By Jack Brubaker
Updated Aug 30, 2022
State and municipal governments frequently spread salt on roads to keep them safe. By lowering the ice’s melting point—an effect known as freezing‑point depression—salt helps keep traffic flowing. The same principle makes for a fun and educational science experiment that can range from a quick demonstration to a detailed project complete with mathematical predictions. All you need is a saucepan and a thermometer.
When a solid dissolves in water, it breaks into tiny particles. For sugar, these are individual molecules; for salts such as sodium chloride, they are charged ions. These particles disrupt the ability of water molecules to arrange into a solid as the temperature approaches freezing, thus lowering the freezing point. This phenomenon is universal across liquids, not just water.
Choosing the right measurement is key. Rather than asking which sample freezes first, focus on the temperature at which each solution reaches the solid state. This approach reveals how impurities alter the freezing point and provides a clearer, more scientifically meaningful result.
The established equation for freezing‑point depression is:
ΔT = –k · m
where k is the solvent’s molal freezing‑point depression constant and m is the solution’s molality (moles of particles per kilogram of solvent). For water, k equals 1.86 °C·kg mol⁻¹. Sucrose (C₁₂H₂₂O₁₁) has a molecular weight of 342.3 g mol⁻¹, so the formula simplifies to:
ΔT = –1.86 × (grams sucrose / 342.3 / kg water)
Example: Dissolve 10 g of sucrose in 100 mL (≈0.100 kg) of water. ΔT = –1.86 × (10 / 342.3 / 0.1) ≈ –0.54 °C. Thus, the solution freezes at about 0.54 °C below pure water’s freezing point.
By rearranging the formula, students can determine a solute’s molecular weight experimentally:
MW = (–1.86 × grams sucrose) / (ΔT × kg water)
Many high‑school and college chemistry courses use this method to find the molecular weight of unknown substances. A similar approach works for boiling‑point studies, with the constant k changing to 0.52.
