Ice Cream / Lowering Freezing Point Issues

[sasali]

Hi there -

My daughter is conducting her experiment using the ice cream kit that tests lowering the freezing point of water. We are having some issues and have several questions:

1) To control the solution/solvent across all samples, I got a large jug of distilled water. We began with the control and tested plain water. Her initial results are -8 C, -9 C, and -10 C. In all three cases, there wasn't a slurry or crystals forming. Instead, there was a rim of solid, clear ice around the inside of the tube. (The ice bath was very cold, something in the -15 C range based on my best guess. See #2 below for issues with the thermometer.)

Later, we re-ran the control test with an ice bath measuring -10 C. (This was in case it was too cold before.) We also ran it with 2 different plain water samples - tap and distilled - to see if that would affect anything. After about an hour, nothing happened.

Did we do something wrong? Is distilled water too pure? Are there plastic chemicals/contaminants from the container that could affect results? Why aren't we getting something close to 0 C for the plain water / control sample?

2) I bought her the kit from your site. However, the thermometer in our kit only goes down to -10 C even though the picture shows -20 C. With our control sample "starting" at -10 C, we can't even measure the changes correctly.

Did we get the right thermometer in our kit? Is it possible that our thermometer isn't working right? Is there a different one we could use that would go lower?

3) The first real test sample she ran was the lowest concentration of salt (2.9 g). To begin, she tested the ice bath temp and that was -10 C. In the ice bath, we watched the temperature drop to -11 C with no crystals forming. Then, she added salt to the ice bath because she was worried it wasn't cool enough to freeze it. Within seconds of her adding the salt, she pulled up the test tube and there was a slurry in it! However, the temperature actually rose to -10 C and then stayed there. On her 2nd run, the temperature dropped to about -12 C and nothing was forming. After waiting a while, she added some ice first and then later some salt to the ice bath. Again, within seconds of adding the salt, the slurry appeared and the temperature rose to -10 C.

She and I are both wondering if there is something going on with supercooling that is affecting this experiment. While it's just a sample size of 2, we're seeing results that seem consistent, but don't make sense. Are you able to explain what might have happened?

4) She wanted to expand her test to include additional samples based on her research of various freezing points. She was hoping to test:
Vinegar - 2 different amounts
Rum - 2 different amounts
Isopropyl alcohol - the goal was to see if she could get it to freeze, even with just a few drops

I think she wanted to play around with it and see how these other solutes would affect the results. However, with a thermometer that only goes to -10 C and a control that is already at that level, I don't think she's going to be able to measure anything.

Do you have any suggestions for how to best continue and see usable results?

Thanks so much for your help!

[norman40]

Hello sasali,

I’m assuming that you are working on the project described here:

https://www.sciencebuddies.org/science- ... p013.shtml

Sorry to hear that you’re having problems with the project. The distilled water sample that you tested should have frozen at close to 0 C. Formation of ice around the inside of the test tube isn’t surprising but freezing temperatures of -8 C to -10 C are pretty far off.

From your description it seems that there might be something wrong with the temperature measurements. When measuring the temperature is the thermometer bulb in contact with the walls of the test tube? If so, this might produce lower readings than the temperature of the liquid. To make sure you’re reading the liquid temperature gently stir the liquid with the thermometer while keeping the thermometer bulb away from the test tube walls

If you are certain that you are measuring the temperature of the liquid inside the test tube you might want to try a different thermometer.

I hope this helps and good luck with your project. Please post again if you have more questions.

A. Norman