Which is least compressible?

• A. Air
• B. Gasoline
• C. Water
• D. Steam

Answer: (C)

Compressibility is about how much a substance’s volume changes when pressure is applied. Gases have lots of space between their molecules, so squeezing them makes a big change in volume. Liquids have molecules packed tightly, held together by cohesive forces, so they don’t compress much under pressure. Water, in particular, has very strong intermolecular interactions and a tight structure, making it resist compression more than other liquids and far more than gases. Gasoline is a liquid and will compress more than water under the same pressure, while air and steam—being gases—compress readily. That’s why water is the least compressible option.