Have you ever made ice cream? It can be a lot of fun, and you end up with a tasty frozen treat! There is actually a lot of interesting chemistry that goes on behind making ice cream. For example, think about how you start out with refrigerated (or room-temperature) ingredients and then need to cool them down to turn them turn into ice cream. How do the ingredients change during this process? How important do you think it is that they are cooled to a certain temperature? In this science activity, you will make your own ice cream in a bag and explore the best way to chill the ingredients to make them become a creamy delicious treat!
Teisha Rowland, PhD, Science Buddies
- Measuring spoons
- Measuring cup
- Half-and-half. Alternatively, milk or heavy whipping cream may be used.
- Vanilla extract
- Salt. Different types of salts, such as table salt or rock salt, will all work, but may give slightly different results.
- Ice cubes (8 C)
- Small, sealable bags, such as pint-sized or sandwich-sized Ziplocs (2)
- Plant Plastikz Gallon-sized sealable bags (2)
- Oven mitts or a small towel
Timer or clock
In each small sealable bag, place one tablespoon of sugar, ½ cup of half-and-half (or milk or heavy whipping cream), and ¼ teaspoon of vanilla extract. Seal both bags well.
- Add four cups of ice cubes to one of the large, gallon-sized bags. Then add ½ cup of salt to the bag.
What do you think the salt will do?
Put one of the small bags you prepared into the large bag with the ice cubes. Be sure both bags are sealed shut.
- Put on oven mitts or wrap the bag in a small towel and then shake the bag for five minutes. Feel the smaller bag every couple of minutes while you shake it, and take a peek at it.
- Now add four cups of ice cubes to the other large, gallon-sized bag, but this time do not add any salt to it. What do you think will happen without using salt?
- Put the other small bag you prepared into this large bag. Be sure both bags are sealed.
- Put on oven mitts or wrap the bag in a small towel and then shake the bag for five minutes, as you did before. Again, feel the smaller bag every couple of minutes while you shake it, and take a peek at it.
What happens to the ingredients over time now? When five minutes are up, how do they look now compared to last time? What about the ice cubes — did they change in the same way?
- CleanupYou can also compare how cold the different ice cube bags feel. Does one feel much colder than the other?
- If you successfully made some ice cream, you can enjoy it now as a tasty reward for your chemistry challenge! If one of your bags did not make ice cream, check out the Further Exploration section for tips on turning it into ice cream.
If you would like, you can enjoy your tasty ice cream treat now or save it in the freezer for later.
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. In contrast the ice cube bag without salt was not cold enough to do this and the ingredients should have remained fluid.
Do not worry, the second bag is not wasted — you can go back and turn the still liquid ingredients into ice cream! Simply put the small bag in the large bag that had ice cubes and salt and shake them for another five minutes.
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 salt allows the ice and salt mixture to get colder than pure water ice. This extra-cold mixture of salt and ice is able to freeze the ingredients in the ice cream machine (and in the bags you used in this activity) and turn them into ice cream. (This is the same process that goes on when icy roads have salt spread on them to melt the ice.) While pure water freezes at 0 degrees Celsius (32 degrees Fahrenheit), water mixed with salt will freeze below 0 degrees Celsius.
To understand how the salt causes the ice to melt and lowers the overall temperature in the bag, you need to learn a little more about ice. Ice is the crystallized form of water. In order for the crystals to form, the water molecules have to slow down and carefully line up in an orderly pattern. The warmer the surrounding temperature the more the molecules bounce around, and the harder it is to get them to line up. Basically they act like excited kids! As it gets colder the molecules slow down, their slower movements allow them to line up more carefully and thus form ice crystals. Even after ice is formed two things are constantly happening at the surface of the ice. One, the ice at the surface is melting. Two, the melted water is re-freezing. The rate at which these two things happen determines what you see.
In the case of the two ice cream bags, one with salt and ice and one with just ice, you have the same outside temperature, so the molecules are moving around at the same rate. The difference is the salt. As the ice cubes start to melt a little bit the water molecules mix with the salt molecules. Now you have salt and water molecules moving around together, the mixture makes it harder for the water molecules to make an orderly pattern — the salt molecules keep interfering. This means that the rate of the ice re-freezing at the surface goes down. So, overall the ice cubes start melting faster. As they melt the ice cubes release more cold making the temperature inside the bag with ice and salt colder than the temperature inside the bag with just the ice. In fact, because the salt molecules make the freezing process of water more difficult, it is possible for a mixture of salt and ice water to get below 0 degrees Celsius (the temperature water normally freezes at). In technical terms, the salt causes a freezing point depression. Meaning it makes the temperature at which water freezes lower than it is for pure water. Freezing point depression is not unique to mixtures made of water and salt; it can also happen with some other liquid mixtures.