What is the difference between m and mL in the First Law of Thermodynamics? Othey are the same (and equal) m is the mass that goes through a phase change (solid-liquid-gas) while mL is the total mass of the system mL is the mass that goes through a phase change (solid-liquid-gas) while m is the total mass of the system m is the initial mass of the system and ml is the mass that is lost during the process