The texture of homemade ice cream is more akin to soft serve than the hard scoop consistency you might be used to from your favorite scoop shop or store-bought brand.

Freezing point depression is not unique to mixtures made of water and salt it can also happen with some other liquid mixtures. Meaning it makes the temperature at which water freezes lower than it is for pure water. In technical terms, the salt causes a freezing point depression. 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). As a result, the temperature inside the bag with ice and salt is colder than the temperature inside the bag with just the ice.

As it is melting, the ice absorbs heat and thus lowers the temperature. So, overall the ice cubes start melting faster. This means that the rate of the ice re-freezing at the surface goes down. 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. As the ice cubes start to melt a little bit the water molecules mix with the salt molecules. 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 rate at which these two things happen determines what you see. Even after ice is formed two things are constantly happening at the surface of the ice. 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. The warmer the surrounding temperature the more the molecules bounce around, and the harder it is to get them to line up.

In order for the crystals to form, the water molecules have to slow down and carefully line up in an orderly pattern. 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. 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. Emulsions are the combination of two liquids that normally do not mix well like fats and water. In creating your ice cream you also made an emulsion. (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. 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. The salt allows the ice and salt mixture to get colder than pure water ice. 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. In contrast the ice cube bag without salt was not cold enough to do this and the ingredients should have remained fluid.ĭo 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.

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. 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. If you would like, you can enjoy your tasty ice cream treat now or save it in the freezer for later.

Ice cream making