Kinetic Particle Model of Matter: Question 5
Syllabus 2.1.2
Smoke particles suspended in air are observed through a microscope, moving in a continuous, random zig-zag path. Which statement correctly explains this motion?
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Worked solution
Step 1: Identify what is actually being observed
The bright specks seen through the microscope are the (comparatively large) smoke particles. They are not moving under their own power. Something must be pushing them around.
Step 2: Identify what is causing the motion
The air surrounding the smoke particles is made of molecules that are far smaller and far more numerous than the smoke particles, and these molecules move extremely fast and completely randomly in all directions. Huge numbers of these molecules strike each smoke particle every second.
Step 3: Explain why the motion is random and constantly changing
Because the molecular collisions come from random directions, the number and force of impacts on one side of a smoke particle will, at any instant, rarely balance those on the opposite side. This produces an unbalanced resultant force on the smoke particle, which keeps changing direction as the pattern of collisions changes moment to moment, giving the zig-zag path observed.
Why the other options are wrong
| Option | Problem |
|---|---|
| A | Smoke particles do not move under their own power in straight lines; they are pushed around by external collisions. |
| B | Smoke particles in a smoke cell are too dilute (too far apart) for collisions between them to explain the constant, random motion. |
| D | Evaporation and condensation play no role in this phenomenon; the motion is caused by molecular collisions, not phase changes. |
Final answers
- Correct explanation: option C, random, unequal collisions from fast-moving air molecules produce an unbalanced, constantly changing force on each smoke particle.