Required Practical: The Effect of Temperature on the Rate of Decay

The pH of milk decreases as it decays. This is because the bacteria present in milk convert lactose to lactic acid, which makes the milk more acidic.

To investigate the effect of temperature on the rate of decay:

1. Pour 15 cm³ of fresh milk into three different beakers.

2. Mark one beaker with 10°C, another with 20°C and the last with 30°C.

3. Use universal indicator paper to determine the pH of each beaker of milk and record it in your results table, under 0 hours.

4. Cover the top of each beaker with cling film for incubation to make sure there are no holes.

5. Record the pH of each beaker of milk.

• 24 hours after the starting time
• 48 hours after the starting time
• 72 hours after the starting time

Your results table should look something like this:

Calculating the Rate of Change

The equation to calculate the rate of change is:

So using the results table above, first we calculate the change in value. We do this by subtracting the previous value from each value.

Remember, as we started at 0 hours, we do not need to calculate the change in value for this column.

Now, we divide the change in value by the change in time.