Freezing Point Depression
When making ice cream in two plastic bags, you typically use ice and salt in the larger bag to lower the temperature around the smaller bag that holds the ice cream ingredients. Salt, usually rock salt, is added to the ice because it lowers the freezing point of water. This is called freezing point depression.
Here’s how it works:
- Normally, water freezes at 0°C (32°F), but when salt is added, it interferes with the ability of water molecules to form ice crystals.
- The more salt you add (up to a certain point), the lower the freezing point of the water becomes. This can drop the temperature to as low as -21°C (-6°F), allowing the mixture to become colder than ice alone.
Heat Transfer
As the salt lowers the freezing point of the ice, the ice begins to melt. To melt, the ice needs to absorb energy (heat). In this case, it absorbs heat from its surroundings, including the ice cream mixture inside the smaller bag. This process is called endothermic reaction, where heat is absorbed, leading to a rapid cooling of the cream mixture in the smaller bag.
Formation of Ice Crystals
As the ice cream mixture loses heat, it begins to freeze. Stirring or shaking the bag helps prevent the formation of large ice crystals, which would make the ice cream grainy. The constant motion ensures the formation of smaller ice crystals, resulting in smooth, creamy ice cream.
Summary of the Process:
- The salt lowers the freezing point of the ice, making the temperature around the inner bag cold enough to freeze the ingredients.
- Heat is drawn out of the cream mixture into the ice-salt solution, cooling it and causing it to freeze.
- Shaking the bags helps to churn the mixture, ensuring smooth texture and even freezing.
This simple but effective method uses basic scientific principles to create ice cream without the need for special equipment!