Yes, the density of a substance generally changes when it freezes. Here's why:

* Density is mass per unit volume. When a substance freezes, its molecules become more ordered and pack more tightly together.

* Exceptions: While most substances become denser when they freeze, water is a notable exception. Water expands when it freezes, which is why ice floats. This is because the hydrogen bonds in water create a more open, crystalline structure when it's in the solid state.

Here's a summary:

* For most substances: Density increases when freezing.

* For water: Density decreases when freezing.