Which statement about solids is true?

• A. They have no definite shape.
• B. They flow to fill containers.
• C. They are always in motion with equal energy.
• D. They maintain a definite shape and volume.

Answer: (D)

Solids keep a definite shape and volume because their particles are tightly packed in a fixed, orderly arrangement and are held together by strong intermolecular or ionic bonds. The particles can only vibrate in place, not slide past one another, so the overall shape and size stay the same unless you add enough energy to melt or deform the material. This is why solids resist changes in shape and volume.

As a contrast, liquids have particles that can slide past each other, so they flow and take the shape of their container but keep a roughly fixed volume. Gases have even more freedom and will fill both the shape and the volume of their container. The important idea is that in a solid, motion is limited to vibrations about fixed positions, which is why the definite shape and volume are maintained.