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Q & A: Freezing points of various liquids

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Most recent answer: 01/02/2016
Q:
WHAT IS THE DIFFERENCE DETWEEN THE FREEZING POINT OF WATER AND OTHER LIQUIDS?
- amanda (age 13)
norfolk V.A. U.S.A
A:
Different substances freeze at different temperatures because the molecules that make them up are different. Some kinds of molecules have stronger forces holding molecules to each other than other kinds of molecules. In water, for example, the positively charged hydrogen end of the molecule electrostatically attracts the negatively charged part of neighboring water molecules to form "hydrogen bonds". These are responsible for the cohesion of water molecules in the liquid and the arrangement of water molecules in ice.

Other kinds of substances have different freezing points. For example, nitrogen molecules have only very weak attractive forces between each other (although the bond between the two nitrogen atoms in an N2 molecule is very very strong). Consequently, nitrogen freezes at a much lower temperature than water. It even liquifies at a much lower temperature than water: -195.8 °Celsius. It freezes solid at -209.86 °C. Water in comparison, freezes at zero degrees Celsius. We use liquified nitrogen here at the Physics van for lots of great demonstrations because it is so very cold.

Mercury is a poisonous liquid at room temperature and freezes at -38.87 °C. Benzene, a flammable, poisonous, carcinogenic hydrocarbon (C6H6), freezes at 5.5 °C. Butane liquifies at -0.5 °C and freezes at -138 °C (it liquifies at higher temperatures when it's under pressure). Most substances have solid, liquid, and gas phases under different temperature and pressure conditions. Some, like oil and glass, don't have clear demarcations between solid and liquid phases. They get hard when they get cold, but there's no real temperature you could call the "freezing point".

Water with some salt in it will have a lower freezing point than pure water. Check out some of the other answers on water and salt to find out how this works.

Tom

(published on 10/22/2007)

Follow-Up #1: detective work on freezing a slurpee

Q:
My granddaughter swears that the red liquid left in a Burger King slurpee cup had no alcohol. I tasted no obvious alcohol. Is BK slurpee so full of suagr that it does not freeze when left overnight in the car when the outside temp is 18' F. In my mind, when drinking a slurpee one mostly likely consumes the syrup and the crushed ice remains. I don't envision consuming enough of the ice that none remains to freeze. Any comment will be appreciated.
- Virginia (age 67)
Oshkosh WI USA
A:

Those BK Icee's have a lot of sugar, but it's still less than 100gm/liter. That liter incudes lots of ice too though, so I don't know how high the concentration is in the syrup. As you say, we don't know whether what was left was pure syrup or had some melted ice in it. Also, I don't know if the sugar is mostly sucrose or fructose, which has about half the molecular weight and thus does a better job per gram of lowering the freezing point. High fructose corn syrup is very highly concentrated (only about 1/4 water) and must have a very low freezing point, but I haven't been able to find data on that. So I'm not sure if lowering the freezing point to 18°F with sucrose and fructose is even possible. However, there can be other complications. How cold did it actually get inside the car? Could the slurpee have stayed a little supercooled? Could enough water have evaporated while the car was still warm to increase the sugar concentration significantly?

So maybe an experiment would be the best way to answer your question. You and your granddaughter can take some of the same type of slurpee and get some liquid from it, making sure that its red color density matches the suspected drink. (The red dye and the sugar concentrations should track each other, since neither evaporates or goes into ice.) Leave it out in the same car under the same conditions and see what happens. In case it does freeze, you should try several repeats with approximately similar conditions, since in this country we have the "innocent until proven guilty" principle. (In some other countries, adolescents are legally allowed to drink.)

Regardless of the truth or falsity of the particular accusation you're making, maybe the exercise of doing a real experiment together will be valuable. 

Mike W.


(published on 01/02/2016)

Follow-up on this answer.