22.11 - Food Tests, Indicators & Reading the Results

What a food-test question looks like

• Nearly every Cambridge practical paper asks you to describe a food test and give its result.
• A typical question is worth 2 marks, and they always split the same way.

A typical question

A student wants to find out whether a sample of milk contains reducing sugar.

Describe the test the student would carry out, and give the result if reducing sugar is present. [2]

How the two marks split

1. Name the reagent and, where the test needs it, the method - Here, add Benedict's solution and heat the mixture in a hot water bath (1 mark).
2. Give the positive result - The colour change that shows the nutrient is present, here green, yellow, orange, or brick-red (1 mark).

The same knowledge tested two other ways

1. Naming the correct test for a named substance - e.g. "which test would show that a drink contains protein?"
2. Reading given results backwards - Working out which nutrient a sample holds from a set of results you're given.

The food tests and their colours

• There are five food tests to know.
• For each, exam marks are given for three things: naming the reagent, saying what you do with it, and giving the positive colour - so learn all three parts of every test.
• The positive colours below are the full range the mark scheme accepts, not a single shade.

The five tests

Three points worth pinning down

1. Benedict's needs heat - It's the only test that needs heating, so "add Benedict's" without "and heat" can lose the method mark.
2. The Benedict's colour range is the point - A little reducing sugar gives green, a lot gives brick-red, so the colour tells you roughly how much is present. The mark scheme credits any point along that range.
3. Vitamin C runs backwards - Instead of a colour appearing, the blue DCPIP is decolourised, so a positive result is the loss of colour, not a new one.

Negative results

• If the nutrient is absent, the reagent simply stays its starting colour:
  • Iodine stays orange-brown
  • Benedict's stays blue
  • Biuret stays blue
  • DCPIP stays blue
• Naming that starting colour is sometimes what a question wants.

Indicators for carbon dioxide, and reading results back

• Two indicators detect carbon dioxide - the gas that respiration produces and photosynthesis uses.
• The second skill is working a result backwards: given the colours, deduce which nutrient the substance contains.

The two carbon dioxide indicators

1. Limewater - Goes from clear to milky or cloudy when carbon dioxide is bubbled through it. This is the standard test for carbon dioxide gas.
2. Hydrogencarbonate indicator - Red in ordinary air. Its colour reports which way the carbon dioxide level has moved:
   • Turns yellow when carbon dioxide rises - e.g. next to respiring organisms.
   • Turns purple when carbon dioxide falls - e.g. near a photosynthesising plant.

Deducing the nutrient from a result

• Instead of "here is a substance, what colour will you see?", the question gives you the colours and asks what the substance must be.
• A common version prints a table of drinks with their test results and asks which drink contains a particular nutrient, and to justify it.
• Match each positive colour back to its nutrient, then state the deduction by quoting the colour that told you.
• Example: drink C gives a purple colour with Biuret and turns Benedict's orange on heating, so drink C contains protein and a reducing sugar - "drink C contains protein, because it turned Biuret purple, and reducing sugar, because Benedict's went orange".
• The justification - naming the colour change - is what earns the mark, not the nutrient on its own.

Common exam questions

Question 1

Describe how you would test a food sample for starch, and give the result if starch is present.

Add a few drops of iodine solution to the sample; if starch is present the orange-brown iodine turns blue-black.

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Question 2

A student tests a sample for reducing sugar and writes "add Benedict's solution, and it turns red". What has the student left out, and how could the result be described better?

The method is missing the heating step - Benedict's must be heated in a hot water bath. The result is better given as the full range: the blue changes to green, yellow, orange, or brick-red depending on how much sugar is present.

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Question 3

Which test would show that a drink contains vitamin C, and what would a positive result look like?

Add the drink drop by drop to blue DCPIP solution; a positive result is that the DCPIP is decolourised and goes colourless.

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Question 4

Carbon dioxide is bubbled through limewater. What would you see, and what does it show?

The limewater turns milky or cloudy, which shows that carbon dioxide is present.

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Question 5

A table shows that drink B turns Biuret purple but does not change Benedict's solution when heated. Which nutrient does drink B contain, and how do you know?

Drink B contains protein, because it turned Biuret purple; it contains no reducing sugar, because Benedict's stayed blue when heated.

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Question 6

Why is the vitamin C test described as working "backwards" compared with the test for starch?

Because a positive result removes a colour instead of producing one: the blue DCPIP is decolourised, whereas in the starch test a colour (blue-black) appears.