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Heavy water D₂O melts at 3.8°C vs H₂O at 0°C. Explain. Step 1: D is twice as heavy as H, so D₂O molecules are ~11% heavier. Step 2: Heavier molecules vibrate more slowly (lower zero-point energy). Step 3: O–D···O hydrogen bonds are slightly stronger than O–H···O. Step 4: More thermal energy needed to break the lattice → higher melting point. This is the kinetic isotope effect in action.