Making ice cream in a bag works by creating a super-cooled environment that allows the liquid ice cream mixture to freeze despite not being in a freezer. The key is salt's effect on the freezing point of water.
The Science Behind It
Here's a breakdown of the process:
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Ice and Salt: The outer bag contains ice and salt. When you add salt to ice, you lower the freezing point of the water. Pure water freezes at 0°C (32°F). Saltwater freezes at a lower temperature.
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Melting Process: Because the ice is now above its freezing point (due to the added salt), it begins to melt. This melting process requires energy.
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Energy Absorption: The energy required for the ice to melt is absorbed from the surroundings, specifically the flavored milk mixture in the inner bag. This transfer of heat causes the temperature of the milk mixture to drop significantly.
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Freezing: As the milk mixture loses heat to the melting ice bath, it eventually reaches its freezing point and transforms into ice cream. The constant shaking helps to create small ice crystals, resulting in a smoother texture.
In essence, the salt lowers the freezing point of the ice, forcing it to melt and draw heat away from the ice cream mixture, causing it to freeze. The bag acts as a container to keep the ice, salt, and ice cream mixture separate, while facilitating the transfer of heat.
