The postulate that molecules are in constant random motion best explains why liquids

Analyze the question: The kinetic theory of matter says particles (molecules) are in constant random motion. We need to pick the property of liquids that this motion best explains.

Concept: In a liquid, the molecules move randomly and slide past one another. The forces between them are strong enough to keep them close together, but not strong enough to hold them in fixed positions.

Evaluating each option:

• A. Can undergo solidification: This depends on cooling, which lowers the kinetic energy of the molecules until intermolecular forces lock them in place. It is explained by energy change, not by random motion.
• B. Maintain their volumes: This is due to the intermolecular forces of attraction holding the molecules close together, not to random motion.
• C. Are incompressible: This is because the molecules are already closely packed with very little space between them.
• D. Have no characteristic shape: Because the molecules are constantly moving randomly and sliding over one another, they are not fixed in position. A liquid therefore flows and takes the shape of its container.

Answer: The random, constant motion of molecules best explains why liquids have no definite shape of their own.

Exam tip: Match each property to its cause:

• Random motion → no fixed shape, flow, diffusion
• Intermolecular attraction → fixed volume
• Close packing → incompressibility