NCERT Solutions Curiosity Chapter 3 Activity 3.7: Let us investigate — Test for proteins
Book page 49 & 50 Updated on2026-09-05
Q1.
Make a paste or powder of the food item using pestle and mortar. Put about half teaspoon of each food item in a separate clean test tube. Add 2–3 teaspoons of water to each test tube and shake them well. Add two drops of copper sulfate solution to each test tube using a dropper. Now, take another dropper and add 10 drops of caustic soda solution to each tube (Fig. 3.8). Shake well and leave the test tubes undisturbed for a few minutes. What did you observe? Did the content of some test tubes turn violet?
Answer
Yes — in some test tubes the contents slowly turn violet, and those foods contain protein.
The test in one line: reagents — copper sulfate solution (2 drops) and caustic soda solution (10 drops); result — a violet (purple) colour where protein is present.
What you see
Colour before the protein test
A milky white or pale suspension of the food in water
On adding the two reagents
The mixture first becomes pale blue, the colour of the copper sulfate
Positive result
Within a few minutes the pale blue changes to violet or purple → protein is present
Negative result
The mixture stays pale blue (or slightly greenish) with no trace of violet → protein is absent
Precautions given in the book — follow them strictly:
These chemicals are harmful and need to be handled with care. Do not touch any of them unless asked to do so.
If any chemical gets spilled on your body, immediately wash the affected area with water.
Do not put any of these chemicals into your mouth, or try to smell them.
This activity may be demonstrated by the teacher.
The reagent and the positive result of each of the three tests you perform in Activities 3.5, 3.6 and 3.7.
Why the food must be crushed first: the reagents can only reach the protein if the food is broken open and mixed with water. A whole grain of soya bean would show almost nothing; the same grain ground in a mortar and shaken with water gives a clear violet.
Q2.
This violet colour indicates the presence of proteins in the food item. Write your observations in Table 3.3. What conclusions can you draw from Table 3.3? Which food items show the presence of more than one nutrient?
Answer
Here is the complete Table 3.3 with all three tests together.
Food item
Starch (iodine → blue-black)
Fat (oily patch)
Protein (violet)
Nutrients found
Potato
Yes
No
No
Starch only
Cucumber
No
No
No
None of the three
Boiled rice
Yes
No
No (a trace)
Starch
Boiled gram (chana)
Yes
No
Yes
Starch + protein
Peanuts
No
Yes
Yes
Fat + protein
Bread / chapati
Yes
No
Yes (small)
Starch + some protein
Butter
No
Yes
No
Fat only
Coconut
No
Yes
No
Fat
Soya bean / peas
Yes (small)
Yes (soya)
Yes
Protein-rich
Conclusions from Table 3.3:
A single food usually contains more than one nutrient — boiled gram shows starch and protein, peanuts show fat and protein.
No single food contains all the nutrients. That is why one dish can never make a balanced diet.
The tests only detect starch, fat and protein. Vitamins and minerals need a well-equipped laboratory, so a negative result here does not mean the food is useless — cucumber gives water, minerals and roughage.
Foods showing more than one nutrient: boiled gram, peanuts, soya bean, bread/chapati, and milk if you test it.
Q3.
Which food items show the presence of both proteins and fats?
Answer
Peanuts show the presence of both proteins and fats — this is the example given in the book itself.
Other foods that pass both tests:
Soya bean — one of the richest plant sources of protein, and it also gives oil.
Almonds, cashew, walnut, til and sunflower seeds — nuts and seeds are generally rich in both.
Coconut — mostly fat, with a small amount of protein.
Milk, paneer, cheese and curd — protein with milk fat.
Egg and fish — protein with fat.
What this proves: any food which we eat may contain multiple nutrients. Foods are not sorted into neat boxes labelled “protein” or “fat” — we call a food a protein food or an energy food only according to which nutrient is present in the largest amount.
Did you know? This is why a handful of roasted peanuts is such a good school snack — it gives body-building protein and stored energy from fat at the same time, and it costs very little.
Q4.
Is there a food item that lacks any of these nutrients? Which of these foods do you consume daily? Try to find out other foods that are good sources of starch, fats, and proteins.
Answer
Yes — cucumber. It shows no blue-black with iodine, no oily patch and no violet colour, so it contains almost no starch, fat or protein. Other such foods are lauki, tomato, radish, watermelon and green leafy vegetables.
Does that make cucumber useless? Not at all. It is mostly water, and it gives roughage, vitamins and minerals — components that our three simple tests cannot detect. Remember that some nutrients can be detected only in a well-equipped laboratory.
Which of these do I consume daily: rice or chapati (starch), dal or milk (protein), cooking oil (fat), and a vegetable — a typical Indian thali therefore covers all three every day.