* Molecular Motion: As temperature increases, molecules within the liquid gain kinetic energy and move faster. This increased movement causes them to spread out further, leading to a larger volume.

* Volume Expansion: Since the volume increases while the mass remains the same, the density (mass/volume) decreases.

Exceptions:

While this is the general rule, there are some exceptions. For example, water exhibits an unusual behavior between 0°C and 4°C. In this range, water's density actually increases with temperature. This is due to the unique hydrogen bonding structure of water molecules.

Summary:

In most cases, a rise in temperature leads to a decrease in the density of a liquid due to increased molecular motion and volume expansion.