The mass of the fully melted mass and the initial solid will be the same. So, the mass of the melt is equal to M.
Mass is the amount of matter contained within a substance. Since only the phase changed and the amount of matter is still the same, the mass of the molten phase and the solid phase will remain the same.
We are correct to say that in the heating process no mass was destroyed or added in melting the solid.
A simple phase change that preserved the mass only occurred.
