SPEC 4.2.2.1 • ORGANISATION — FOOD TESTS (RP4)
Food Tests
Three named reagents, six exact colour changes, and one risk-assessment rule the examiner flagged as the biggest mark-loss area on the June 2024 paper.
The Risk-Assessment Trap
The June 2024 Paper 1 had a 6-mark food-test plan question with risk assessment. The examiner report flagged the same error across most papers.
Examiner report — exactly what didn't score
Many students listed "wear goggles" or "tie hair back" — generic lab-safety phrases that did not link to the method being used. Generic safety = zero credit on risk assessment.
The rule that scores
Every safety step must be paired with a specific named hazard from the experiment. Not "wear goggles" — instead, "wear goggles to protect against Benedict's reagent spitting when heated." Specific hazard + linked control = the mark.
In the next 10 minutes, you'll know the three named tests, the six colour changes, and how to write a risk assessment that scores Level 3.
Learning Objectives
RP4 uses qualitative reagents — you describe a colour change to identify whether a substance is present (yes/no), rather than measure how much. By the end of this lesson, you'll be able to:
Objective 1
Describe the three named food tests (iodine, Benedict's, Biuret) — the reagent, the method, and the substance each one detects.
Objective 2
State the colour change for a positive result — naming BOTH the initial reagent colour AND the positive colour.
Objective 3
Write a risk assessment for RP4 that links each safety step to a specific named hazard.
SPEC REF 4.2.2.1 • RP4 • PAPER 1 • HIGHER TIER
The Three Named Tests
The AQA spec names exactly three food tests for RP4. Memorise the reagent, method, and colour change for each.
Iodine Test — Starch
IODINE TEST — METHOD
Add a few drops of iodine solution to a sample of the food at room temperature. If starch is present, the colour changes from yellow / orange / brown (the original iodine colour) to blue-black.
Positive result
yellow/orange/brown → blue-black = starch IS present.
Negative result
Stays yellow/orange/brown = starch is absent (or below detectable level).
Mark-scheme phrasing
MS Jun 2024 accepts both "blue-black" and "black" and "dark blue". The safest write-up uses "blue-black" — it appears in every accepted variant.
Benedict's Test — Reducing Sugars
BENEDICT'S TEST — METHOD
Add Benedict's reagent to a sample of the food. Heat in a water bath to at least 60 °C. If reducing sugar is present, the colour changes from blue to green / yellow / orange / brown / brick-red (depending on how much sugar there is).
Critical mark-scheme rule
HEATING IS ESSENTIAL — without it, Benedict's reagent doesn't react. The mark scheme says: heat to at least 60 °C. Writing "heat" alone is too vague — name the temperature OR say "boil" OR "hot water bath."
Why the range of colours?
The amount of sugar determines the final colour: green = trace amounts → yellow → orange → brown → brick-red = high amounts. Any of these colours scores the positive-result mark.
Biuret Test — Protein
BIURET TEST — METHOD
Add Biuret reagent (sodium hydroxide + copper sulfate) to a sample of the food at room temperature. If protein is present, the colour changes from blue to purple / lilac.
Positive result
blue → purple/lilac = protein IS present.
Negative result
Stays blue = protein is absent.
The classic confusion
Don't mix up Biuret (protein, purple) with Benedict's (sugar, heated). Examiner reports flag this every year. Memory aid: Biuret → Big molecule (protein). Benedict's → Boil (heat).
Why Heat Only Benedict's?
Of the three tests, only Benedict's needs heating. The other two work at room temperature.
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Why heat is needed
The reaction between Benedict's reagent and reducing sugar only proceeds at high temperature. Without heating, sugar in the food won't react — and you'll get a false negative even though sugar is present.
Fair-test controls — RP4 method essentials
For a valid comparison between food samples: use the same volume of reagent for each test, the same volume of food extract, and include a control with no food (water + reagent only) — to show what a negative result looks like. For solid foods, crush the food and mix with a small volume of water to make an extract before testing.
Two Important Notes
1. "Reducing sugar" — what Benedict's detects
Benedict's detects reducing sugars only — including glucose, fructose, lactose, and maltose. Sucrose (table sugar) is not a reducing sugar — it gives a NEGATIVE result with Benedict's even though it's a sugar.
For GCSE, you don't need the chemistry — just know that the test detects glucose and similar sugars, not sucrose.
2. Lipids — spec scope clarificationYou may have been taught the ethanol-emulsion test or Sudan III stain for lipids. These are commonly taught but the AQA spec for RP4 specifies only three reagents: Benedict's, iodine, Biuret. Mark schemes for RP4 questions do not credit a lipid test. Useful knowledge — but not assessed under this RP.
Risk Assessment — Three Named Hazards
This is the most-failed part of RP4 questions. Each hazard must be specific to the experiment, not a generic lab rule. The template below maps directly to the MS Jun 2024 indicative content.
Hazard 1 — Burns
Risk: hot water bath or hot glass causes skin burns / scalds.
Control: use a water bath (not direct flame); wait for equipment to cool; use test-tube tongs.
Hazard 2 — Spitting
Risk: Benedict's reagent can spit out of the tube when heated.
Control: wear eye protection; point test tube away from face.
Hazard 3 — Irritant
Risk: Benedict's reagent and iodine solution are skin/eye irritants.
Control: wear gloves; clean spills immediately; use a dropper bottle.
The pattern that scores
Specific hazard from the experiment + control linked to that hazard. Generic phrases ("wear goggles" / "tie hair back") alone score zero — they could apply to any practical.
