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:

0 hours24 hours48 hours72 hours
10°C6.56.46.25.9
20°C6.56.15.54.7
30°C6.55.04.74.7

Calculating the Rate of Change

The equation to calculate the rate of change is:

A formula for calculating the rate of change. It reads: "Rate of change equals Change in value divided by Change in time."

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

24 hours48 hours72 hours
10°C6.5 – 6.4 = 0.16.4 – 6.2 = 0.26.2 – 5.9 = 0.3
20°C6.5 – 6.1 = 0.46.1 – 5.5 = 0.65.5 – 4.7 = 0.8
30°C6.5 – 5.0 = 1.55.0 – 4.7 = 0.34.7 – 4.7 = 0

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.

24 hours48 hours72 hours
10°C0.1 ÷ 24 = 0.004170.2 ÷ 24 = 0.008330.3 ÷ 24 = 0.0125
20°C0.4 ÷ 24 = 0.01670.6 ÷ 24 = 0.0250.8 ÷ 24 = 0.0333
30°C1.5 ÷ 24 = 0.06250.3 ÷ 24 = 0.01250 ÷ 24 = 0

Then graph your results.

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