“It’s NEVER too cold for ice cream.” -Anonymous
Have you ever wondered how making ice cream the old-fashioned way actually works? Well, let’s say one night, you put both a water bottle and a container of vanilla ice cream in the freezer. The next day, when you open the freezer, the water bottle is frozen and rock-hard. But what about the ice cream? Why does the ice cream remain solid-like but still slightly soft? This is because ice cream has a lower freezing point than water. Ice cream contains many particles, which makes it harder for the water molecules in the ice cream to push the particles out of their way and become solid to form ice. The lowering of the freezing point of a liquid by adding compounds to it is known as freezing point depression. This is exactly what occurs in the process of making ice cream!
To get an idea of what freezing point depression is and how it works, refer to the diagram below:
The vapor pressure of the solution is lower than that of the pure solvent. Therefore, the freezing point is also lowered.
You can also see in this picture of a heating curve, the curves for a pure liquid and solution (liquid with added impurities):
See how the freezing point of the solution is lower than the freezing point of the pure liquid. As I stated before, this is because the solution has added particles into it that the pure liquid does not have, making it more difficult for water to reach the order and organization required to form ice.
To better understand this concept, you can think of freezing point depression in the process of making ice cream. Here’s a recipe for how to make ice cream (you can try it out!):
One sandwich-size Ziploc bag Larger Ziploc bag Sugar Cream or milk Vanilla extract, or other flavoring Rock or coarse salt Measuring cup and spoons Crushed ice cubes or “party” ice
1. Make up the ice cream mixture by adding to the smaller bag:
– 100mL (1/2 cup) of cream or milk
– 25mL (5 tsp) of sugar
– A few drops of vanilla or other flavoring
2. Fill the larger bag half full with ice.
3. Sprinkle 100mL (1/2 cup) of rock or coarse salt over the ice.
4. Seal and place the small bag inside the larger one surrounded by the ice mixture. Seal the big bag.
5. Time one-minute intervals. At the end of each minute, flip the bag over on the other side. Repeat the “flippings” about 10 times.
6. After 10 times, begin flipping the bag over every 30 seconds for 5 minutes.
7. Check to see if your ice cream is the right consistency. If it is not, continue flipping the bag at 30-second intervals for another 5 minutes.
Pay particular attention to step 3, which is highlighted in a cyan color. This will be the focus of our discussion.
While making ice cream, heat must be removed from the water in the cream to solidify. However, because of all the particles ice cream contains, the temperature must drop below 0 degrees Celsius in order for the mixture to become solid. Therefore, pure ice cannot be used to freeze ice cream, because it’s freezing point is 0 degrees Celsius, which is higher than the freezing point of ice cream (which is around -3 degrees Celsius).
A solution to this problem is the addition of rock salt to ice, which lowers its freezing point. This is because energy is required to form the hydrogen bonds necessary for ice, and when impurities such as salt are added to water or ice, they prevent water from making hydrogen bonds and achieving solidity. Hence, the water must become even colder before it freezes. So how does this affect the immediate temperature of the ice? Adding salt to ice causes the temperature of the brine solution to decrease dramatically, because as the ice melts, the “heat” of the ice mass is preserved by lowering the temperature (this is called latent heat). As a result, the temperature is lowered to below the freezing point of pure water, and the solution can be used to freeze ice cream.
When the cream in ice cream cools down and loses its heat to the salt and ice solution, ice crystals begins to form. Water freezes out of a solution in its pure form as ice. In a sugar solution such as ice cream, the initial freezing point of the solution is lower than 0 degrees Celsius due to these dissolved sugars and the impure particles in the ice cream, as I mentioned before. As ice crystallization begins and water freezes out in its pure form, the concentrations of the remaining solution of sugar is increased due to this removal of water, and hence the freezing point of the ice cream is further lowered. This process is shown here:
This process of freeze concentration continues to very low temperatures. Even at the typical ice cream serving temperature of -16 degrees Celsius, only about 72% of the water is frozen. The rest remains as a very concentrated sugar solution.
Thus when temperature is plotted against % of water frozen, we get the phase diagram shown below:
This helps to give ice cream its ability to be scooped and chewed at freezer temperatures. Air content also contributes to this ability.
Still find the concept of Freezing Point Depression difficult to comprehend? Check out the video below for a live demonstration and explanation of the concept with ice cream:
Author: Jamie Lee