warm drink is the one giving something up, and the ice is the one taking it.
What the ice takes the heat for is melting. To turn solid ice into liquid water you have to prise apart the rigid lattice the molecules are locked into, and that costs a startling amount of energy — about 334 joules for every single gram, the latent heat of fusion. Strangely, none of that energy makes the ice hotter. Every joule goes into breaking bonds, not into raising the temperature, which is why a glass of ice and water sits patiently at 0 °C and refuses to budge while ice remains. The drink, meanwhile, keeps feeding its warmth into that melting process and cools as it does. Only when the very last sliver of ice has melted does the temperature start to climb again.
That latent-heat trick is why ice is such a good coolant, and why a slushy half-melted drink feels colder for longer than one chilled in the fridge to the same temperature. The 334 joules-per-gram is also why frozen things thaw so slowly, why ice keeps a cooler box cold for a whole afternoon, and why an unstirred drink with ice in it will hover near 0 °C until the cubes vanish. Your drink isn't being given cold. It's being relieved of heat, one melting gram at a time.
Your drink is not given cold, it is relieved of heat one melting gram at a time, which is why a slushy drink stays cold for so long.
- 1Fill a glass with ice and a splash of water, stir, and read its temperature with a kitchen thermometer every minute.
- 2It locks onto 0 °C and stays there — while ice remains, all the room heat coming in is spent melting it, not warming the water.
- 3The instant the last piece of ice melts, the temperature finally starts to rise. You have just watched latent heat on a kitchen thermometer.
Common questions
Every joule of incoming heat goes into breaking the ice's bonds rather than raising temperature, so the mix pins at 0 °C. Only once the very last sliver of ice has melted does the temperature start to climb again.
A half-melted slush still has melting left to do, and melting is where almost all the cooling power hides. So it holds a drink near 0 °C far longer than one chilled in the fridge to the same temperature.
The 334 joules per gram means a block of ice must absorb a huge amount of warmth before it even fully melts. That large per-gram energy cost is what makes ice such an effective, long-lasting coolant.