Suggest why in an experiment for rusting the water level increases.

• A. Oxygen is used up, and water is used to take its place.
• B. Water evaporates.
• C. Water condenses.
• D. The water level increases due to temperature rise.

Answer: (A)

In rusting, iron reacts with oxygen (and water) to form rust, so oxygen is gradually used up in the reacting space. If you place the reaction in an inverted, water-filled tube, the amount of gas inside that space decreases as oxygen is consumed. With less gas and the same external pressure, water is pushed up into the space to fill the void, so the water level inside the inverted tube rises. This rise specifically reflects the consumption of oxygen, not evaporation, condensation, or a simple temperature effect.