Brownian motion explains

Step 1: Analyze the question The question asks what Brownian motion is used to explain. Brownian motion is the random, zigzag movement of tiny particles (such as pollen grains or smoke particles) suspended in a fluid.

Step 2: The concept Robert Brown, a botanist, observed pollen grains moving erratically in water. The explanation is that the invisible molecules of the fluid are in constant, random motion. They collide with the visible particles from all sides, and the unequal impacts push the particles in irregular directions. This is also seen with smoke particles in air under a microscope.

So Brownian motion is evidence for the kinetic theory of matter: it shows that molecules of liquids and gases are in constant, random motion.

Step 3: Check each option

• A. Boyle's law: This relates the pressure and volume of a gas. Brownian motion is not used to explain it directly.
• B. Collision theory: This explains reaction rates, based on effective collisions between reacting particles. It is not what Brownian motion demonstrates.
• C. Laws of chemical combination: These are explained by Dalton's atomic theory, not Brownian motion.
• D. Molecular motion in liquids and gases: This is correct. Brownian motion is direct evidence that fluid molecules move randomly and continuously.
• E. Vibrational motion in liquids: Brownian motion shows random translational motion, not vibration. Vibration is characteristic of particles in solids.

Step 4: Answer The correct option is D. Molecular motion in liquids and gases.

Tip: Link "Brownian" with "random zigzag motion of particles in fluids (liquids and gases)". Whenever you see pollen grains or smoke particles, think "evidence of molecular motion". Solids only vibrate, so they do not show Brownian motion.