* Density is mass per unit volume.
* Thermal expansion causes an increase in volume. The molecules of the substance move further apart as they gain energy from heat.
* Mass remains constant. The amount of matter in the substance doesn't change.
Since the volume increases while the mass stays the same, the density (mass/volume) decreases.
Example: Imagine a balloon filled with air. When you heat the air inside, it expands, causing the balloon to become larger. The air inside still has the same mass, but it now occupies a bigger volume, making its density lower.
