# Salt in home-made ice pack

• February 17th, 2010, 05:40 PM
Karl Engel
Making a simple ice-pack, I added some salt to water in a couple of zip-lock bags & put in freezer... but now wonder if this is counter-productive: sure the freezing point is colder, but so is the melting point once it warms up, so is there any benefit?
• February 17th, 2010, 06:17 PM
Pong
Only that the bag conforms (to your body) better because it isn't a solid hunk.

On second thought: you have increased the density a bit too, so you get "more cold" per volume.
• March 13th, 2012, 09:35 PM
Scibob
Ice packs are used for lowering temperature so melting point is mute as the temperature of the melted salt water is still colder than normal H2O & will still cool the area being treated... I have enough salt in mine so it just has to be kneaded a little when removed from the freezer and it is still "gelled" after 30 minutes. I use an old fashioned ice bag but like the gelled affect for conformability.
• March 14th, 2012, 05:45 AM
Harold14370
Let's say we have two packs of ice, one salt and one fresh, at the freezing point of salt water, which is about 0 degrees F. We warm both of them up to the freezing point of fresh water, 0 degrees C, 32 degrees F and melt the ice.

The heat absorbed will be equal to the latent heat of fusion plus the specific heat capacity of the water (or ice) multiplied by the mass multiplied by the change in temperature.

In the case of the salt ice, the specific heat capacity of water would be the one to use, whereas the fresh water ice will undergo its temperature change in the frozen state. The specific heat capacity of ice is about half that of water, so it would seem there may be a slight advantage to your scheme. Slight, because most of the heat absorbed will be in the form of latent heat of fusion.
Heat capacity - Wikipedia, the free encyclopedia

However there are also practical considerations. If you are using your ice pack to keep food and beverages cold in a cooler, the salt ice is a bad idea. It will probably freeze your food and drinks, then after it has melted, it won't do much good and the food will quickly warm up. Whereas the fresh water ice will keep the food and drinks at 0C (32F) for long periods of time, which is ideal. Also, since the interior of the salt ice cooler will be at 0F for a long period of time, the cooler will absorb more heat due to the greater delta-T across the insulated walls of the cooler.

If you are using the ice to keep ice cream frozen, then the salt ice would probably be a good idea.