Q1.
Take a small quantity of the food items such as a slice of potato, cucumber, bread, some boiled rice, boiled gram, crushed peanuts, oil, butter and crushed coconut. Place a small piece of each item on a separate dish. With the help of a dropper, put 2–3 drops of diluted iodine solution on each food item (Fig. 3.7).
Answer
The test in one line: reagent — dilute iodine solution (a light brown or yellowish-brown liquid); result — a blue-black colour where starch is present.
| What you do and see | |
|---|---|
| Reagent used | Dilute iodine solution, taken with a dropper — 2 to 3 drops only |
| Colour before the test | The natural colour of the food — white rice, pale potato, brown bread, yellow butter |
| Positive result | The spot turns blue-black almost at once → starch is present |
| Negative result | No blue-black appears; the drop keeps its own brown/yellow iodine colour on the food → starch is absent |
How to do it properly:
- Use a separate dish for each food so that the drops do not mix.
- Keep the pieces small and put the drops on the cut surface, not on the skin.
- Note the colour of each food before adding iodine — otherwise you cannot tell that a change happened. This is why Table 3.3 has a “before iodine test” column.
- Wash your hands afterwards, and do not taste anything after adding iodine.
Tip: Test a piece of plain white paper as well. Most paper contains starch, so it also turns blue-black — a neat check that your iodine solution is working.