The Salty Secret: Ultimate Guide to Homemade Ice Cream

With blistering summer temperatures on the rise, ice cream is the perfect way to cool off. Here is a homemade ice cream recipe that relies on—what we call in chemistry—the colligative properties of solutions.

Materials:

• Milk

• Heavy Cream

• Sugar

• Vanilla Extract

• Table Salt (Sodium Chloride)

• Ice

• 1-Gallon Ziploc Bag

• 1-Quart Ziploc Bag

• Gloves (optional)

Steps:

1. Add equal parts milk and heavy cream (around 1/2 cup) to the quart-sized Ziploc bag. Add 1/4 cup of sugar and 1/4 teaspoon of vanilla extract. Seal the bag and mix thoroughly.

2. Put enough ice into the gallon-sized Ziploc bag to fill it about halfway. Add about 1/2 to 1 cup of table salt.

3. Place the quart-sized Ziploc bag inside the gallon-sized Ziploc bag. Seal and shake/mix the bag's contents for around 10 minutes. You may want to wear gloves for this portion of the process since the bag will become extremely cold.

4. When the ice cream has solidified and reached an ice-cream-like consistency, open the gallon-sized Ziploc bag and remove the quart-sized Ziploc bag.

5. Open the smaller bag, stick a spoon in, and enjoy!

One of the challenges of making ice cream at home is getting the ingredients cold enough to freeze. This is largely due to the ice cream mixture's low freezing point, which can be attributed to the fat molecules that hinder the particles in the solution from crystallizing into a solid. Since the ice cream mixture freezes at a lower temperature than ice melts, using ice by itself will generally not absorb enough heat to freeze the mixture: the ice will melt before ice cream is made. However, we can use the chemistry of colligative properties to solve this problem.

Freezing point depression is a colligative property that describes a drop in the freezing point of a solution when a substance is dissolved in it. When something like table salt (Sodium Chloride) is dissolved in solution, it dissociates into the sodium and chloride ions: positively and negatively charged atoms that will interact with the molecules in solution. These ions will exert ion-dipole forces, which are strong attractive forces, on the water molecules in the solution. These forces make it difficult for water molecules to crystallize, lowering the temperature at which the solution will freeze.

So how is freezing point depression used in making ice cream? By adding table salt to the bag of ice, we depress its freezing point, allowing water to exist at temperatures below its standard freezing point of 0° C. This ensures that when the ice melts, the water is still at cold enough temperatures to freeze the ice cream mixture and form the creamy goodness that we long for on hot summer days.