Here on Oahu this weekend, Sacred Hearts Academy is hosting it’s 19th annual Science Symposium for Girls. It’s a day full of fun science-y workshops that are free to attend for girls in 5th through 8th grade. Dr. Kira Krend, my roommate and a kick-butt, PhD-wielding biology teacher at Sacred Hearts Academy, is one of the awesome science teachers and professionals helping out with this year’s symposium. So, a few weeks ago, we sat around the house brainstorming fun, short science experiments that the girls might enjoy. When I mentioned my favorite lab from high school chemistry, making ice cream, my roommie’s eyes lit up. When she told her bosses her idea, they loved it — and so did the local news. They chose her symposium workshop as the feature for their advanced coverage. Which, of course, meant only one thing: the protocol had to be perfect.
Here’s the general protocol for the experiment:
- Mix together 1/2 cup of half & half (or 1/4 cup of cream and 1/4 cup of milk), one tablespoon of sugar, and 1/4 tablespoon of vanilla and put in a quart-sized ziploc
- Fill a gallon ziploc 1/3 of the way with ice
- Add salt (rock salt or large granules, ideally)
- Place the ice cream ziploc inside the bag of ice and seal the bag
- Mix the ice/water around the inner bag for ten minutes
…and voilà! The bag of liquid ingredients is frozen into delicious ice cream!
Kira had never made ice cream in a bag before, so she wanted to make sure everything ran smoothly for the cameras, and since I was the one who suggested the lab in the first place, she turned to me for advice. Problem was, it had been over a decade since my last attempt at ice cream making. As it turns out, there are many variations on the recipe, from what type of cream to use to how much salt to add to the ice to achieve maximal freezing temperature. So what did two scientists decide to do? Experiment, of course!
First things first: how much salt? We decided to take the recipe and try three different amounts of salt to ice ratios. Using two solo cups of ice in each gallon ziploc, 
we added either 1/8, 1/4, or 1/2 a cup of sea salt. We then placed the same premixed ice cream recipe in a sealed bag inside, and got to mixing. Ten minutes (and pairs of freeeeezing hands) later, we compared the consistency of the ice cream in each bag. The one with the least salt was visibly less solid, while the 1/4 and 1/2 cup tests were roughly the same. So, we erred on the side of extra salt, and set the amount to 1/2 a cup.
Then it was time for the real test: taste. The various recipes we had suggested either half & half or milk and cream, and we decided to try a fun third option: chocolate milk instead of regular milk. We also decided that we’d be holding our bags with hand towels this time, to save our hands. Another 10 minutes later it was time for the taste test… yum!
The end result was not surprising: the recipes with cream and whole milk tasted creamier. But, the difference was pretty minimal, and since half & half is easier to buy in bulk, she decided to go with that. For the record, the chocolate milk also worked, and added a nice, subtle chocolate flavor to the ice cream. It was my personal favorite:
But perhaps it’s time to answer the real question: why does this work?
For those who have never tried making ice cream in a bag, it’s one of the easiest and tastiest science experiments you can do at home, taking advantage of what chemists call Freezing Point Depression.
We say that pure water freezes at 32 degrees Fahrenheit (0 degrees Celsius), but it is actually an equilibrium point: at that temperature, liquid water freezes as quickly as ice melts, so to create ice, we have to continue to remove heat. When the water isn’t pure, however, things are different. Other particles (for example, salt) disrupt the process of freezing because they disrupt the forming of the crystalline structure that is ice. Thus, the freezing temperature of a solution is lower than pure water.
When you add the salt to the ice cubes in this experiment, the salt particles interact with the water molecules, beginning the process of melting. But, the lower the freezing point of something, the more energy has to be absorbed by it to melt; which is why, though it might seem strange, the moment you add salt, ice-and-forming-water’s temperature drops to colder than freezing and starts absorbing heat from its surroundings. In fact, adding salt to ice water can lower the temperature to as low as -15 °C! To chemists, processes that require heat, like melting ice, are known as endothermic. Meanwhile, the water that is forming in the bag is just as cold — that is, below freezing. Because liquids conduct heat more readily than solids, the super-cooled water sucks heat away from the ice cream faster than the ice would alone, making it possible to make ice cream without the machine.
So there you have it: a little bit of delicious chemistry you can do at home. Have fun with it!
Special thanks to Joan Kaufmann, my high school chemistry teacher (who still teaches at my high school!) for doing this in our chemistry class — and for not failing me, even though I wrote the entire lab report in an “Irish” accent (I wish I was kidding. The report began “Methinks it was a wee bit chilly to be faring an ice cream labbin today”. Methought I was cool).
UPDATE: see the kids on TV for yourself!
Maybe my understanding of this is incomplete, but I’m under the impression that you don’t have to lower the temperature of liquid water at 0°C at all to form ice, you just need to remove heat. The resulting ice will still be exactly 0°C. Perhaps that’s a macro-level view, though.
Tom: yes, you are correct. In that sentence, I was not being a good chemist and distinguishing between the difference of cooling down and removing heat. My point was that liquid water (or for that matter, a ice/water mixture) at 0 degrees C with no inputs or outputs will never become solid ice – more energy needs to be removed from the system.
Tom: yes, you are correct. In that sentence, I was not being a good chemist and distinguishing between the difference of cooling down and removing heat. My point was that liquid water (or for that matter, a ice/water mixture) at 0 degrees C does not become solid ice; more energy needs to be removed from the system.
It says “at that temperature, liquid water freezes as quickly as ice forms”
Should be “at that temperature, liquid water freezes as quickly as ice melts”
“”I was not being a good chemist and distinguishing between the difference of cooling down and removing heat””
That is physics, btw.
The optimum ratio is when all the ice and all the salt is used up to make a saturated solution of salt in water. Because saturation concentration for common salt is 25 % (almost independent of temperature) the optimum ratio is 1 to 3 by weight. One does not need any “theory” or math for that,
just one small piece of fact knowledge :=)
Alternatively one can use the CRC-Handbook, which should be ready for use in any kitchen
Georg
This is awesome! I’m *definitely* trying it this weekend 🙂 Thanks Christie!
Go easy when you mix the ice/water mixture around the inner bag; if the inner bag breaks, you’ll get salty water in your ice cream mixture and that won’t taste very good!
here is a cool experiment to try at home with your kids.
Grow a pet TickleMe Plant that moves and closes its leaves when you Tickle It. This is a real plant and rather easy to grow
See video http://www.ticklemeplant.com