By Joy Campbell | Updated Aug 30, 2022
Understanding the freezing behavior of liquids is fundamental in physics and chemistry. The presence of solutes, temperature, and other factors can alter the freezing point, making some liquids freeze faster than others.
Set up a simple investigation by having students compare orange juice, tea, water, and milk. Each student receives 3–4 cups, fills them half‑full with a different liquid, and places them in the freezer. Every 25 minutes a toothpick is inserted to test for ice. Students record the order of freezing and compare the results to their hypothesis. Typically, plain water freezes first because it contains no dissolved solutes.
Students prepare three cups of tap water: one unchanged, one with one tablespoon of sugar, and one with one tablespoon of salt. After thorough mixing, the cups are frozen and checked every 30 minutes. The expected outcome is that tap water freezes first, followed by sugar water, with salt water freezing last due to the greatest freezing‑point depression.
To explore the Mpemba effect, students fill one cup with hot water (≈80°C) and another with cold water (≈5°C). Both are placed in the freezer and examined every 25 minutes. Contrary to intuition, hot water often freezes sooner because dissolved gases are expelled, reducing the number of nucleation sites.
Using identical containers, one cup is filled with fresh water, the other with salt water. Both are cooled to the same initial temperature, then frozen. Observations taken every 30 minutes consistently show fresh water freezing faster, confirming that solutes lower the freezing point.
These experiments provide hands‑on evidence of colligative properties and reinforce scientific reasoning. For more background on freezing‑point depression, see the American Chemical Society’s overview at acs.org.
