supercooling and snap freezing

[julie1]

I have not been able to cool the distilled water to below freezing - the lowest temperature I have gotten it to is 40F. I use a glass bowl with plastic cup in the middle with about 1/2 inch of distilled water in it, surround that with ice and sprinkle the ice with 2 T of salt. The water starts at around 60F. I have done it 6 times now. Can't figure out what I'm doing wrong.

[deleted-71588]

You probably have several things causing problems with getting your water below the freezing point.
1) Many plastics are good thermal insulators, so heat transfer from the water through the plastic to the ice/salt mix will be slow. Try using a SMALL stainless or alumnium cup. Metals are excellent thermal conductors.
2) Are you pre-crushing your ice? Over the years, I've made my share of homemade ice cream using hand-crank or electric motor cranked processes. If you just pour a bag of ice cubes into the container, it takes forever if you are the one cranking. If you attack the bag of ice with a hammer and crush it into small pieces, and layer it with salt, it goes a lot faster. The smaller ice pieces have more surface area for the salt to react with so the thermo chemical reaction happens faster.
3) Are you turning the cup or just letting it sit? By turning the cup, you cause the distilled water inside and the salt water outside to flow against the sides. This increases fluid convection (look up convection if you don't know that word).
4) What is the temperature of the ice/salt bath? Glass is a good thermal conductor so you maybe loosing your cooling to the room. You may need to provide thermal insulation around the bowl and switch to a plastic bowl. The old icecream makers used wood (which is a good thermal insulator) and were often wrapped with newspaper. Newer electric cranked ones use a plastic bucket with foam insulation and decorative wood slats on the outside.
5) Just sprinkling the salt on the top of the ice isn't going to react as fast as layering ice, sprinkling salt, more ice more salt, more ice, more salt.
-Craig

[julie1]

Thanks Craig that helped a lot. But now have a new problem. Whenever I try to supercool the distilled water it turns into slushy ice. I look at the temp on the therometer is 40 degrees! please tell e what I am doing wrong or need to change.
thanks
julie1

[deleted-71588]

We can't diagnose your issue without knowing your current setup. What changed from your original setup?

Where did you take the 40 F temperature reading? If you have slushy ice in the distilled water, then the distilled water can't be 40 F. If you are taking the temperature of the salt water/ice mixture, it had to be lower than 40 F at some point for the distilled water to get slushy.

Is your thermometer accurate in this temperature range? Try measuring a glass filled with ice with just enough water to barely float the ice. It should read very close to 32 F.

Are you running out of ice or salt in your brine? Are you losing too much cooling to the suroundings? What is the room temperature? What size and quantities of things does your setup consist of?

[julie1]

You're right - new thermometer is getting us much closer! It got down to 32.9F after 15 minutes. This is in a 70 degree room. We've tried several variations: plastic mixing bowl with thin disposable plastic cup; replaced plastic cup with aluminum can but it starts to freeze with that. The ice/salt mixture is melting very little if at all.

[julie1]

I did it! We put the cup in a small cooler - plastic inside and insulated on the outside - filled that with crushed ice + salt. There was no freezing, but the temperature wouldn't go below 33.1. So I put a towel on top of the ice, around the cup but not over it, to keep the cold from escaping. The temp dropped almost immediately - down to 27.5 with no freezing at all!! Awesome!

[deleted-71588]

Congratulations on the demonstration of the effect. Unfortunately, a demonstration is NOT a Science Fair Project.

You probably learned a lot about things that can affect heat transfer. Now it is time to try and turn this into a real Science Fair Project. You should look at https://www.sciencebuddies.org/science- ... ndex.shtml and read up on the "Scientific Method" and "Your Question" and come up with a testable hypothesis, run the experiments and figure out if you could prove or disprove your hypothesis or if your testing ended up without a answering your hypothesis.

Keep up on your quest to doing a great Science Fair Project.