Organisation of an Ecosystem » Required Practical: Temperature and the Rate of Decay

The aim of the practical

You have learned that decay is faster in warm conditions. In this required practical you investigate the effect of temperature on the rate of decay of fresh milk by measuring a change in pH.

Why milk? When milk goes off, microorganisms make acids and the milk becomes more acidic, so its pH falls. A falling pH is a sign that decay is happening, and the faster it falls, the faster the decay.

Waiting days for real milk to go off is slow, so the practical uses a quick model. The enzyme lipase is added to the milk. Fresh milk contains fat, and lipase breaks it down into fatty acids. This makes the pH fall, so lipase is a quick model of the pH change in decay.

Key terms:

• pH indicator: a chemical that changes colour as the pH changes. Here it is phenolphthalein, which is pink in alkaline conditions and colourless when the pH falls.
• Water bath: a container of water kept at a set temperature, so that the milk stays at that temperature.

The equipment

The method

1. Set up water baths at different temperatures, such as 10, 20, 30, 40 and 50 °C. Check each one with a thermometer.
2. Use a syringe to put the same volume of milk, sodium carbonate solution and phenolphthalein into a test tube. The mixture is pink.
3. In a second test tube, measure the same volume of lipase solution.
4. Stand both tubes in the first water bath for a few minutes, so that both reach the temperature of the water.
5. Pour the lipase into the milk mixture and start the stopwatch at once.
6. Keep the tube in the water bath. Stop the stopwatch when the pink colour disappears.
7. Record the time. Repeat at each temperature, and repeat each temperature at least three times.

The lipase breaks down the fat and makes fatty acids. The fatty acids use up the sodium carbonate, so the pH falls. When the pH is low enough, the indicator turns colourless. The time taken for this to happen tells you the rate.

If you use a pH probe, you record the pH at regular times, such as every 30 seconds, until it stops changing.

Variables

• Independent variable: the temperature of the water bath.
• Dependent variable: the time taken for the colour to disappear (or the change in pH).
• Control variables: the volume and concentration of milk, sodium carbonate, indicator and lipase, and the type of milk.

Your hypothesis can use what you know about enzymes: decay will be fastest at a warm temperature, because the enzymes and microorganisms work best there. At a very high temperature the enzymes are denatured, so the decay slows right down.

Calculating the rate

A short time means a fast reaction, so time and rate are opposites. To turn a time into a rate, use:

rate = 1 ÷ time

With a pH probe you can find the rate as the change in pH divided by the time. If the pH falls from 9.0 to 6.0 in 120 seconds, the rate is 3.0 ÷ 120 = 0.025 pH units per second.

Recording and graphs

Draw a results table before you start. Put units in the headings, for example Temperature (°C) and Time (s). Add columns for each repeat, the mean and the rate.

• Plot temperature (independent variable) on the x-axis and the rate (or mean time) on the y-axis.
• Choose scales that use most of the graph paper, and label both axes with units.
• Plot the points carefully, then draw a smooth curve or a line of best fit.
• Reading a graph works both ways. You can turn a table into a graph, and you can read values from a graph back into numbers.

A graph of rate against temperature for this practical usually rises to a peak and then falls as the high temperature denatures the enzyme.

Safety

Evaluating the investigation

• Was the temperature steady? The mixture can cool or warm while you work, so keep the tube in the water bath and check with the thermometer.
• Was the end point clear? It is hard to say exactly when pink becomes colourless. A pH probe is more accurate, because it gives a number and removes opinion.
• Did the lipase and milk reach the temperature first? If you mix them before they are warmed, the reaction starts at the wrong temperature.
• Were the results repeated? Repeats let you spot an odd result and find a mean.
• Was the range wide enough? More temperatures, closer together, show the best temperature more clearly.

Exam-style question

A student investigated the effect of temperature on the rate of decay of milk. Lipase was added to milk containing sodium carbonate and phenolphthalein, and the time for the pink colour to disappear was recorded.

(a) Explain why the colour of the mixture changed. [2 marks]

(b) At 30 °C the colour took 40 s to disappear. Calculate the rate. [1 mark]

(c) Suggest one improvement that would make the results more accurate. [1 mark]