Which statement about the theory of molecular motion is true?

Which statement about the theory of molecular motion is true?

• A. All matter is composed of molecules that are stationary
• B. Temperature does not affect molecular motion
• C. The density of matter is determined solely by chemical composition
• D. The rate of molecular motion influences the density of matter

Answer: (D)

Molecular motion is tied to temperature, which affects how tightly matter can be packed. Molecules are never truly at rest; as temperature rises, they move more vigorously and tend to push apart, causing the material to expand. Since density is mass per volume, increasing volume with the same mass lowers density. This effect is especially clear in gases, where heating markedly reduces density, and it also occurs in liquids and solids through thermal expansion, though to a lesser extent. So, the rate of molecular motion indeed influences density because changes in motion lead to changes in how much space the matter occupies. The other statements don’t fit because matter isn’t stationary, temperature does affect motion, and density isn’t determined solely by chemical composition—it also depends on temperature, phase, and how the material expands.

Molecular motion is tied to temperature, which affects how tightly matter can be packed. Molecules are never truly at rest; as temperature rises, they move more vigorously and tend to push apart, causing the material to expand. Since density is mass per volume, increasing volume with the same mass lowers density. This effect is especially clear in gases, where heating markedly reduces density, and it also occurs in liquids and solids through thermal expansion, though to a lesser extent. So, the rate of molecular motion indeed influences density because changes in motion lead to changes in how much space the matter occupies. The other statements don’t fit because matter isn’t stationary, temperature does affect motion, and density isn’t determined solely by chemical composition—it also depends on temperature, phase, and how the material expands.