Food Tests for Starch and Fats, Grade 9 Practical Task 1
The Grade 9 practical task asks you to write a hypothesis about glucose, and then never gives you a glucose test.
Practical task 1 is titled "investigate the presence of starch and fats and oils in food". Both methods it supplies are for starch and for grease. Then question 1, worth six marks, tells you to hypothesise about "starch and glucose", and question 7 asks you to compare your conclusions with that hypothesis.
This page gives the task as it should be run, the prescribed 20 marks reproduced exactly, the glucose test the task forgot, and one correction to the method that decides whether your class sees blue-black or a brown smudge.
Where this fits in the curriculum
| Subject | Natural Sciences |
|---|---|
| Grade | 9 |
| Topic | Topic 5, Digestive system |
| Status | Practical task 1. Formally assessed |
| Marks | 20, prescribed. Raw 30 scaled by two thirds |
The correction that decides whether it works
The printed method for starch reads:
Step 1: Grind the food sample into a paste. Step 2: Add the paste to a test tube with ethanol to make a mixture. Step 3: Add a few drops of light brown iodine solution.
Grind it with water, not ethanol.
| With water | With ethanol | |
|---|---|---|
| Does the starch disperse? | Yes | Poorly |
| The positive result | Immediate, strong blue-black | Weak, slow, easy to miss |
| Reading a faint positive | Clear | Masked by iodine's own brown colour |
Ethanol belongs to the grease test lower on the same page, where its job is to dissolve fats and oils. It has been copied into the starch method by mistake, and it is the reason classes report that "the iodine did not work".
Test 1: starch, with iodine
- Grind a small piece of the food with a little water into a paste
- Put the paste in a test tube or on a spotting tile
- Add two or three drops of iodine solution
- Blue-black means starch is present. If the iodine simply stays brown, there is none
It happens immediately. If nothing has changed in ten seconds, nothing will.
Test 2: fats and oils, the grease spot
- Grind the sample if it is solid
- Mix it with a little ethanol in a test tube and shake
- Filter into a clean beaker
- Put one drop of the filtered liquid onto white paper
- Let the ethanol evaporate. Two minutes is usually enough
- Hold the paper up to the light. A translucent mark that stays after drying means fats or oils
The drying step is the whole test. A wet patch of ethanol or water is also translucent, and a class that judges the paper straight away finds fat in everything.
The glucose test the task forgets
If your textbook asks for a hypothesis about glucose, you need Benedict's solution, and it needs heat.
- Put about 2 ml of the food solution in a test tube
- Add an equal volume of Benedict's solution
- Stand the tube in a beaker of hot water from a kettle for five minutes
- Read the colour
| Colour after heating | Glucose |
|---|---|
| Stays blue | None |
| Green | A trace |
| Yellow to orange | Moderate |
| Brick red | A lot |
Use hot water from a kettle, never a Bunsen burner. Ethanol is on the bench in the other half of this practical, and it is flammable. This is the most important safety line on the page.
Benedict's answers "how much", iodine only answers "yes or no". That difference is the same idea as the pH practical: a single-colour test sorts, a graded test ranks.
What you should get
| Sample | Starch | Fats and oils |
|---|---|---|
| Bread | Strong blue-black | Slight, if buttered |
| Potato | Strong blue-black | None |
| Maize meal | Strong blue-black | None |
| Peanut | Slight | Strong grease spot |
| Sunflower oil | None | Strong grease spot |
| Cheese | None to slight | Strong grease spot |
| Apple | None | None |
Include an apple. It is sweet and it gives no starch result, which is the cleanest way to show that sugar and starch are not the same thing, and it sets up the Benedict's extension.
How the marks work
This task has an official mark allocation, so we have not invented one.
| Section | Raw marks |
|---|---|
| Write a hypothesis | 6 |
| Variables | 4 |
| Fair test | 2 |
| Starch results | 6 |
| Grease results | 6 |
| Conclusion | 2 |
| Compare with your hypothesis | 2 |
| Improvements | 2 |
| Raw total | 30 |
| Scaled, two thirds | 20 |
A learner whose hypothesis mentions glucose has not made a mistake. The printed question asked for it. Mark the rest of the hypothesis on its merits.
If it does not work
| Problem | Cause | Fix |
|---|---|---|
| A brown smudge instead of blue-black | Ethanol in the starch test | Grind with water. This one fix saves the lesson |
| Every sample shows a grease spot | The paper was judged while still wet | Dry it fully, then hold it to the light |
| No sample shows a grease spot | Too little sample, or it was not filtered | More sample, shake properly, filter |
| Everything turns blue-black | The mortar was not rinsed. Starch clings | Rinse between samples |
| The Benedict's tube stays blue | No sugar, or the water was not hot enough | Fresh kettle water, five full minutes |
| Two groups disagree on the same food | Different amounts of water used to grind it | Fix the volume of water for the whole class. It is a controlled variable |
Safety, which the printed task does not give at all
There is no safety panel anywhere on this task in the textbook, and ethanol appears in both halves of it.
- Ethanol is flammable. No open flames in the room while it is out, and keep the bottle stoppered
- The glucose extension uses a kettle water bath, never a burner
- Iodine stains skin, clothing and benches permanently, and it is harmful if swallowed
- Nobody eats any sample. Everything has been ground in shared apparatus
- Check your class allergy list before choosing samples, particularly for nuts
- Goggles on, and wash hands afterwards
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, Part A the prescribed 20 marks plus a 20-mark extension. Part A reproduces the assessed task exactly, including the scaling line. Part B covers the glucose test, the ethanol error and a food label check
- Marking memorandum, with expected results for eight common foods and how to mark a hypothesis that mentions glucose
Related practicals
- The pH scale and indicators, Grade 9. The other formally assessed Grade 9 task, also with a prescribed 20 marks
- Plant and animal cells under a microscope, Grade 9. Term 1, and it uses the same iodine
- Reactions of acids, Grade 9
What you need to run it
The apparatus is ordinary and we have it. The chemicals are the problem, and we would rather say so than pretend.
A pestle and mortar at R55 does the grinding, and the 80 mm size is right for a food sample. Filter paper, funnels, test tubes and beakers are standard laboratory glassware, and most of it is already in the store cupboard.
Iodine solution is not currently in our catalogue and we are sourcing it. Until then a pharmacy sells it, and it is the one chemical this task genuinely cannot do without. Ethanol and Benedict's solution are also being sourced.
Safety goggles are R26 and there are 76 on the shelf. Ethanol is flammable and iodine stains eyes as readily as it stains benches, so this is not a practical to run without them.