0620 · 1.2Diffusion

Diffusion and concentration gradients

0:00When a perfume bottle is opened, its scent can eventually reach someone across the room. Some substance from the bottle must travel through the air to reach their nose. Diffusion helps explain this spreading, although air currents can also carry the scent. The explanation comes from moving particles, and the same idea shows why some gases spread faster than others. The starting point is kinetic particle theory. Substances contain tiny particles, and those particles are constantly moving.

0:26In gases, particles are widely separated and move rapidly in random directions. In liquids, particles are close together but can move past one another. This movement allows substances to spread without anyone stirring them. Concentration describes how much of a substance is present in a particular volume. Imagine two equal spaces containing different numbers of scent particles. The space containing more has a higher concentration of that substance, and the other space has a lower one.

0:54We compare equal volumes so that the difference genuinely reflects concentration. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration by random motion. The word net is essential because individual particles do not all travel in the same direction. Some move towards the more concentrated region. Overall, however, more move towards the less concentrated region. In our diagram, there are more particles on the left than on the right.

1:21Each particle moves randomly, without knowing where the concentration is lower. Because more start on the left, more can cross towards the right. Movement happens both ways, but the overall transfer is from higher to lower concentration. Think of two groups of people wandering randomly across an imaginary line. If one side starts much more crowded, more people are available to cross from that side. They do not need instructions to spread out.

1:46This analogy explains the overall pattern, although real particles move because they already possess kinetic energy. As diffusion continues, the difference in concentration becomes smaller. Eventually, the substance is evenly distributed throughout the available space. The particles still move randomly, so diffusion has not made them stop. Equal concentrations mean there is no overall transfer in one direction. On average, movement across an imaginary boundary balances in both directions. This explains why diffusion does not require stirring.

2:19Stirring moves larger portions of a liquid together and can make mixing happen faster. Diffusion comes from the random movement of individual particles. Similarly, wind carries air from place to place, while gas diffusion can occur even without a wind blowing through the space.

Diffusion in liquids and gases

2:33A liquid example uses a small crystal of potassium manganate(VII) placed in still water. As it dissolves, particles of the dissolved substance enter the surrounding liquid. Their concentration is initially highest near the crystal. Random movement spreads them through the water, so the colour gradually extends beyond the starting region. Dissolving and diffusion describe different parts of this example. Dissolving allows the substance to become part of the solution. Diffusion then spreads its dissolved particles from higher to lower concentration.

3:04The spreading colour provides visible evidence of this movement. Do not explain it by saying that water particles become coloured particles. This next part is for the Extended paper. Different gases can diffuse at different rates, even when their temperatures are the same. Relative molecular mass compares the masses of their molecules. A gas with a lower relative molecular mass diffuses faster than one with a higher relative molecular mass under comparable conditions.

3:30At the same temperature, the molecules of different gases have the same average kinetic energy. Kinetic energy is the energy associated with movement. A lighter molecule must move faster, on average, to have the same kinetic energy as a heavier molecule. This connects lower relative molecular mass with faster diffusion, rather than treating the relationship as an unexplained rule. A classic worked example uses ammonia and hydrogen chloride in a glass tube.

3:55Cotton wool at opposite ends releases the two gases at the same time. They diffuse through the air inside the tube. Where they meet, they react to form solid ammonium chloride, which appears as a white ring. Suppose we must predict where that ring forms and explain our answer. First, compare the relative molecular masses given in the question. Ammonia has a relative molecular mass of seventeen, while hydrogen chloride has a value of thirty-six point five.

4:20Ammonia therefore has the lower relative molecular mass and diffuses faster at the same temperature. Next, connect diffusion rate to distance travelled during the same time. Ammonia spreads farther along the tube before meeting hydrogen chloride. The meeting point is therefore nearer the hydrogen chloride end. That is where the white ammonium chloride ring forms. A common mistake is to place the ring nearer the ammonia end because ammonia is faster.

4:46The white ring is a new solid formed by a reaction, not one of the original gases becoming visible. Diffusion brings the reactant particles together, while the reaction produces the observable ring. This distinction links the observation to the explanation. Both gases are harmful, so this demonstration requires appropriate laboratory precautions and must not be attempted at home.

Answering diffusion exam questions

5:07In an examination, describe means state what happens, while explain means give the reason. For the liquid example, describe the colour spreading through the water. Then explain that dissolved particles move randomly. There is net movement from higher concentration near the crystal to lower concentration elsewhere. Eventually, the dissolved substance becomes evenly distributed. For the gas tube, a clear explanation gives each connection separately. Ammonia has a lower relative molecular mass than hydrogen chloride.

5:36At the same temperature, its molecules move faster on average, so it diffuses faster. Ammonia travels farther before the gases meet. The white ammonium chloride ring therefore forms nearer the hydrogen chloride end. Practical paper questions may also ask you to record observations or interpret a diagram. Keep an observation separate from its explanation: the ring’s position is observed, but the difference in molecular speeds is inferred. Avoid saying that particles deliberately move towards empty space.

6:05Their movement is random, and diffusion can occur into regions already containing other particles. The central idea is that random particle movement can produce a predictable overall change. Diffusion gives net movement from higher to lower concentration until the substance is evenly distributed. Particle movement continues even when there is no net transfer. For gases at the same temperature, lower relative molecular mass means faster diffusion, explaining the position of the ammonium chloride ring.