How can the freezing of surface waters make the water that is left unfrozen denser

Explanation:

When an aqueous solution freezes, the unfrozen solution becomes denser. That's because some of the water has been removed as ice. However, the solutes remain dissolved in a solution with a smaller liquid volume. The concentration of the solution increases, so the density increases.

this is because water expands as it gets cold hence floating leaving the unfrozen water dense

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