Colligative Properties
Colligative Properties are ways that adding a substance to a liquid changes how it freezes, boils, or behaves, based on how many particles you add.
When salt dissolves in water, the salt particles crowd into the liquid and get between the water molecules. Those extra particles make it harder for water molecules to break free and boil or freeze. Adding more salt means more particles in the way, so the water needs more heat to boil and more cold to freeze. The number of particles added — not what kind they are — is what changes the boiling and freezing points.
Explaining colligative properties by grade level
When you add salt to water before freezing it, the salty water stays liquid longer than plain water. The salt gets in the way of the ice forming. The more salt you add, the harder it is for the water to freeze. That is why people put salt on icy roads in winter.