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Scrumptious Science: Making Ice Cream in a Bag

A chilly, edible chemistry challenge from Science Buddies

Observations and results
Did the ingredients that were shaken in the large bag with the ice cubes and salt turn into ice cream whereas the ingredients that were shaken with ice cubes alone (and no salt) remain fluid, and not harden into ice cream?
 
You should have seen that the ice cubes in the large bag with salt melted much more, and felt much colder, than the ice cubes in the large bag without salt. Because it was cold enough (several degrees below freezing), the ice cube bag with salt should have been able to cool the ingredients enough to harden them and turn them into ice cream whereas the ice cube bag without salt wasn't cold enough to do this, leaving the ingredients fluid. (If you put the fluid ingredients back in the bag with ice cubes and salt and shook them for about five minutes, the ingredients should have been cooled sufficiently to turn into ice cream!)
 
If you have ever made ice cream with an old-fashioned hand-crank machine, you probably packed a mixture of ice and rock salt around the container holding the cream. The ice–salt combination gets colder than pure water ice and can freeze the ingredients in the ice cream machine (and in the bags you used in this activity), turning them into ice cream. (This is the same process that occurs when icy roads have salt spread on them to melt the ice, keeping the roads less slippery at lower temperatures.) Although pure water freezes at 0 degrees Celsius (32 degrees Fahrenheit), water mixed with salt will only freeze at colder temperatures below 0 degrees C.
 
Cleanup
If you haven’t already, you can enjoy your tasty ice cream treat now or save it in the freezer for later.
 
More to explore
Boiling-Point Elevation and Freezing-Point Depression, from Iowa State University
Make Ice Cream in a Baggie, from About.com Chemistry
Fun, Science Activities for You and Your Family, from Science Buddies
Chemistry of Ice Cream–Making: Lowering the Freezing Point of Water, from Science Buddies
 

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