Freezing Point vs. Amounts of Material

Does freezing point of a substance depend on the amount you have?
- Anthony (age 14)
Omaha, NE, USA

For big amounts the freezing point doesn't depend on the amount.

For extremely small amounts, in principle, there's no well-defined freezing point since true phase transitions (like liquid <--> solid) only exist in the limit of infinitely large systems. In other words, if you had say 10 water molecules together, they wouldn't be clearly in an ice-like or liquid-like state. 

In practice, for in-between amounts you can get something very much like a phase transition in a drop so small that the freezing temperature is slightly lowered. For example, a drop of water about 100nm across would (in equilibrium) go pretty abruptly from liquid-like to solid-like at a temperature just below 0°C. (That's around 10,000,000 molecules.) The reduction in the melting/freezing point goes as the inverse of the diameter of the drop. It's called the Gibbs-Thomson effect, as described here: https://en.wikipedia.org/wiki/Gibbs%E2%80%93Thomson_equation. By the time you get to a drop big enough to see, the effect is extremely small.

Mike W.

(published on 12/12/2015)