Apentix/AP Biology/Unit 1: Chemistry of Life/Question
AP Biologymediummcq1 pt

A lake freezes from the top down while fish survive in the colder but liquid water below. Which explanation is correct?

A.Cold water keeps sinking until it turns to vapor beneath the surface layer.
B.Wind pushes newly formed ice aside because ice barely adheres to shorelines.
C.Hydrogen-bonded open spacing in ice leaves it less dense than liquid water, so it floats and insulates the water beneath.
D.Salt expelled during freezing raises the density of freshwater ice above that of water.

Explanation

Core Concept

As water cools toward 4 degrees Celsius it grows denser, but further cooling favors directional hydrogen bonds that lock molecules into an open hexagonal lattice. Average molecular spacing in that lattice exceeds that of the jostling liquid, so ice is roughly 8 percent less dense and floats. Surface ice then shields deeper water, which stays near 4 degrees Celsius, allowing fish and other organisms to overwinter. If ice sank, ponds would freeze solid from the bottom up and most freshwater aquatic life could not persist.

Correct Answer

CHydrogen-bonded open spacing in ice leaves it less dense than liquid water, so it floats and insulates the water beneath.

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