Q.Find out a qualitative test for proteins, fats and oils, amino acids and test any fruit juice, saliva, sweat and urine for them.

A qualitative test is a simple procedure in which a chosen reagent is added to a sample and a characteristic colour change (or other visible change) tells you whether a particular biomolecule is present.

• Proteins: the Biuret test. Add a few drops of dilute copper sulphate solution to the sample along with sodium hydroxide; a violet or pink colour develops if protein (peptide bonds) is present.
• Amino acids: the ninhydrin test. Heat the sample with ninhydrin reagent; a purple/blue colour develops if free amino acids are present.
• Fats and oils: the Sudan III test (grease-spot test). Shake the sample with Sudan III dye and water, or simply rub it on paper; a red-stained oily layer, or a translucent greasy spot that does not dry or disappear, indicates the presence of fat/oil.
• Carbohydrate (starch), for comparison: the iodine test — a drop of iodine solution turns blue-black in the presence of starch, because starch's coiled helical chains trap the iodine. …

This is an activity you carry out yourself, so the core of the answer is knowing the correct, established test for each class of biomolecule, so that you can actually perform and interpret it.

Proteins — the Biuret test. Add a small amount of dilute sodium hydroxide to the sample, followed by a few drops of dilute copper sulphate solution. If protein is present, the peptide bonds joining the amino acids react with the copper ions in alkaline solution to give a violet or pink colour. No colour change means no protein.

Amino acids — the ninhydrin test. Add ninhydrin reagent to the sample and gently heat it. Free amino acids react with ninhydrin to produce a purple or blue colour (proline and hydroxyproline give a yellow colour instead, as an exception worth knowing). This test detects free amino acids, distinct from the Biuret test, which detects the peptide bond found in whole proteins.

Fats and oils — the Sudan III test (grease-spot test). Sudan III is a dye that stains fats and oils red because lipids are insoluble in water but dissolve the dye. A simpler version of the same idea is the grease-spot test: rub or spot a small amount of the sample onto plain paper; a fatty sample leaves a translucent, greasy mark that does not evaporate or dry out the way a watery sample would.

Carbohydrates (starch), for comparison — the iodine test. Starch's chains coil into a helical shape that traps iodine molecules, producing a distinctive blue-black colour when iodine solution is added; the colour disappears once the starch has been broken down (for example, by the amylase enzyme in saliva).

Now test the four fluids. Apply each named test to fruit juice, saliva, sweat and urine, and record what you observe:

• Fruit juice is generally rich in sugars, so it is a good sample in which to look for reducing sugars/carbohydrate. …

Method: Choosing the Right Named Test for Each Biomolecule Class

A "find out a qualitative test" question is only properly answered when you name the actual, established test — a generic description like "a colour appears" or "it doesn't dissolve in water" is not enough, because the whole point of a qualitative test is that it is a specific, reproducible, named procedure with a specific, predictable result.

The efficient way to build this kind of answer is to keep a fixed pairing in mind between each biomolecule class and its standard test: proteins → Biuret test (violet/pink, because it detects the peptide bond), amino acids → ninhydrin test (purple/blue, because it detects the free amino group), fats and oils → Sudan III test or the simpler grease-spot test (because lipids are hydrophobic and either take up the dye or leave a persistent oily mark), and starch → the iodine test (blue-black, because starch's helical coils trap iodine). Notice that each test targets a specific chemical feature of the molecule (a bond, a functional group, a physical property, or a shape) — recalling WHY the t …