Somebody asked me about this on Twitter, so here you go:
Some whiskey drinkers love the purity of the flavour of their whiskey. They may want it chilled, but they might not want the pure beverage to be diluted by melting ice. A company has tried to come up with a solution to this problem with a product called Whiskey Stones. They are small ice-cube sized cubes of soapstone, that a whiskey drinker chills in the freezer, and then puts in his glass of whiskey to chill it. It’s a neat idea, but how effective are the stones at chilling the drink compared to regular ice cubes?
Presuming that we take a standard two ounce shot of 80 proof whiskey, which is about 55 grams, from a bottle at room temperature (19 deg C), and then use a standard freezer at -15 deg C to prepare both ice and the Whiskey Stones, what is the difference in cooling of the beverage? I will use the same volume of Whiskey Stones (which are 11 mL each, so I’ll use 3) and ice cubes in my two scenarios. 33 mL of Whiskey Stones are about 96 grams, and 33 mL of ice cubes are about 33 grams.
When you take two things with different temperatures, and put them together, energy will flow from the warmer thing to the colder thing, until both things are the same temperature. This is one of the implications of the second law of thermodynamics. If you have a perfectly-insulated glass, all the energy that comes out of the whiskey as it chills is transferred to the ice as it warms up and melts, or to the stones as they warm up, until a temperature equilibrium is reached.
The amount of energy it takes to warm something up depends on the material it is made of. For ice, it takes 2.11 joules (J) of energy to warm up 1 gram (g) by 1 degree C. For water, it takes 4.18 J. For whiskey, which is a mixture of alcohol and water, it takes about 3.4 J. Conversely, as something cools, it gives up energy according to the same figures. Water gives off 4.18 joules per gram as it cools one degree.
Additionally, as a solid melts into a liquid, it also absorbs energy, without changing temperature. It takes 334 J of energy to change 1 g of ice into 1 g of water at 0 deg C.
Any energy that comes out of the whiskey goes into the ice, water or stones.
To see how much energy is absorbed by the ice, I will calculate how much energy it takes to warm ice at -15 degrees up to zero, and then melt it all into water.
First, assuming 33 mL of ice starting at -15, it takes 15 x 33 x 2.11 = 1044 J to warm it up to zero degrees. Next, to melt that ice into water, it takes 334 x 33 = 11022 J. That means that to convert all the ice to water, it takes 12066 joules of energy.
How cold does the whiskey get if we remove 12066 joules of energy by transferring it into the ice? The change in temperature is equal to the energy transferred, divided by the specific heat of whiskey times the mass of the whiskey. T = 12066 / (55 x 3.4) = 64 degrees of temperature change. If we started at 19 degrees, the whiskey can’t drop all 64 degrees, because the second law of thermodynamics says that energy will flow from a hot thing to a cold thing until the two things are the same temperature. That means that the whiskey will reach 0 deg C before all the ice melts, and then it will stay at that temperature.
For the stones, I have to take a different approach. I know that any energy that comes from the whiskey as it cools, goes into the stones as they warm up. If the final temperature is called F, I know that 19 deg -F = the change in energy of the whiskey, divided by the whiskey’s specific heat multiplied by the mass of the whiskey. I also know that 15 deg + F = the change in energy of the stones, divided by the stones’ specific heat multiplied by the mass of the stones. In both equations, the change in energy is equal but opposite, so that allows me to solve both equations for F, which turns out to be 7.6 degrees C.
That means the coolest the whiskey can get using the three stones is 7.6 degrees, but with the ice, it’s zero degrees C.