Molecular Motion & Diffusion

Ink spreads in water on its own because of diffusion: molecules move by themselves. They travel from high to low concentration, and higher temperature makes diffusion faster. Brownian motion is pollen jittering as unseen water molecules strike it, evidence of molecular motion. Raise the temperature slider and watch diffusion speed up.

Why Ink Spreads on Its Own

Temperature25℃

💡 What is Diffusion?

①Molecules of a substance move on their own

②They move from high to low concentration

③Higher temperature → faster molecules → faster diffusion

④Perfume scent filling a room is also diffusion!

Evidence: Brownian Motion

🔍 What is Brownian Motion?

①Brown observed pollen under a microscope (1827)

②Pollen on water moved erratically

③Cause: invisible water molecules colliding from all sides

④Direct evidence that molecules really move!

Molecular Motion vs Temperature

Motion vs Temperature

Temp ↑ → molecular speed ↑ → diffusion ↑

Higher temperature → larger kinetic energy

Diffusion Direction

High concentration → low concentration

Diffusion continues until concentrations equalize

🌡️ Diffusion in Daily Life

①Sugar dissolves faster in hot water (temp↑ → diffusion↑)

②Perfume spreads through a room (gas diffusion)

③Ink dropped in a cup spreads (liquid diffusion)

④Gas diffusion is much faster than liquid

Exam Key Points

Core Summary

Diffusion: spontaneous motion from high to low concentration

Higher temperature → faster diffusion; gas > liquid > solid

🎯 Exam Key Points

①Diffusion: molecules move from high to low concentration

②Temp↑ → motion↑ → diffusion↑

③Brownian motion = indirect evidence of molecular motion

④Gas diffusion > liquid because of larger spacing

⑤Diffusion happens regardless of phase (solid/liquid/gas)

Examples and Unit Test

Example 1

When you drop ink into water, it spreads on its own. What is this called, and what happens to the spreading speed if you raise the water temperature?

When molecules move on their own and spread from high concentration to low concentration, it is called diffusion.

The higher the temperature, the faster molecules move, so the diffusion speed increases.

▸ Diffusion / it gets faster at higher temperature

Diffusion goes from high to low concentration, and it is faster at higher temperature.

Example 2

Why does Brownian motion, the irregular movement of pollen floating on water, happen?

Around the pollen, invisible water molecules are moving constantly.

These water molecules strike the pollen irregularly from all sides, so the pollen moves irregularly; this is evidence that molecules really move.

▸ Because water molecules strike the pollen from all sides (evidence of molecular motion)

Brownian motion is indirect evidence that invisible molecules are moving.

Unit-test style

Which statement about molecular motion and diffusion is correct?

①Diffusion goes from low concentration to high concentration

②The higher the temperature, the faster the diffusion

③Diffusion happens fastest in solids

④Brownian motion is evidence that molecules are at rest

⑤Diffusion happens even when molecules do not move

▸ ② The higher the temperature, the faster the diffusion

At higher temperature molecular motion becomes more active, so the diffusion speed increases.

Diffusion goes from high to low concentration (①), it is fastest in gases (③), Brownian motion is evidence that molecules move (④), and diffusion happens through molecular motion (⑤).

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State Changes

Reflection & Refraction

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