An ice cube floats in water, on top of which a layer of oil (less dense than water) has been poured. After the ice has melted completely, how do the water-oil interface level and the topmost oil surface change, respectively?

rise and fall
fall and rise
rise and not change
not change and fall

Correct answer: not change and fall

Solution

The water produced by melting exactly replaces the volume of water the submerged part displaced, so the water-oil interface stays put. The portion of ice that protruded into the oil no longer displaces oil, so the oil column drops, lowering the top oil surface.

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