# Which property of physical changes explains why matter is conserved in a physical change?

Do you mean $\text{physical change}$ or $\text{chemical change}$?
Mass is always conserved in a chemical change, which process involves involves the rearrangement of atoms and molecules. Of course most physical changes also conserve mass: when $10 \cdot g$ of liquid water is frozen, you are left with $10 \cdot g$ of solid ice.
Of course, there are also nuclear reactions (I am bit reticent, because I don't know whether I should put this under the $\text{physical}$ umbrella). In nuclear fusion and fission, matter is not conserved, and can be converted into energy, with the result that the energy output/input dwarfs those observed in chemical reactions.