A real cubic expansivity is the increase in volume of a liquid per unit volume per degree rise in temperature when heated in an inexpansible vessel

How can volume expand in an inexpansible vessel? It can't.

Maybe it was half full?

True.

The term "cubic expansivity" refers to the coefficient of volume expansion, which represents how much a substance's volume increases per unit volume per degree rise in temperature. In the case of a liquid, the real cubic expansivity specifically refers to the increase in volume of the liquid when heated in an inexpansible (unexpanding) vessel.

To determine whether the given statement is true, we need to understand the definition of real cubic expansivity. If a liquid's volume increases per unit volume per degree rise in temperature when heated in an inexpansible vessel, then it aligns with the definition of real cubic expansivity.

Therefore, the statement is true.