Boiling point elevation

A cooking tip your grandma might give you is to add salt when cooking pasta. But why is that? Does it make the pasta taste better or cook faster?

When salt is dissolved into water, the normal boiling point of water is actually raised making it require more energy to reach its normal boiling point. Ex. Instead of boiling at 100C, it now may boil at 110C.

But why is that?

When solute is added to a solution, the mixture of solute and solvent causes the vapor pressure to be lowered (less willing to go into the gas phase), thus requiring more added energy to reach the gas phase. The added solute disrupts the water molecules interaction, making it have less pressure. The solute molecules also “cover” parts of the water molecule, making the water molecules vapor pressure to drop.

What about freezing point depression?

Freezing point depression is when the freezing point lowers, in other words, instead of water freezing at 0C, water will freeze at -10C. In order for freezing point to lower, more energy must be take n away from the compound to Screen Shot 2015-08-14 at 10.38.03 AMreach the phase change.

 

Salt is used to melt ice in the winter quite regularly, but why is that possible?

When salt is placed on ice, the salt is then dissociated into Na+ and Cl-. The solvation process then occurs as the positive and negative sides of the ions are attracted to each other. As more ions are dissolved, this causes impurities in the water molecule structure. The impurities caused by the dissolved Na+ and Cl- creates more of a “disturbance”, which makes it more difficult to freeze. As a result, additional energy must be required to taken out from the solution for it to freeze, thus lowering the freezing point.

Living in Canada, our temperatures are a lot harsher than our U.S neighbors. Although NaCl is useful in many occasions, it may not be useful in Canadian climates such as -20C temperatures and colder. But why are there de-icers that are rated -30C while the main ingredient is not NaCl but CaCl2. To answer that question, try to understand what would happen when CaCl2 dissolved into solution. The CaCl2 will be separated into 3 separate ions instead of 2 ions in NaCl. The disruption caused by CaCl2 will be greater than NaCl causing a even lower freezing point! COOL!