IGCSE Biology 0610: Factors Affecting the Rate of Diffusion

A student investigates diffusion using a scent-free blue food dye. She places one drop of the dye at the bottom of four identical tall glass cylinders, each filled with the same depth of still water, and does not stir any of them. Two cylinders are placed in a warm water bath at 40 degrees Celsius, and the other two are placed in a fridge at 4 degrees Celsius. She records the time taken for the coloured dye to become visible at a mark two-thirds of the way up each cylinder.

(a) State the factor being investigated in this experiment, and state one variable that has been kept constant between all four cylinders. [2]

(b) The dye reaches the mark faster in the cylinders in the warm water bath than in the cylinders in the fridge. Explain this result in terms of the movement of the dye particles. [2]

(c) State two factors, other than temperature, that affect the rate of diffusion. For one of these factors, suggest a change to the investigation that would increase the rate at which the dye reaches the mark. [2]

Part (a): Identifying the investigated factor and a controlled variable

The factor being investigated is temperature: two cylinders are kept warm and two are kept cold, while everything else about the cylinders is the same. A variable that has been kept constant is the depth of water in each cylinder (the size of the dye drop and the distance to the mark are also kept the same across all four cylinders).

Part (b): Explaining the faster diffusion in warm water

All particles are in constant, random motion, and this motion is faster when the particles have more kinetic energy. In the warm water bath, both the dye particles and the water particles have more kinetic energy than in the fridge, so they move faster and more randomly. This means the dye particles spread out, diffuse, from the concentrated drop at the bottom to the less concentrated water further up the cylinder more quickly, so the dye reaches the two-thirds mark sooner in the warm water bath.

Part (c): Other factors affecting the rate of diffusion

Besides temperature, the rate of diffusion is also affected by the concentration gradient and the distance the particles must travel (and, in other contexts, by surface area). Taking concentration gradient as the example: using a larger or more concentrated drop of dye would create a steeper concentration gradient between the drop and the surrounding water. A steeper concentration gradient increases the rate of diffusion, so the dye would reach the two-thirds mark faster than with the original, smaller drop.

Final answers

(a) Investigated factor: temperature. Controlled variable: depth of water (or size of dye drop, or distance to the mark).
(b) Warm water gives the particles more kinetic energy, so they move faster and more randomly, making diffusion quicker.
(c) Concentration gradient and distance also affect diffusion rate; increasing the size/concentration of the dye drop increases the concentration gradient and so speeds up diffusion to the mark.