Decay is the breakdown of dead plants and animals and their waste by microorganisms such as bacteria and fungi. Three factors affect the rate of decay. A warmer temperature speeds up decay because microorganisms respire, grow and reproduce faster, up to an optimum. If it gets too hot the enzymes in the microorganisms are denatured and decay slows. Water is needed because microorganisms need moisture, so decay is slow in dry conditions. Oxygen is needed for aerobic respiration, so decay is fastest when oxygen is available.
Gardeners and farmers try to provide the best conditions for rapid decay of waste biological material. The compost produced is used as a natural fertiliser for growing garden plants or crops. Where there is no oxygen, anaerobic decay happens and it produces methane gas. Biogas generators use this to make methane, which can be burnt as a fuel.
Required practical 10 investigates the effect of temperature on the rate of decay of fresh milk. Samples of milk are kept at different temperatures. Microorganisms in the milk make acid, so the pH falls as the milk decays. You measure the pH change, and the faster it falls, the faster the rate of decay. Keep the volume of milk, and the time, the same in each sample.
To find a rate of change, divide the change in pH by the time taken. For example, a fall of 2 pH units in 4 hours is a rate of 0.5 pH units per hour.