Exploring the Science Behind Making Ice Cream at Home

Video ice cream in a bag science


Have you ever tried making your own ice cream? It can be a super fun activity that not only satisfies your sweet tooth but also uncovers the fascinating chemistry behind creating this frozen treat. Just think about it: you start with ordinary ingredients at room temperature, and by cooling them down, they magically transform into creamy, delicious ice cream. Ever wondered how this happens? How important is it to cool the ingredients to a specific temperature? In this article, we’ll guide you through making your very own ice cream in a bag while exploring the best method to chill the ingredients for that perfect consistency.


What Happened?

You probably noticed that the ice cubes in the large bag with salt melted much more quickly and felt a lot colder than the ice cubes in the large bag without salt. Why is that? Well, the ice cube bag with salt, being several degrees below freezing, had the ability to cool the ingredients enough to harden them into ice cream. On the other hand, the bag without salt couldn’t achieve the same level of coldness, resulting in the ingredients remaining in their liquid state.

Don’t worry if your second bag didn’t yield ice cream at first. You can easily fix that! Simply place the small bag containing the still-liquid ingredients into the large bag with ice cubes and salt, and give it a good shake for another five minutes.

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If you’ve made ice cream using an old-fashioned hand-crank machine, you may recall adding a mixture of ice and rock salt around the container holding the cream. The salt actually enhances the freezing process by lowering the temperature of the ice and turning it into an extra-cold mixture. This super-chilled combination of salt and ice is what freezes the ingredients in the ice cream machine (and in the bags you used for this activity) and ultimately transforms them into delicious ice cream. (Incidentally, the same principle is employed when salt is spread on icy roads to melt the ice.) While pure water freezes at 0 degrees Celsius (32 degrees Fahrenheit), water mixed with salt can freeze at temperatures below 0 degrees Celsius.

When you made your ice cream, you inadvertently created an emulsion. Emulsions are a blend of two liquids that don’t typically mix well together, such as fats and water. In your ice cream, the fat molecules from the cream blend perfectly with water, ice crystals, sugar, and small pockets of air, resulting in a scrumptious cold treat.

Digging Deeper

To understand how salt causes the ice to melt and lowers the overall temperature in the bag, we need to delve a little deeper into the science of ice. Ice represents the crystallized form of water. For these crystals to form, water molecules must slow down and carefully align themselves in an orderly pattern. The higher the surrounding temperature, the more the water molecules bounce around, making it challenging for them to align. Think of them as excitable kids! As the temperature drops, the molecules slow down, facilitating a more precise alignment and the formation of ice crystals.

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Even after ice is formed, two processes continually occur at the surface of the ice. First, the ice at the surface starts melting. Second, the melted water re-freezes. The rate at which these processes happen determines what we observe.

In the case of our ice cream bags, one with salt and ice and the other with just ice, both bags were exposed to the same outside temperature, so the water molecules moved at the same rate. The crucial difference lies in the presence of salt. As the ice cubes in the bag with salt begin to melt, the water molecules mix with the salt molecules. Now, you have salt and water molecules moving together, disrupting the orderly pattern of the water molecules. This interference from the salt molecules reduces the rate of re-freezing at the ice surface, causing the ice cubes to melt faster overall. While melting, the ice absorbs heat, leading to a decrease in temperature. Consequently, the bag containing ice and salt ends up colder than the bag with only ice. In fact, the presence of salt can lower the freezing point of water below its usual 0 degrees Celsius temperature. This phenomenon is known as freezing point depression, which occurs not only in water-salt mixtures but also in certain other liquid combinations.

For Further Exploration

  • If one of your bags didn’t yield ice cream initially, try placing it back in the large bag with ice cubes and salt, and shake it for an additional five minutes. Did it turn into ice cream this time? What do you think caused the difference in results?
  • Experiment with different ingredients like milk or heavy whipping cream instead of half-and-half. How does using different dairy products affect the texture and flavor of the ice cream?
  • Explore making ice cream in a bag using various types of salts. Do you notice any differences in the final product when using table salt, rock salt, or other types? Can you provide an explanation for your observations?
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