Three test-tubes A, B and C were set up in a water bath to investigate the digestion of the fat in milk. Each tube contained an indicator that turns from pi...
Question 1 Report
Three test-tubes A, B and C were set up in a water bath to investigate the digestion of the fat in milk. Each tube contained an indicator that turns from pink to white as fatty acids are produced. The apparatus and the results are shown below.
| Tube | Contents | Time for colour change / min |
|---|---|---|
| A | milk + lipase | 8 |
| B | milk + lipase + bile | 3 |
| C | milk + bile only | no change |
(a) State which tube showed the fastest chemical digestion of the fat. [1]
(b) Explain why the colour changed faster in tube B than in tube A. [3]
(c) Explain why there was no colour change in tube C. [2]
(d) Suggest two variables, other than temperature, that should be kept constant in this investigation. [2]
Answer Details
(a) Tube with the fastest chemical digestion [1]. Tube B [1], which changed colour in the shortest time (3 minutes).
(b) Why the colour changed faster in tube B than in tube A [3]. Tube B contains bile as well as lipase [1]; bile emulsifies the fat, breaking it into tiny droplets and greatly increasing its surface area [1]; so the lipase can produce fatty acids faster and the indicator turns from pink to white sooner [1].
(c) Why there was no colour change in tube C [2]. Tube C contains no lipase [1]; bile only emulsifies the fat physically and cannot chemically digest it, so no fatty acids are produced and the indicator does not change [1].
(d) Two variables to keep constant (other than temperature) [2]. Any two of: volume/concentration of milk; volume/concentration of lipase; volume of bile; starting pH [2]. Keeping these the same makes the comparison of the tubes fair.
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