KS3 · Biology
Food tests for nutrients
Iodine turns ground-up potato blue/black, but meat stays brown/orange. That 'nothing happened' is an answer too.
Read the result
What does the colour tell you?
Each card is a food, the reagent it was tested with and the colour it ended up. Start with starch: which results say yes, which say no, and which can't tell you at all? Pick a card, then pick where it goes.
Iodine solution is the test for starch. It starts brown/orange.
Still to sort
Starch present (0)
Iodine turned blue/black.
No starch (0)
Iodine stayed brown/orange.
Where the line is: Staying brown/orange is not a failed test. Iodine stayed its starting colour, and that is the answer: no starch.
This test can't tell you (0)
The reagent used tests for something else.
Where the line is: 'Can't tell' is different from 'no'. A reagent that stayed its starting colour gives a 'no' for the food it tests for. It gives no answer at all about anything else.
Sorted them all? Switch to Question 2 and sort the same results by how much reducing sugar they show.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
What you need to know
- A food test shows whether a specific type of food is present in a sample. A reagent is the chemical used for the test: it changes colour to show whether the food substance is there.
- Reagents can test for large food molecules, including protein, fat and carbohydrates such as starch. They can also test for smaller molecules, such as the sugars made when starch is broken down.
- Starch is a large, complex carbohydrate. The body has to break it down into much smaller molecules, called sugars, before it can be absorbed.
- Iodine test for starch: iodine solution is brown/orange and turns blue/black if starch is present. If it stays brown/orange, there is no starch.
- Benedict's test for reducing sugars (such as glucose, or fructose found in fruit): Benedict's solution is blue. After heating, it stays blue (none) or turns green/yellow (traces), orange (moderate) or brick red (large amount).
- Both tests are qualitative: they show whether a substance is present, or an approximate range. A quantitative test gives a measurable number.
The big picture
A food test uses a reagent, a chemical that changes colour to show whether a type of food is present. Iodine tests for starch: brown/orange turns blue/black if starch is there. Benedict's tests for reducing sugars: after a water bath, blue turns green/yellow, orange or brick red as the amount rises. A reagent that stays its starting colour means 'not present'. Both tests are qualitative: presence or a rough range, never an exact figure.
Key points
Worked example
Problem
A student has a mystery food and wants to answer two questions: does it contain starch, and does it contain reducing sugar? The iodine result stays brown/orange. The Benedict's result, after the water bath, is green/yellow. What can the student conclude, and what can't they?
⚠ Watch out
Mixing up the two methods: heating the iodine test, or doing the Benedict's test on a spotting tile. Only the Benedict's test goes in a boiling tube in a water bath. And don't read 'no colour change' as a failed test. It means the food substance isn't there.
Memory hook
Only Benedict's gets the hot bath, and its colours go from 'cold' to 'hot': blue (none), green/yellow (traces), orange (moderate), brick red (lots). Iodine stays cool. Brown/orange means no starch, and blue/black means starch is there.
Check yourself
Cover the page. Name both reagents, the colour each starts and the colour that means 'yes'. Which test needs a water bath? Why can't brick red give an exact amount?
Flashcards
(14)What does a food test show?
What is a reagent?
Which reagent tests for starch?
Iodine solution: starting colour, and colour if starch is present?
Iodine stays brown/orange. What does that tell you?
Which reagent tests for reducing sugars, and what colour is it to start with?
Give two examples of reducing sugars.
What does each Benedict's colour show?
Which step is only in the Benedict's test?
What is a suspension?
Why wear goggles for both food tests?
Qualitative or quantitative: what's the difference?
Why does the body break starch down?
Iodine test on bread, potato and meat: what happens?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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How this lesson was checked. This KS3 Biologylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 29 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.