An aqueous solution is warmed from 20 °C to 40 °C in a sealed flask with no evaporation. Which concentration value changes?

Core Concept

Molarity divides moles of solute by liters of solution, and liquid volume expands slightly as temperature rises. The mole count stays fixed while the volume grows, so the molarity drops slightly on warming. Molality, mass percent, and mole fraction are all built only from masses or mole counts, which warming cannot change. That volume-free construction is exactly why chemists switch to molality for work across wide temperature ranges.