See full version: How Cold Does Ice Get With Salt
Some interesting science occurs when you mix salt and ice. Salt is used to help melt ice and prevent it from re-freezing on roads and walkways, yet if you compare the melting of ice cubes in fresh water and salt water, you'll find ice actually melts more slowly in the saline and the temperature gets colder. How can this be? How cold does salt make ice?
When you add salt to ice (which always has an outer film of water, so it's technically ice water), the temperature can drop from freezing or 0 °C to as low as -21 °C. That's a big difference! Why does the temperature get lower? When ice melts, energy (heat) must be absorbed from the environment to overcome the hydrogen bonding holding the water molecules together.
Melting ice is an endothermic process whether there is salt involved or not, but when you add the salt you alter how readily water can refreeze back into ice. In pure water, ice melts, cools the surroundings and water, and some of the energy that is absorbed is released again as the water returns to ice. At 0 °C ice melts and freezes at the same rate, so you don't see ice melting at this temperature. here
Salt lowers the freezing point of water via freezing point depression. Among other processes, the ions from the salt get in the way of water molecules aligning to crystallize into ice. When salted ice melts, the water can't refreeze as readily because the saline isn't pure water anymore and because the freezing point is colder. As more ice melts, more heat is absorbed, bringing the temperature down even lower. This is great news if you want to make ice cream and don't have a freezer. If you put the ingredients in a bag and place the bag in a bucket of salted ice, the drop in temperature will give you a frozen treat in next to no time!
Finally, the test was carried out by quickly lifting the lids to see where the ice melt to water ratio was at once every 12hrs. It’s also important to note that the average daytime temperature during this test was around 95 degrees and in the mid 70’s overnight. The coolers were left in the shade as they would probably be at most campsites.
This will help in two ways. First, and the most obvious is you will have more ice in your cooler. Second, this will reduce the initial flow of air and transfer of heat around your ice. As the ice melts into water, this will also cause less air to fill the gap between the surface of the water and top of the ice chest. here
This ice chest comes in around $300 at the time of this article. You can check the current price on Amazon here . (Amazon link) here
The cool thing about this is that this is nothing special about salt. Pretty much anything that dissolves in water will drop the melting point. For instance, you could use sugar or even alcohol. Salt is just used on roads because it's inexpensive.
It's also possible that as the ice started to melt, the salted cube was sitting high in a pool of very dense saltwater, while the unsalted one was sitting low in a pool of plain water. Then the newly melted (and cold) water would sit on top of the denser saltwater, keeping the ice cold. In the plain case, the ice would contact more water, probably helping conduct heat to the ice.
mike w. here
So why did your student see that the regular ice actually melted faster when we'd expect them to have melted at the same rate? This is hard to say without seeing the experiment itself. Most likely it was something incidental to the set-up. It's possible that he used so much salt that the undissolved salt actually worked like an insulator around the ice, keeping it from melting as quickly. here
You know that you can sprinkle salt on an icy road or sidewalk to help keep it from becoming icy, but do you know how salt melts ice? Take a look at freezing point depression to understand how it works.
Salt melts ice essentially because adding salt lowers the freezing point of the water. How does this melt ice? Well, it doesn't, unless there is a little water available with the ice. The good news is you don't need a pool of water to achieve the effect. Ice typically is coated with a thin film of liquid water, which is all it takes.
Pure water freezes at 32°F (0°C). Water with salt (or any other substance in it) will freeze at some lower temperature. Just how low this temperature will be depends on the de-icing agent. If you put salt on ice in a situation where the temperature will never get up to the new freezing point of the salt-water solution, you won't see any benefit. For example, tossing table salt (sodium chloride) onto ice when it's 0°F won't do anything more than coat the ice with a layer of salt. On the other hand, if you put the same salt on ice at 15°F, the salt will be able to prevent melting ice from re-freezing. Magnesium chloride works down to 5°F while calcium chloride works down to -20°F.