Which state of matter has no defined shape and no defined volume?

• A. Gas
• B. Liquid
• C. Solid
• D. Plasma

Answer: (A)

In this question, the key idea is how matter behaves in terms of shape and volume. In the gas state, particles are spread far apart and move freely, so there are no fixed boundaries holding the matter in place. Because of this, a gas will expand to fill whatever container it’s put in, adopting the container’s shape and volume. So there isn’t a defined shape or a defined volume on its own—the container determines both.

For contrast, solids keep a definite shape and volume because their particles are tightly packed and vibrate in place, while liquids have a definite volume but take the shape of their container, since their particles flow but don’t compress to change volume. Plasma behaves like a high-energy, ionized gas, so it also lacks a fixed shape or volume in many contexts, but in basic descriptions the standard match for this prompt is gas.