Neighborhood Nature

Our Family's Nature Blog

Salty, Super-Cooled, Heat-Sucking Slush — and Homemade Ice Cream February 21, 2009

Filed under: Geology,Puzzles and Mysteries,Seasons,Water,Winter — saltthesandbox @ 8:16 am
Tags: , , , , , ,

We have a couple of inches of new snow in the neighborhood this morning, so I thought I’d tell you about something “cool” we did with snow. It’s great to do this with new snow, but even if the snow in your yard is already melting or turning gray, that’s fine.

This is as much science as nature, so here’s our research question: What happens when you add salt to snow? The kids in our house that day voted three to one for this hypothesis: “It will get warmer.” Why? “Because salt melts snow.”

Was the majority correct? Let’s do an experiment to find out!

Here’s our experimental apparatus:

We used a large bowl of snow, a bunch of salt, a large metal spoon, and a thermometer left over from Nature and Science Club.

We used a large bowl of snow, a bunch of ice-melting salt, a large metal spoon, and a thermometer left over from Nature and Science Club.

First we took the temperature of the pure snow:

The thermometer reads 0 on the left side (Centigrade) and 32 on the right (Fahrenheit).

The thermometer reads 0 degrees on the left side (Centigrade) and 32 degrees on the right (Fahrenheit).

The snow was at the freezing point of water: 32 degrees Fahrenheit, 0 degrees Centigrade.

Next we mixed a bunch of salt into the snow:

The metal spoon was strong enough to mix the salt and snow without breaking.

The metal spoon was strong enough to mix the salt and snow without breaking.

We stuck the thermometer into the mixture, watched, and waited. Here’s what happened:

The thermometer read _ degrees Centigrade, _ degrees Fahrenheit.

The thermometer read -22.5 degrees Centigrade, -9 degrees Fahrenheit.

Three of four kids were very surprised — the mixture had gotten colder — much colder! Their hypothesis was wrong — but why?

Ethan, who remembered doing this experiment with our old school’s Nature and Science Club, explained it this way: “Because the salt lowers the freezing point of water.” That’s a good start, but remember one more thing: It takes heat to melt snow, and that heat has to come from somewhere. As the salty mixture reached a new balance between liquid and frozen water, heat was sucked out of the mixture,and its temperature dropped. Steve Spangler puts it like this:

When salt is added to the ice (or snow), some of the ice melts because the freezing point is lowered. Always remember that heat must be absorbed by the ice for it to melt. The heat that causes the melting comes from the surroundings.

So, we had this very cold mixture of salty, heat-sucking slush. We couldn’t just let it sit there — we had to put it to use. As Ethan remembered from Nature and Science Club, salty slush is just to thing to suck the heat from a mix of cream and sugar — to make ice cream!

We looked up directions for homemade ice cream on the Web, and kind of followed the ones on Steve Spangler’s website. However, we couldn’t find the vanilla, so we used chocolate sauce instead:

This is what we used to make homemade chocolate ice cream.

This is what we used to make homemade chocolate ice cream.

We mixed the half-and-half, sugar, and chocolate sauce inside a heavy-duty zippered bag:

We mixed the half-and-half, sugar, and chocolate in a zippered bag.

We squeezed and sloshed the bag to make the mix.

Then we put the sealed bag of ice cream mix inside a second zippered bag packed with salty slush. The most important direction was to keep the ice cream mix (half-and-half, sugar, and flavoring) separate from the salty slush:

Make sure the bag of ice cream mix is sealed really tight!

Make sure the bag of ice cream mix is sealed really tight!

And then we held the bag of slush with a really warm glove and carefully shook and jiggled it for 15 minutes.

Make sure that your bare hand never touches the salty slush, and the bag of ice cream mix doesn't leak.

Make sure that your bare hand never touches the salty slush, and the bag of ice cream mix doesn't leak.

When we were done. it looked like this:

It looked good enough to eat -- so we did!

It looked good enough to eat -- so we did!

Everybody had a taste:

Aaron like it!

Aaron liked it! And so did everybody else.

The ice cream was super smooth, because thousands of tiny ice crystals formed as the super-cooled salty slush sucked heat from the mix.

 

3 Responses to “Salty, Super-Cooled, Heat-Sucking Slush — and Homemade Ice Cream”

  1. Tess Says:

    AWESOME experiment – and looks delicious! I went on Steve Spangler’s website after reading your post and found tons of experiments we want to try!! I also bought this praying mantis kit for my kids (http://www.stevespanglerscience.com/product/praying-mantis) I think it will be a BLAST to observe them. The ones my kids see our yard are always full grown. Thanks for a great post!

    • Thanks for the comments, and good luck with the praying mantis kit! I hope they supply good directions for obtaining lots of tiny insects to feed this mantis babies. Maybe some day I’ll write about our experience raising 200 baby toads from eggs. Once they grew legs, we had to catch thousands of tiny bugs to feed the toadlets. We did it by sweeping a butterfly net through short grass in lawns and parks. (If you do it in tall grass and weeds, you sometimes catch predatory bugs that will eat your babies!)

  2. […] Salty, Super-Cooled, Heat-Sucking Slush — and Homemade Ice Cream […]


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