Why does the relation C1V1 = C2V2 hold across a dilution?

1. The molar amount of solute equals the molarity multiplied by the volume in litres, written n = ML. 2. Write that once for the solution before dilution and once for the solution after it. 3. Adding solvent does not change how much solute is present, so the two amounts are equal. 4. Setting them equal gives M1L1 = M2L2, generalised as C1V1 = C2V2. 5. The volume clearly changes during dilution, since that is the whole point of adding solvent. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.3 Molarity_