Interesting. The problem with adding saltwater as top-off is that the salt doesn't evaporate. So if someone were to use frozen saltwater to cool a tank, and then dump the SW in the tank - over time it will cause a problem.
So while an occaisional addition will not cause a problem - it is something new people need to be aware of
Q3. At what temperature did the salt water begin to freeze. Why is this lower than the pure water.
A3. The temperature again will depend on the students results. We are not sure why salt water freezes at a lower temperature - please let us know if you have a good explanation!
Blue is the saltwater
Salt water with a salinity of 35 ppt (parts per thousand), the average salinity of the open ocean, freezes at 28.5oF.
If salt reduces the freezing point of water to 15?F, what happens when it's 10?F outside?
Higher concentration of salt will help to lower the freezing point. So, if the air temp is 10?F, you might be able to melt ice by using more amounts of salt. But when temps are expected to dip below 15?F and especially below zero, road crews will switch over from rock salt (NaCl) to Calcium Chloride (CaCl). Magnesium Chloride (MgCl) is less common but can also be used. While rock salt reduces the freezing point to about 15?F, Calcium Chloride will effectively melt ice until -20?F! Magnesium chloride will effectively melt ice to about 5?F(1) .
Also, Calcium Chloride is a great deicing agent because it releases heat as it dissolves. This makes it a very quick acting deicer. However, calcium chloride is more expensive than rock salt, so it is typically only used when temperatures get near or below freezing (2).