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Difficulty: MediumPostulates of Kinetic Theory and States of Matter

When smoke particles suspended in air are observed under a microscope, they display a continuous, random zigzag movement known as Brownian motion. According to the kinetic theory of matter, what causes this phenomenon?

  1. Unbalanced, rapid collisions between invisible air molecules and the larger smoke particlesAnswer
  2. B
    Thermal convection currents created within the container carrying smoke particles along fixed streamlines
  3. C
    Electrostatic repulsion occurring constantly between identical electric charges on adjacent smoke particles
  4. D
    Gravitational attraction continuously pulling smoke particles downward against upward buoyant forces

Answer

Unbalanced, rapid collisions between invisible air molecules and the larger smoke particles
According to the kinetic theory of matter, gas particles are in continuous, rapid, and random motion. Smoke particles, though microscopic, are significantly larger than individual air molecules. At any given moment, air molecules strike a smoke particle unevenly from various directions, creating a net unbalanced force that causes the smoke particle to change direction abruptly and produce Brownian motion.

Step-by-Step Solution

1
Recall the fundamental postulate of kinetic theory regarding particle motion in gases
Gas molecules (such as air) are in constant, rapid, and random straight-line motion.
Gas particles possess thermal kinetic energy proportional to the absolute temperature.
2
Analyze the impact of moving gas molecules on suspended microscopic particles
Because air molecules hit a smoke particle unequally from different sides at any given instant, the net unbalanced impulse shifts the particle's direction constantly.
This continuous molecular bombardment directly demonstrates the existence of perpetual molecular motion postulated by kinetic theory.

Key Concept

Brownian motion as empirical evidence for continuous random molecular motion
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Topics

Postulates of Kinetic Theory and States of Matter