Last week the Year 11 Biology students investigated the concept of surface area to volume ratio and its effect on diffusion rate. They used blocks (3 different sizes) of agar jelly to simulate the shape of organisms. The jelly was a pink colour due to the presence of sodium hydroxide (a base) and phenolphthalein indicator. Phenolphthalein is an indicator that is pink in a basic solution, but colourless in a neutral or an acidic solution.
When the blocks are placed in an acid solution, the acid diffuses into the jelly, causing it to change from pink to colourless. The time taken for a block to completely lose the pink colour is a measure of the rate of diffusion of acid into the jelly. Students were asked to formulate an hypothesis on which sized agar block would go clear first. Each object was a different size, with the largest block measuring 20mm x20mm x 20mm, medium at 20mm x20mm x10mm and the smallest block was 20mm x 20mm x 5mm.
Can you hypothesis which one became clear first? If you selected the smallest object you are correct! The reason is that it has the largest surface area to volume ratio.
