Wednesday, January 23, 2013

Why whiskey stones don't cool as well as ice.

While they sound like something you might find in the Skymall catalog, whiskey stones have been touted as a way to cool drinks without the annoyance of dilution that you get from melting ice cubes.  It's true that they don't dilute your beverage of choice, and you get to make jokes about having drinks "on the rocks".  However, for real physics reasons these stones just aren't that effective at cooling your drink down.  To see this, let's consider how much energy it takes to warm four of these stones from -40 \(^{\circ}\)C to room temperature (25 \(^{\circ}\)C).  Each stone is around 8 cm3, and granite has a density of 2.7 g/cm3, and the specific heat of granite is 0.79 J/gK.  Combining, that means that warming four of those stones to room temperature would take around 4400 J.

Now consider an equivalent volume of ice starting at -40 \(^{\circ}\)C.  Ordinary ice has a density of 0.917 g/cm3, and a specific heat of roughly 2.05 J/gK.  Warming four 8 cm3 ice cubes up to 0 \(^{\circ}\)C takes 2400 J.  However, converting ice from solid to liquid requires a latent heat, in this case 334 J/g.  So, just melting those ice cubes requires 9800 additional Joules.  Without even worrying about warming up the resulting water, the ice cubes are able to take up almost three times as much energy just by warming up to the melting point and melting.  So, while it's true that ice can dilute your beverage, it is much better at cooling things (if that's what you want to do), thanks to the latent heat, the energy required to change phases.

A lack of understanding of specific heats and so forth is quite common.  Even the article I linked above about whiskey stones says "Another obscure advantage of whiskey stones is they freeze quickly. Granite ice cubes are ready to go after 20 to 30 minutes in the freezer, whereas water needs hours to freeze into ice cubes."  That's not an advantage - it tells you that the heat capacity of your whiskey stones is low compared to the water equivalent.

11 comments:

Anzel said...

However, a few thoughts:
- When we typically drink whiskey, we don't let the ice melt fully, yes? So we'd only use a fraction of the latent heat.
- Issues of specific heat aside, would there be any useful comment about relative heat transfer rates? I'd imagine that, given a cube of metal with a similar specific heat as water, the metal cube would cool faster as it transfers heat faster. Also, don't different materials have different convective heat-transfer coefficients with the local air?
- Small typo: The equivalent volume of ice should start from -40 \(^{\circ}\)C, not 40 \(^{\circ}\)C.

Alex said...

Nice piece of physics. The only question remaining is why andyone in the world would want to drink cold whisky!??! This is like drinking red wine cold - absolutely pointless. Most of the essential aromas that give a whisky its intriguing flavor are don't evaporate off at low temperatures, so the whisky is far more bland. A proper course in how to drink and enjoy whisky would be far more useful!

David said...

Another approach is cubes of plastic containing water, which you can freeze, and then avoid the dilution issue.

Douglas Natelson said...

Anzel, you're right, we only use a fraction of the latent heat. A better calculation would actually do the heat balance between the coolant and the liquid to be cooled, but I was lazy. Thermal conductivity will affect the rate, but (in the limit that heat transfer to the environment is small) not the final temperature. My dim recollection from my mechanical engineering days is that material differences in convective heat transfer for smooth surfaces are small. (I'd bet that hydrophobic coatings would reduce convective heat transfer to water, though.)

Alex, I agree!

David, that's some A-1 thinking. I have some "freezer-cups" at home somewhere like this. The cup is basically one of those liquid-containing freezer packs that happens to be shaped like a beverage mug.

3Davideo said...

Why not freeze whiskey into cubes, and then you can have the latent heat without the dilution?

(not an alcohol drinker so I don't know if you could actually freeze whiskey)

MB's Blog said...

I have a question. Normally when we watch TV, there is always some ambient lighting so as to reduce the strain on the eye of the viewer. What is the physics behind it?

Is it that the the photons emitted from a tv screen are of high energy and the photons emitted from a lighting source (tubelight, bulb etc) are of lower energy and the collision of both these causes high energy photons to lose their energy and the eye doesn't get strained ? But if energy changes, then wavelength will change and a blue in a dark room will appear less blue in a room with ambient light. Am I correct on this ?

Also , with ambient light on , will there be a reduction in no of photos of blue light reaching my eye as on average I will recieve photons from both ambient source and screen whereas with light off only photons from screen will hit my eye.

Suppose if I have a laptop at a distance of 4 feet from eye and a projector which projects image on a wall and I view it at 4 feet . Will there be any difference between the brightness of the two if the wall is as ?

Rich Thomas said...

Douglas,

Nice piece of science on the subject. I just wanted to add that when I wrote that granite freezing is a relative advantage, having already acknowledged their lack of cooling power. Even if their ability to chill something is modest, having the ability to do that in short order is a plus. Imagine if they had only modest chilling power and took two hours to freeze!

Our Jake Emen came back to this subject and wrote a scathing op-ed about whiskey stones. Click my name link to see that one.

Douglas Natelson said...

Hello Rich - Thanks for your comment. I actually had seen the scathing post before the original :-)

Bryan Sanctuary (Dr.) said...
This comment has been removed by the author.
Bryan Sanctuary (Dr.) said...

Ok, so a lot of people like cold drinks, but that only started about 1950 and refrigerators came out at the same time as TV, with the message "Have a cold one" which wrecked North American beer. Any bad drink tastes better cold and carbonated: test, drink warm flat coke.

Along the same theme, ice in bad scotch makes it taste better, but good scotch should not be iced.

Having said that, some drinks are better cold and I have see devices that use latent heat to cool. We have a cork stick that you can take from freezer and plunge into a bottle of white wine and in 10 minutes is a at a passable temperature.

Anonymous said...

They are Soap Stone not granite. Never meant to replace ice.
They are meant to put a slight chill into a dram.
If you cover the stones with booze they will not work. you have to keep the liquid volume at an appropriate ratio. figure 1 stone for 1 Oz.