IGCSE Chemistry 0620: How Temperature Affects the Rate of Diffusion

A school technician sets up two identical, tall glass cylinders, P and Q, each filled with the same volume of still water and left undisturbed on a bench. Cylinder P contains water at $20\,^{\circ}\text{C}$ (room temperature) and cylinder Q contains water at $60\,^{\circ}\text{C}$ . At exactly the same moment, the technician gently lowers one small crystal of potassium manganate(VII), a deep purple solid, into the bottom of each cylinder, without stirring or otherwise disturbing the water in either cylinder. Over the next hour, the technician watches a purple colour spread upward from each crystal.

(a) Define diffusion. [1]

(b) State, with a reason based on the kinetic particle model, in which cylinder, P or Q, the purple colour spreads through the water faster. [3]

(c) Even in the faster of the two cylinders, the purple colour takes many minutes to spread through the whole depth of water, rather than mixing instantly. Suggest why this is, in terms of the particles present. [2]

Part (a): Defining diffusion

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, as a result of the random motion of the particles.

Part (b): Comparing cylinders P and Q

The two crystals are identical in size and are dropped in at the same moment, so the only difference between the two cylinders is the temperature of the water.

Particles in the hotter water (cylinder Q, $60\,^{\circ}\text{C}$ ) have a greater average kinetic energy than particles in the cooler water (cylinder P, $20\,^{\circ}\text{C}$ ), so they move faster, on average. This applies both to the water particles and to the particles that leave the dissolving crystal.

Faster-moving dissolved particles spread away from the crystal, and mix in among the surrounding water particles, more quickly. So the purple colour from the crystal in cylinder Q spreads through the water faster than the purple colour in cylinder P.

The purple colour spreads faster in cylinder Q.

Part (c): Why diffusion in the liquid still takes a long time

Even in the hotter cylinder, the dissolved particles do not travel to the top of the cylinder in a straight line at full speed. This is because the water itself is made of particles that are packed close together, filling the whole cylinder.

As the dissolved particles from the crystal move, they constantly collide with the surrounding water particles, and each collision knocks them off course in a new, random direction. This means the particles follow a long, random, zig-zag path rather than a direct route upward, so their overall (net) progress through the water is far slower than the actual speed at which they move between individual collisions.

Final answers

(a) Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration, due to the particles’ random motion.
(b) The colour spreads faster in cylinder Q: higher temperature gives the particles more kinetic energy and a greater average speed.
(c) Frequent random collisions with the closely packed water particles make the dissolved particles follow a zig-zag path, so their net spreading through the liquid is much slower than their speed between collisions.