thermodynamics - How do whisky stones keep your drink cold?

From a discussion in the DMZ (security stack exchange's chat room - a place where food and drink are important topics) we began to question the difference between how ice and whisky stones work to cool drinks.

Both are frozen, but when ice is placed in a drink it slowly melts, using energy from the drink, thus cooling it.

But whisky stones don't melt, so how do they cool the drink?

Answer

Ice cubes have three distinct cooling effects:

1. The cube, initially at sub-zero temperature, absorbs some heat to reach fusion point (0⁰C).


2. The cube absorbs more heat to switch phase: it takes some energy to turn 1 kg of ice at 0⁰C into 1 kg of liquid water at 0⁰C.


3. The water absorbs some heat to become warmer than 0⁰C.

The three effects occur more or less successively, although not necessarily simultaneously throughout the ice cube. But the idea remain the same.

For ice, the bulk of the cooling comes from the melting. Let's put some figures on it. Heat capacity of ice is 2.06 kJ·kg^-1·K^-1, meaning it takes 2.06 kJ to transform 1 kg of ice at -12⁰C into 1 kg of ice at -11⁰C. For liquid water, that's 4.217 kJ·kg^-1·K^-1. The latent heat, i.e. energy used for turning ice into liquid water (at constant temperature) is 333 kJ·kg^-1. Imagine that you have some beverage at room temperature, which you want to lower to 8⁰C with ice cubes. The ice cubes come from the freezer and are initially at -18⁰C. The three cooling effects amount to, per kg of ice:

1. Raising ice temperature to 0⁰C: 18×2.06 = 37.08 kJ.



2. Melting the ice: 333 kJ.


3. Raising the water temperature to 8⁰C: 8×4.217 = 33.736 kJ.

So, in this example, the melting contributes to about 82% of the cooling.

Non-melting stones work only on heat capacity. So they are effective only insofar as a material with high heat capacity is used -- but, in practice, water (and ice) have a quite high heat capacity, higher than stones, so the cooling effect of such stones is necessarily quite reduced compared to ice cubes. On the other hand, since there is no dilution effect, you can put a lot of stones in your glass.

Reusable ice cubes are actually much better at cooling things, because they do melt -- but they do so internally, in a sealed envelope, thus not spilling into your drink. Since they use latent heat of phase transitions, they are as good as true ice cubes. Although they do lack in aesthetics.

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But what business have you torturing perfectly fine Whisky with unnatural coolness ?