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An ice cube floats in water, and a layer of oil (less dense than water) is poured on top of the water. As the ice melts completely, what happens to (i) the level of the water-oil interface, and (ii) the uppermost level of the oil?

1. rise and fall
2. fall and rise
3. rise and not change
4. not change and fall

Correct answer: not change and fall

Solution

The ice displaces water (below interface) and oil (above interface). When the submerged (water) portion melts, it exactly fills the water displaced - water level (interface) stays the same. When the portion in oil melts, it becomes water that sinks below the oil, so oil is no longer displaced upward by that volume. The upper oil level falls. The interface: the melt from the oil-submerged part adds water volume below the interface, causing interface to RISE. Overall: interface rises, oil top falls.

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