Which statement best describes water in fuel tanks?

• A. Water is heavier than avgas and jet fuel and will settle toward the bottom.
• B. Water is lighter than fuels and will float on top.
• C. Water dissolves completely in fuel.
• D. Water evaporates quickly.

Answer: (D)

Water in fuel tanks follows the property that water is denser than aviation fuels, so it will settle to the bottom of the tank rather than mixing with the fuel or rising to the top. Because water and fuel are not miscible, water does not dissolve into the fuel, so you won’t get a single uniform solution. Also, water doesn’t evaporate quickly in this context; its boiling point is much higher than typical fuel temperatures, so it remains as a liquid at the bottom unless drained.

In short, the water layer at the bottom due to its higher density best describes what happens in fuel tanks. The other statements—water floating on top, dissolving completely, or evaporating quickly—don’t match the actual behavior of water in fuel.