The trick on this activity is to buy materials right so that you don't get soaked in the process.
A few up-front suggestions that will greatly help:
1. Get some other chemistry or science teachers on board to do the activity at the same time. It makes it a lot more fun, distributes the work and cost load, and avoids those fellow teachers hearing "how come we're not making ice cream too?" all day long.
2. If you are an "extra credit" teacher, have students bring in paper towels, napkins, spoons, toppings, etc. Usually you end up with lots of extras that you can use for classroom supplies next year.
3. If at all possible, do the activity outside. It will greatly alleviate collateral damage to your room, and the custodians will thank you.
4. Buy your milk, sugar (in 20 or 50lb bags), freezer zip locks, plastic cups and spoons, and vanilla at Costco, Sams Club, or other warehouse store. Ask them if they will donate - many stores will.
5. Stock up on ice from the cafeteria a few days in advance. Find a couple of freezers (or use the cafeteria's), load up some plastic wastebasket bags with ice, and save them for lab day. Ice is one of the more costly materials, and if you can get it for free you can save significan cost.
Another great source of ice is the training room up at the gym - ask a coach if there is one available, and use it during the day. Just send up a TA or a couple of football players at the beginning or end of each period.
6. Another great cost saver I figured out after a few years of doing this: DON'T use rock salt from the grocery store ($1.00/lb). Simply buy a few bags of coarse water softener salt (also available at the grocery store, usually up at the front). They come in 40 lb bags, cost about $4.50 apiece, and are the exact same thing at a tenth the cost!
Day of the Lab Procedure:
I put everything on a large lab cart and wheel it outside each period, and then back in at the end of each period for restocking. The sugar and salt comes directly out of the large bags.
Out in our quad there are some large, raised cement planter areas. I wheel the cart around the planter, spreading the materials out in "stations" so that the students don't get to crowded at any one area. They start with a quart zip-lock bag, then move over to the milk. 1 cup of whole milk goes into the bag. Next is the sugar stop - they put in 1/3 cup. Next is the vanilla stop - about 1 teaspoon (or a couple of squirts from a berel pipet). Now they close the bag, removing most of the air in the process. MAKE SURE IT IS SEALED TIGHTLY and is not leaking!
Roll the bag around for a minute or two to dissolve the sugar, then place it into a gallon zip-lock freezer bag. Now move to the ice stop and place 2-3 cups of ice into the gallon bag, enough to cover the milk bag. Then go over to the salt station and add about 1/2 cup salt to the ice. Zip the whole thing shut, removing most of the air.
Now the shaking begins. Tell students to shake for about 10-15 seconds, then set it down for a minute or two. Then repeat. If they shake it constantly you will end up with lots of cases of frostbite on fingers. Shake, set it down. Repeat this process for about 10-15 minutes, until it is the consistency of frozen yogurt.
At this point they dump the ice/salt mixture into a 5 gallon bucket that I have setting nearby. Scoop the ice cream into a paper or plastic cup, add toppings if you have them, and enjoy!
You can reuse the gallon zip-locks, although I don't like the hassle and just throw them away. I place a couple of large trash bags outside and everything goes into them. There are sewer drains outside in the quad, and I dump all of the salt/ice mixture down into them.
-Quart zip loc freezer bags - one per student
-Gallon zip lock freezer bags - one per student
-Whole milk - 1 cup per student (about 2 gallons per class of 30)
-Sugar (granulated) - 1/3 cup per student (one 50lb bag from Costco should cover 2 teachers X 6 periods)
-Vanilla (cheap stuff) - about 1 tsp (5 mL) per student
-Coarse water softener salt - 1/2 cup per student (three or four 40lb bags for 2 teacher's classes)
-plastic spoons
-plastic cups
-napkins
-garbage bags
-measuring cups, or marked plastic cups
