Make Ice Cream in a Bag

Introduction

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!

Materials

• Measuring spoons
• Measuring cup
• Sugar
• 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)
• Gallon-sized sealable bags (2)
• Oven mitts or a small towel
• Timer or clock
• Video instructions are available in English and Spanish
  
  
  

What Happened?

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.

In creating your ice cream you also made an emulsion. Emulsions are the combination of two liquids that normally do not mix well like fats and water. In your ice cream the fat molecules in the cream are perfectly mixed with water, ice crystals, sugar, and small pockets of air to form a delicious cold treat.

Digging Deeper

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 it is melting, the ice absorbs heat and thus lowers the temperature. As a result, the temperature inside the bag with ice and salt is 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.

For Further Exploration

• If one of your bags did not make ice cream, try putting it back in the large bag that had ice cubes and salt and then shake them for five minutes. Did it turn into ice cream? Why do you think you got the results that you did?
• Instead of half-and-half, you could use milk or heavy whipping cream to make ice cream in this activity. How does making ice cream with half-and-half compare to making it using milk or heavy whipping cream?
• You can make ice cream in this activity using different types of salts, but you may get different results. How does ice cream made in a bag with table salt compare to ice cream made with rock salt or some other type of salt? Can you explain your results?

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