Mark-Scheme Rule — Name BOTH Colours
The mark scheme awards a separate point for the initial reagent colour AND for the positive colour. Naming only the final colour loses a mark.
TestWrong (1 mark loss)Right (full marks)
Iodine"iodine turns blue-black""iodine changes from yellow / orange / brown to blue-black "
Benedict's"Benedict's turns brick-red""Benedict's changes from blue to green / yellow / orange / brown / brick-red "
Biuret"Biuret turns purple""Biuret changes from blue to purple / lilac "
Sentence template that scores every time
"[Reagent] changes from [initial colour] to [positive colour], showing [substance] is present."
PEE Worked Example — 6-Mark Plan
A student wants to test a sample of natural yoghurt to find out whether it contains reducing sugars AND protein. Describe the chemical tests they would use to detect both substances. Include a risk assessment for one of the tests in your answer. 6 MARKS
Step 1 — Read the command words. "Describe" (state the steps) + risk assessment (specific hazards + linked controls). Level 3 requires BOTH halves.
Step 2 — Plan. Test A: Benedict's for sugar. Test B: Biuret for protein. Risk assessment: pick the test with the biggest hazard (Benedict's — heated).
Model answer
Test for reducing sugar: Add Benedict's reagent to a sample of yoghurt. Heat in a water bath at at least 60 °C. If reducing sugar is present, the colour changes from blue to green / yellow / orange / brown / brick-red.
Test for protein: Add Biuret reagent to a sample of yoghurt at room temperature. If protein is present, the colour changes from blue to purple / lilac.
Risk assessment (Benedict's test): Benedict's reagent can spit out of the test tube when heated — wear eye protection and point the test tube away from face when heating.
Mark-scheme tick
✅ Benedict's named · ✅ heat ≥60 °C · ✅ both colours named (blue → brick-red range) · ✅ Biuret named · ✅ both colours named (blue → purple) · ✅ specific hazard + linked control. 6 / 6 marks.
Examiner Alerts
Generic safety scores zero
The single biggest mark loss on RP4 questions: students writing "wear goggles" or "tie hair back" without linking to a specific hazard. The examiner report from June 2024 directly called this out. Always: specific hazard from THIS experiment + control linked to that hazard.
Name BOTH colours
The mark scheme awards two separate marks for each test — one for the initial reagent colour, one for the positive colour. Writing only the positive colour loses one of those marks. "Iodine turns blue-black" → only half the answer.
Don't mix up Biuret and Benedict's
Examiner reports flag this every year — students writing "Biuret for sugars" or "Benedict's for protein." Memorise: Biuret = Protein (purple); Benedict's = Boil for sugars.
Specify the temperature for Benedict's
"Heat" on its own is too vague. The mark scheme accepts "at least 60 °C", "boil", or "hot water bath." Use the number — it's the safest answer.
Common Errors
WrongRight
"wear goggles" (generic)"wear goggles to protect against Benedict's spitting when heated "
"iodine turns blue-black""iodine changes from yellow / orange / brown to blue-black"
"test for sugar" (no qualifier)"test for reducing sugar " (Benedict's doesn't detect sucrose)
"boil iodine"iodine works at room temperature — only Benedict's needs heating
"Biuret tests for carbohydrates"" Biuret tests for protein ; Benedict's tests for sugars"
"heat the Benedict's""heat Benedict's to at least 60 °C in a water bath"
Action
Screenshot this table. Every row is a real examiner-flagged error.
Practice Question
A student tested a sample of egg white using two food tests and recorded the results:
Iodine test: no colour change (stayed yellow / orange / brown)
Biuret test: changed from blue to purple
(a) What do these results tell us about the egg white? Give two conclusions. 2 MARKS
(b) Describe the test the student could use to find out if egg white also contains reducing sugar. Include the heating requirement and the expected colour change for a positive result. 3 MARKS
a
Egg white does not contain starch — the iodine stayed yellow / orange / brown (no positive result). (1 mark)
Egg white does contain protein — the Biuret changed from blue to purple (positive result). (1 mark)
b
Add Benedict's reagent to a sample of egg white. (1 mark)
Heat in a water bath to at least 60 °C. (1 mark)
If reducing sugar is present, the colour changes from blue to green / yellow / orange / brown / brick-red. (1 mark)
Total: 5/5. Note for (a): the mark scheme demands precision — "no starch" earns the mark; "doesn't change colour" doesn't. For (b): all three elements (reagent + heating + both colours) are creditworthy points.
Recap — Three Takeaways
1. The three named tests
Iodine (starch, room temp): yellow/brown → blue-black. Benedict's (reducing sugars, heat ≥60 °C): blue → green/yellow/orange/brown/brick-red. Biuret (protein, room temp): blue → purple/lilac.
2. Always name BOTH colours
Mark scheme awards a separate mark for the initial reagent colour AND for the positive colour. Sentence template: "[Reagent] changes from [initial colour] to [positive colour], showing [substance] is present."
3. Risk assessment = specific hazard + linked control
"Wear goggles to protect against Benedict's spitting when heated" — specific. "Wear goggles" — generic. The first scores, the second doesn't. Three named hazards: burns from heating, Benedict's spitting, irritant reagents.
You're set
Three tests, six colour changes, three hazards. You'll handle any RP4 question on Paper 1.
Next Up
Lesson 2.05 — Effect of pH on Amylase (Required Practical 5)
You know the food tests. Next: the second required practical on this topic — investigating how pH affects the rate of amylase activity.
The continuous-sampling method, the iodine spot test, the equilibration step — and the rate = 1 ÷ time calculation that the spec demands.
SEE YOU IN LESSON 2.05.
Next: The Risk-Assessment Trap