We were dancing at a club, it was busy, and I was overheating. So I asked for some water - but what they gave me was 90% ice, topped off with water. It did an awesome job at cooling me down.
Later on, I got thinking - How much better is ice at cooling you down than liquid water?
While liquid water does an admirable job cooling you down at a dance party, clearly ice is better. But how much better?
To make this fair on liquid water, we'll skew our test a bit in water's favor, comparing:
- the coldest possible liquid H2O (liquid water at 0°C)
- to the warmest possible solid H2O (ice at 0°C)
- for cooling down a hot person (slightly over 37°C)
Liquid water cools you down in two ways:
Liquid water is usually less than 37°C when you drink it. Once it is inside your body, the liquid water's temperature rises.
Temperature rises need energy, and that energy is taken from your body heat, cooling you down.
You can sweat the water out, where it vaporises (turns to gas). Vaporisation requires energy, some of which is taken from your body heat, thus cooling you down.
As anyone who's ever been to a concert can testify, sweating is useless when there's no wind, and humidity is at 100%. So we ignore that effect here.
It takes 4.18J to heat one gram of water by 1°C. Ingesting water at 0°C, you will raise its temperature by 37° to body temperature.
4.18J/g x 37° = 155J/g
The energy to heat up the water comes from your body heat, so you will lose 155J per gram of water you drink.
Ice melts, forming liquid water. It takes a lot of energy to melt the ice - you have to partially break hydrogen bonds - one of the strongest types of bonds in chemistry.
And, once that gram of ice is melted, you're left with a gram of water at 0°. Which, as shown above, cools you down by a further 155J.
melting energy + heating energy = total 334J/g + 155J/g = 489J/g
Drinking one gram of 0°C water will cool you down by 155J.
Drinking one gram of ice at 0°C will cool you down by 489J.
So, at minimum ice cools you down over three times as well as water. If the ice is colder, or the water is warmer, ice does even better. Prefer ice!