The density of pure water is maximum at which temperature, affecting the vertical cycling of water in winter lakes?

Correct answer: 4°C

Answer: B, 4°C. Pure water has an unusual expansion behaviour: as it cools from higher temperatures to 4°C, it becomes denser, but cooling below 4°C makes it expand slightly and become less dense. Therefore, in a cooling lake, water near 4°C tends to sink, while colder water remains nearer the surface; at 0°C the surface can freeze and ice floats. This density pattern helps explain seasonal mixing and vertical movement of dissolved substances in lakes. A is incorrect because water at 0°C is less dense than water at 4°C. C is incorrect because water is much less dense at its boiling point. D is not the maximum-density temperature and is below the freezing point of ordinary water. Memory cue: “four sinks, zero floats.”