(c) Brownian motion in terms of the kinetic model of matter and a simple…

The idea

Brownian motion is the random, jerky movement of small particles suspended in a fluid. In the standard demonstration, smoke particles suspended in air are viewed through a microscope and are seen to move in irregular zigzag paths. The kinetic model explains this: air molecules move rapidly and randomly, and they collide unevenly with the much larger smoke particles. Because the number and direction of collisions on each side vary from moment to moment, the resultant force on a smoke particle changes randomly, producing the observed erratic motion. The smoke particles are much larger than air molecules, so individual molecular collisions are not seen directly; the smoke particle acts as a visible indicator of invisible molecular motion. MCQ items often ask what the observation shows or why the motion occurs, so focus on the cause rather than describing the path.

What earns the marks

• Brownian motion is the random, jerky movement of small particles suspended in a fluid.
• In the demonstration, smoke particles suspended in air are observed through a microscope.
• Air molecules move rapidly and randomly and collide with the smoke particles.
• Collisions are uneven and vary from moment to moment, so the resultant force on a smoke particle changes randomly.
• The smoke particles are much larger than the air molecules, so the motion of the smoke particles provides evidence for the random motion of the invisible molecules.
• The observation supports the kinetic model of matter rather than showing molecular collisions directly.

Common mistakes and how to put them right

• Saying the smoke particles themselves are moving because they are hot.

  Put it right: The motion is caused by uneven collisions with air molecules, not by the smoke particles' own temperature.

• Claiming that Brownian motion directly shows individual air molecules.

  Put it right: The smoke particles are far larger, and their motion is evidence for molecular motion rather than a direct image of molecules.

• Describing the motion as steady or in one direction.

  Put it right: It is random and changes direction erratically.

• Thinking the smoke particles collide with each other to cause the motion.

  Put it right: The dominant cause is collisions with the much more numerous air molecules.

Exam tips

• State the cause as uneven collisions with rapidly moving air molecules, not simply as random movement.
• Use the word suspended when describing the smoke particles in air.
• If asked what the demonstration shows, link it to evidence for the kinetic model of matter.
• Avoid saying the smoke particles are molecules; they are much larger particles that make molecular motion visible indirectly.