cashew | Monday, July 5th, 2021

Holy fucking shit, people, stop adding salt to your ice bottles.

This rant brought to you by Wiki telling you to make salt water ice to make it last longer.

In my search for DIY ways of cooling down a room that didn't involve blasting the AC constantly, I happened upon a lot of recommendations that all boiled down to creatively "attach ice to a fan and blow". Which, yeah, is pretty easy to conceptualize. When you have hot air passing over cold ice, the heat gradient would transfer energy from high (air) to low (ice) and boom, instant cold air.

However, for some reason, many, many of these sites recommend adding salt to water before freezing it into ice under the mistaken impression that salt water ice will last longer than fresh water ice.

Guys. Guys. You do realize we salt the roads to melt ice for a reason right?

Salt decreases the heat capacity (or specific heat) of liquid water. It also decreases the heat capacity for ice. Less energy is needed to melt salt ice/water and heat up salt ice/water than just regular ice/water.

In the case of cooling the air, you're trying to make the ice last longer, in other words suck up as much energy as possible before rising 1 degree of temperature and reducing the temperature gradient. To last longer you want higher heat capacity, AKA use pure water.

The only reason to use salt is because you're trying to lower the water's temperature to below 0°C. And yes, that will increase the heat gradient and consequently cool the air faster as it travels over colder ice/water, but it will also shorten the lifespan of the ice. And, again, 0°C vs -4°C is not a very high difference when the ambient temperature is 32°C. The air travels so fast over the ice as it's being blown that a few degrees is not going to perceptibly cool the air more. You might as well just go with the longer last 0°C ice water rather than than shorter lasting -4°C salt water.