In an experiment for rusting, why might the water level rise?

• A. Oxygen is used up, and water is used to take its place
• B. Water evaporates
• C. Water is consumed to form rust
• D. The water level should not change

Answer: (A)

The main idea is that rusting uses up oxygen from the air in the space above the liquid. As this oxygen is consumed, the gas in that headspace becomes less, so the pressure there drops. With lower pressure in the space, the surrounding liquid is pushed up into that space to balance the pressure, so the water level rises. Water isn’t being evaporated or consumed in a way that would steadily raise the level; the rise comes from gas being removed and the liquid moving in to fill the space left behind.