What effect does adding NaCl have on the freezing point of water?

Adding NaCl (sodium chloride) to water lowers the freezing point, a phenomenon known as freezing point depression. When NaCl is added to water, it dissociates into sodium (Na⁺) and chloride (Cl⁻) ions. These ions interfere with the ability of water molecules to form the organized crystal structure necessary for ice.

In pure water, molecules can align themselves in a solid structure at 0°C. However, the presence of Na⁺ and Cl⁻ ions disrupts this alignment. As a result, a lower temperature is required for the water molecules to overcome the disruption and form ice. This is why the freezing point of the solution decreases when NaCl is added, making it less than 0°C.

This principle is commonly applied in practical situations, such as using salt to melt ice on roads in winter, as the salt prevents the water from freezing at the usual 0°C.