Final answer:
The mass of the molten iron will still be 100.0 g after it has melted, as the mass of a substance does not change during a phase change due to the law of conservation of mass.
Step-by-step explanation:
The mass of molten iron does not change upon melting; it remains the same as the mass of the solid iron before melting. Therefore, if an iron bar weighs 100.0 g before melting, the mass of the molten iron after it has liquefied will also be 100.0 g. The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction, which includes phase changes like melting. Thus, regardless of the phase change from solid to liquid, the mass remains constant.
It is important to distinguish between mass and volume changes upon melting. While some substances may expand or contract when changing phase, the mass remains unaffected. Temperature changes will also not alter the mass. Therefore, in the case of the iron bar exceeding its melting point of 1535 °C to become molten iron, the mass stays the same at 100.0 g.