- Have you ever noticed how the scent of perfume spreads across a room, or how a drop of ink slowly spreads out in water?
- This process is called diffusion.
- This happens naturally because all particles are in constant random motion.
- Consider a crowded room where people are standing very close together.
- If the doors open to an empty hallway, people will naturally walk out and spread into the free space.
- Over time, the people become more evenly spread out.
- This spreading out is similar to diffusion.
The particle model helps us understand why diffusion occurs in different states of matter.
Diffusion can be described using the particle model: particles are always moving, and over time their random motion causes them to spread out and mix.
These properties of gases, liquids, and solids perfectly describe diffusion:
- Gases:
- Particles are far apart, move very quickly and collide frequently.
- This allows gases to diffuse rapidly.
- Liquids:
- Particles are closer together than in gases, but they still have enough energy to move around each other.
- Diffusion still happens, but usually more slowly than in gases.
- Solids:
- Particles are packed very closely and can only vibrate around fixed positions.
- Diffusion in solids is therefore extremely slow and often negligible at everyday temperatures.
- Try not to confuse diffusion with sedimentation.
- Diffusion: Particles move in all directions due to random motion, spreading out from high to low concentration.
- Sedimentation: Particles settle to the bottom of a container because of gravity, usually when they are large or dense.
- Both processes can happen at the same time, but they are not the same.
- Particles that are diffusing are still moving randomly through the fluid, while sedimented particles have mostly collected at the bottom and are no longer spreading through the fluid.
A classic experiment demonstrates diffusion in gases using ammonia gas (NH₃) and hydrogen chloride gas (HCl).
- The two gases are released at opposite ends of a long glass tube.
- As they diffuse along the tube, they meet and react to form a white solid, ammonium chloride (NH₄Cl).
- The white ring forms closer to the HCl end because NH₃ molecules have a lower relative mass and diffuse faster than HCl molecules.
Diffusion in liquids can be seen using potassium permanganate (KMnO₄):
- A small crystal of KMnO₄ is placed gently in a beaker of water.
- At first, the purple colour is concentrated near the crystal.
- Over time, the colour spreads throughout the water as KMnO₄ particles diffuse between the water particles.
At the same temperature, diffusion is usually much faster in gases than in liquids.
This is mainly because:
- In gases, particles are much further apart, so they can travel longer distances between collisions.
- This allows them to spread out quickly.
- In liquids, particles are much closer together and attract each other more strongly.
- Their movement is hindered, so they move more slowly through the liquid and diffusion is slower.
This is why the scent of perfume in a room spreads quickly to other people, whereas it takes much longer for milk added to coffee to mix evenly if you do not stir it.
Diffusion in a liquid will still occur even if:
- the diffusing substance has a higher density or larger molecular mass than the liquid, or
- the liquid is cold.
However, under these conditions the movement of particles is slower, so diffusion takes longer and may be less obvious to see.
Many ecosystems rely on the diffusion of gases through soil, which is a mixture of solid particles, air and water.
Most organisms in soil need oxygen for aerobic respiration and release carbon dioxide as a waste gas.
- Oxygen from the air diffuses into the air spaces (pores) between soil particles.
- Carbon dioxide produced by microorganisms and plant roots diffuses out of the soil into the atmosphere.
- Diffusion through these air spaces is essential for the survival of roots and many soil organisms.
When soil becomes very compact or waterlogged, the air spaces fill with water and gases cannot diffuse easily:
- Oxygen cannot diffuse into the soil effectively.
- Carbon dioxide can build up in the pores.
- Roots and aerobic microorganisms may not receive enough oxygen for respiration.
If soil oxygen levels fall significantly:
- Plant roots may be damaged or die because they cannot respire properly.
- Aerobic microorganisms decrease, and anaerobic microorganisms (which do not need oxygen) become more common.
- Anaerobic respiration produces substances such as alcohols, methane, sulfides and nitrogen-containing compounds (e.g. ammonium and nitrites), some of which can be harmful to plants and can disrupt nutrient cycles such as the nitrogen cycle.
This is why groundskeepers and turf managers sometimes aerate sports fields by poking small holes in the grass: it allows air to diffuse more easily into the soil and helps maintain healthy root systems.
The rate of diffusion is how fast particles spread from an area of higher concentration to an area of lower concentration. Several factors affect this rate:
- At higher temperatures, particles have more kinetic energy.
- This makes them move faster, so they diffuse more quickly.
Food coloring spreads much faster in hot water than in cold water because the water particles (and the dye particles) move more quickly at higher temperatures.
- Lighter particles move faster (on average) than heavier ones at the same temperature.
- Because they move faster, lighter particles diffuse more quickly.
- Hydrogen gas (H₂) molecules are lighter than carbon dioxide (CO₂) molecules.
- At the same temperature, hydrogen molecules move faster, so hydrogen diffuses more rapidly than carbon dioxide.
- The concentration gradient is the difference in concentration between two regions.
- A steeper concentration gradient (a bigger difference between high and low concentration) makes diffusion faster.
- As diffusion continues and concentrations become more similar, the rate of diffusion gradually decreases.
- In mixtures, substances made of smaller particles tend to diffuse faster than substances made of larger particles.
- Smaller particles can move more easily between other particles.
- As discussed earlier, diffusion happens fastest in gases, more slowly in liquids, and is usually very slow in solids.
- In gases:
- Particles are far apart and move very quickly.
- In liquids:
- Particles are closer together and their movement is more restricted.
- In solids:
- Particles only vibrate in fixed positions, so diffusion is extremely slow.
We can observe diffusion in simple experiments with gases and liquids.
- Set up a long glass tube with a plug of cotton wool soaked in aqueous ammonia (NH₃ solution) at one end and another plug soaked in concentrated hydrochloric acid (HCl) at the other end.
- Both substances give off gas particles: ammonia gas (NH₃) and hydrogen chloride gas (HCl).
- The gases diffuse along the tube and eventually meet.
- Where they meet, they react to form a white solid, ammonium chloride (NH₄Cl), which appears as a white ring inside the tube.
The white ring forms closer to the HCl end because NH₃ molecules are lighter and diffuse faster than HCl molecules.
- Fill a beaker with water.
- Carefully add a few drops of potassium permanganate (KMnO₄) solution (or a small crystal of KMnO₄) to the water.
- At first, the purple colour is concentrated near where the solution was added.
- Over time, the purple colour spreads throughout the water as KMnO₄ particles diffuse between the water particles.
To investigate the effect of temperature:
- Repeat the experiment using cold water and warm/hot water.
- Compare how quickly the purple colour spreads.
You should observe that diffusion is faster in warm water because particles move faster at higher temperatures.
Diffusion is not just seen in the laboratory. It is essential in many everyday and biological processes.
- When someone sprays perfume, tiny gas molecules spread out by diffusion from the area of high concentration (near the spray) into the rest of the room.
- Eventually, they reach your nose and you can smell them.
- In a similar way, pollutant gases such as nitrogen dioxide or methane can diffuse through the atmosphere, affecting air quality over a wide area.
- When someone sprays perfume, tiny gas molecules spread out by diffusion from the area of high concentration (near the spray) into the rest of the room.
- Eventually, they reach your nose and you can smell them.
- In a similar way, pollutant gases such as nitrogen dioxide or methane can diffuse through the atmosphere, affecting air quality over a wide area.