* Velocity decreases.

Here's why:

* Adiabatic process: This means no heat exchange with the surroundings (Q = 0).

* Expansion: The volume of the system increases.

* Conservation of energy: In an adiabatic process, the internal energy of the system decreases as it expands. This is because the system does work on the surroundings (expanding against pressure), and since no heat is added, the energy has to come from the internal energy.

* Internal energy and velocity: Internal energy is related to the kinetic energy of the molecules. A decrease in internal energy means the molecules move slower, resulting in a decrease in velocity.

In summary:

Adiabatic expansion leads to a decrease in internal energy, which in turn causes the molecules to slow down, resulting in a decrease in velocity.