Studyledge

Kinetic Particle Model of Matter: Question 2

Syllabus 2.1.2

Structured Core 5 marks

A student sets up a smoke cell and shines light into it, so that smoke particles floating in the air inside the cell appear as tiny bright specks when viewed through a microscope.

(a) Describe how the smoke particles are seen to move when viewed through the microscope. [2]

(b) Explain, in terms of the particles of the surrounding air, why the smoke particles move in this way. [2]

(c) State what this observation provides evidence for. [1]

Show worked solution Hide worked solution

Worked solution

Part (a): Describing the observed motion

Through the microscope, each smoke particle appears as a bright speck that never stays still and never travels in a smooth path. Instead, it constantly changes both its speed and its direction, tracing out a random, jerky (zig-zag) route with no repeating pattern.

Part (b): Explaining the motion in terms of air particles

The smoke particles are far too large to be moved by any single collision with an air particle, but they are constantly bombarded by huge numbers of air particles from every direction. These air particles move very fast and completely randomly.

Because the collisions are random, at any given moment slightly more of them (or slightly stronger ones) may be landing on one side of a smoke particle than on the opposite side. This creates an unbalanced net force on the smoke particle, pushing it in a particular direction. An instant later, the pattern of collisions has changed, so the unbalanced force acts in a new direction. This constantly shifting, unbalanced force is what produces the random, zig-zag motion of the smoke particle.

Part (c): What the observation is evidence for

Since the smoke particles themselves are not moving under their own power, their erratic motion can only be explained by invisible particles of the surrounding gas striking them randomly and from all directions. This is direct evidence that the particles of a gas are in constant, random motion, in other words, evidence for the kinetic particle model of matter.

Final answers

  • (a) Random, jerky (zig-zag), erratic motion with constantly changing speed and direction.
  • (b) Unequal, random collisions from fast-moving air particles on all sides produce a constantly changing, unbalanced force on each smoke particle.
  • (c) Evidence for the kinetic particle model of matter. That gas particles are in continuous random motion.