NCERT Solutions for Class 6th Science Chapter 3 Activity 3.5: Let us investigate — Test for starch
Book page 48 Updated on2026-09-19
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).
Fig. 3.7, page 48 — redrawn sketch: testing for the presence of starch in various food items. Two students put 2–3 drops of dilute iodine solution on each food item kept in a separate dish.
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.
Q2.
Observe if there are any changes in the colour of the food items. Have they turned blue-black? Record your observations in Table 3.3.
Name of the food item
Colour of the food item for starch test
Oily patch for fat test
Colour of the food item for protein test
Starch present (Yes/No)
Fat present (Yes/No)
Protein present (Yes/No)
Before iodine test
After iodine test
Prediction (Yes/No)
Observation (Yes/No)
Before protein test
After protein test
Potato
Cucumber
Boiled rice
Boiled gram
Peanuts
Bread/ Chapati
Butter
Coconut
Any other
Table 3.3, page 51 — Exploring nutrients present in various food items. Every cell is blank in the book; you fill it as you do the three tests.
Answer
Yes — some of them turn blue-black, and those are the starchy foods. A blue-black colour indicates the presence of starch.
Food item
Colour before iodine test
Colour after iodine test
Starch present? (Yes/No)
Potato
Pale yellow-white
Deep blue-black
Yes
Cucumber
Light green-white
Almost unchanged; the brown drop stays brown
No
Boiled rice
White
Blue-black
Yes
Boiled gram
Cream-brown
Dark blue-black
Yes
Peanuts (crushed)
Cream
Brown drop remains; no blue-black
No
Bread / chapati
White-brown
Blue-black
Yes
Butter
Pale yellow
No change
No
Coconut (crushed)
White
No change
No
Oil
Yellow, transparent
No change
No
Why the colour appears: iodine joins with starch to form a new coloured substance, and that substance is blue-black. No starch, no new substance — so the iodine simply lies on the food keeping its own brown colour. The change is so sharp that a single drop is enough to decide the answer.
Check it yourself: put one drop of iodine on a raw potato and one on a boiled potato. Both go blue-black — cooking does not remove starch, it only makes it softer and easier to digest.