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Name: Toni H.
Status: other
Grade: 4-5
Location: GA
Country: N/A
Date: 11/2/2005


Question:
How do scientists go about measuring the freezing point of liquids? For example, how would the freezing point of salt water be measured (different concentrations of salt)? What would I need if I wanted to measure this? Would you please help us with getting some information or ideas?


Replies:
There are many web sites that provide detailed equipment/materials eeded to measure freezing point depression of water (and other solvents). A "Google" search on the terms: "freezing point depression" and "freezing point depression kit" hit the following (from many others). The first three describe various experimental setups. The second two provide sources of pre-made kits to carry out the experiments. In addition, experimental setups are described in numerous texts on "freshman chemistry" since this is one of the first experiments done in freshman chemistry labs.

http://wwwchem.csustan.edu/chem1112/mwfpdep.htm

http://intro.chem.okstate.edu/HTML/SEXP10.HTM

http://www.sd54.k12.il.us/instruct/resource/science/ScienceKit4.htm

http://store.pasco.com/pascostore/showdetl.cfm?&DID=9& Product_ID=54188&Detail=1

http://sciencekit.com/category.asp_Q_c_E_429325

Vince Calder


I assume you do not want to spend a lot of money. I think you could accomplish this with a freezer (needs to freeze below zero, perhaps to -15 Celsius or so), scientific thermometers (read below zero Celsius), salt ( I would get the non-iodized kind), distilled water (does not have extra ions) and plastic drinking cups (beakers are fine if you have access).

I think I would have several different concentrations, making sure you label each, and do two of each if you want to double-check things or allow for disasters. Have one or two with just distilled water for a control. The obvious way to do this would be to watch it freeze, but that would necessitate opening the freezer repeatedly. I would rely on the fact that the freezing point is equal to the melting point. I think you can freeze them all, then take them out and place the thermometers in each container (if you have a limited number of thermometers, wipe them off between cups so you do not change concentrations in other cups). If you just want freezing/melting point, while the ice is melting, make sure you take the temperature when you have slush, before it all goes to water. This should give you a good melting point/freezing point. Record the data in a chart.

You also have a good opportunity to prepare them for phase change diagrams that they will see in later science classes. Record temperatures over time, including making sure that you let the cups start to warm up as liquid. For recording data, I would create a chart for concentrations, and I would take the temperature periodically. You can then graph the data, with time on the x-axis. You should get a flat line at some point, where the temperature stays the same for awhile----this is your melting/freezing point. This concept is not easy and may be too abstract for that age group, but just introducing it would help them later (I call it "giving them a hook to hang something on"). Have fun,

Mrs. Pat Rowe



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