Tuesday, March 31, 2009

Ice cream in a bag!

This week we did a pretty COOL trick....we made ice cream, without a freezer, without an ice cream machine. How did we do it and what does it have to do with science?

First we demonstrated that ice covered in salt will melt faster than ice alone, and most surprisingly that the liquid salt water is actually COLDER than the frozen ice. We then discussed how phase change, or matter moving back and forth between solid, liquid, and gas states, involves a transfer of energy or heat. In other words, when there is a phase change, heat is absorbed in one direction and released in the other. These ideas and observations formed the basis of our ice cream experiment.

The kids poured the ice cream ingredients (milk, vanilla, and sugar) into a small plastic bag. Then they placed this bag into a larger bag filled with a lot of ice and a lot of salt. The whole business was then wrapped in a towel and then shaken until the milk mixture solidified. The rock salt caused the ice to melt quickly and reach a sub-zero temperature, so that the resulting freezing salt water could quickly absorb heat from the milk mixture, giving you yummy, delicious ice cream or at the very least, a super-cool milk shake.

Here's the procedure in case anyone wants to try it at home.

1. Pour ¼ cup of milk into a sandwich-size zip lock bag.
2. Add 1 teaspoon of sugar and a couple drops of vanilla extract.
3. Close the bag carefully, while getting rid of the extra air.
4. Mush the bag around to mix the contents.
5. Place this bag in another sandwich zip lock bag, remove the extra air, and seal.
6. Fill a 1-gallon zip lock bag with ice and a half a cup of rock salt.
7. Place the double bagged milk mixture inside the larger bag, squeeze out the extra air, and seal.
8. Wrap each bag in a towel and mix, shake, and churn until the inner bag's contents freeze.


* Make sure all bags are closed, especially before you start shaking up the bag of ice and salt.
* Double-bagging the milk mixture is important, to help reduce the odds of salt penetrating the bag and getting into the ice cream (yuck!).
* Err on the side of too much ice and too much salt.
* Try to keep the milk mixture bag in the middle of the ice, and especially in the middle of the melted salt water.
* See what happens if you add different flavors like chocolate syrup or lemon juice.

Thursday, March 26, 2009

Chemistry in a bag

The kids started off with calcium chloride, sodium bicarbonate (see if your kids remember this compound's common name...it's baking soda), water, and phenol red, an indicator for acids. They put all of these materials in a plastic bag, the catch being that the liquids (water and phenol red) were first poured into plastic cups, which themselves were carefully placed in the bag. Why all the extra steps? This allowed us to measure the whole business BEFORE the chemical reactions began. After tying off the bag, the kids allowed the powders and liquids to mix, and that's when things got exciting! The kids observed many changes including: bubbles, the bag inflating, the red liquid turning yellow, and the contents both heating up and cooling down. The kids then measured the weight of the bag with its contents, and they were pretty surprised to find very little if any change. That's conservation of mass for ya! Step 2 of this experiment was breaking down the reaction into parts to see what happens when you combine different pairs of ingredients.

Chugim will run as usual next week for all groups, INCLUDING Tuesday's sessions, which will meet at their usual times.

Sunday, March 1, 2009

General Science: Hunting for bacteria

Here are the results from last week's Bacteria Hunt:

Take-home message:

For the most part soap works, particularly with those bacteria samples taken from the trunk of my avocado tree.