Salt and Its Effects on Boiling and Freezing
Author: L. A. Briggs // Category: boiling point elevation, freezing point depression, recipe, saltPosted April 18, 2011 at 10:59 PM
Have you ever wondered why some people throw salt into their boiling water when they are cooking pasta or vegetables? The answer to this is very simple: salt causes a phenomena called boiling point elevation to occur when it is added to water.
So what does this mean? Well, the salt causes the temperature at which the water boils to increase, meaning that the water is hotter when it is boiling with the salt than it is without it.
Essentially what happens when salt is added to boiling water is that the molecules of salt interact with the water molecules, causing the water molecules to not change phase from liquid to gas, which is what happens when water boils, until a higher temperature has been reached and the interactions between the salt and water molecules can be overcome.
Salt also has an effect on the freezing point of water as well. This effect is called the freezing point depression. This means that the temperature at which water normally freezes at is lower when salt is added than without the salt. This phenomena is often taken into account when making ice cream in some household ice cream makers. This is also why people often throw salt out onto frozen sidewalks in the winter.
Once again the salt molecules interact with the water molecules, causing the restriction of water molecules from making a phase change from liquid to solid, which occurs when water freezes, until the solution has reached a lower temperature and the interactions between the salt and water molecules are overcome.
However, the temperature differences in boiling and freezing of water with salt are not equivalent on both ends. The freezing point depression is greater than the boiling point elevation. Water normally boils at 100°C, but with the addition of salt the boiling point of water increases based on the concentration of salt added to the water (about a 0.17°C increase for every teaspoon of salt added to a quart of water). On the other hand, water normally freezes at 0°C, but when salt is added it freezes at -18°C.
For such as small and common molecule, salt sure does have a lot of power in the kitchen, and not just for flavoring.