Which of the following phenomena is the practical evidence for the existence of the continual motion of molecules.

Step 1: Analyze the question The question asks which phenomenon gives practical (experimental) evidence that molecules are in continual, random motion.

Step 2: Explain the concept Brownian motion was first observed by botanist Robert Brown in 1827. He saw pollen grains suspended in water moving in a zigzag, random way. The grains are large enough to see under a microscope, but they are being constantly bombarded by tiny, invisible water molecules moving at random. The uneven bombardment pushes the grains around.

Smoke particles in air show the same effect. This is why Brownian motion is the classic experimental evidence for the kinetic theory of matter, that is, that molecules are in continual random motion.

Step 3: Check the options

• A. Translational motion: A type of motion (movement from one place to another), not an experiment or observation that proves molecules move.
• B. Rotational motion: A type of motion (spinning), not evidence.
• C. Brownian motion: The visible, random movement of small particles in a fluid caused by molecular collisions. This is the direct evidence.
• D. Oscillatory motion: A type of motion (vibration about a fixed point), not evidence.

Step 4: Answer The correct option is C. Brownian Motion.

Exam tip: Whenever you see "evidence for the continual random motion of molecules," think Brown → Brownian. Also remember the related evidence for molecular movement: diffusion (e.g., the smell of perfume spreading across a room).