Potassium permanganate is placed into a beaker of water
Particles diffuse from an area of high concentration to an area of low concentration
The contents of the beaker are now all the same concentration 
How does diffusion occur
Diffusion occurs when particles spread.
They move from a region where they are in high concentration to a region where they are in low concentration.
Diffusion happens when the particles are free to move.
This happens in gases and for particles dissolved in solutions - but diffusion does not occur in solids. 
Factors influencing faster diffusion
- Larger concentration gradient
- Higher temperature
- Smaller surface area 
Examples of diffusion in small intestine
Products of digestion, dissolved in water, can pass across the wall of the small intestine by diffusion.
Their concentration is higher in the small intestine than their concentration in the blood 
- so the dissolved products pass from the intestine to the blood.
Examples of diffusion in lungs
Gas exchange in the lungs happening in the alveoli
Oxygen and carbon dioxide, dissolved in water, are exchanged by diffusion in the lungs:
- oxygen moves down a concentration gradient from the air in the alveoli to the blood
- carbon dioxide moves down a concentration gradient from the blood to the air in the alveoli 
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