Which reagent is used to test for the presence of starch in a food substance?

Step 1: Analyze the question You're being asked which reagent identifies starch specifically. Each option is a test for something different, so matching each reagent to its use is the key.

Step 2: The concept Food tests in Biology:

| Reagent | Tests for | Positive result | |---|---|---| | Iodine solution | Starch | Brown/yellow changes to blue-black | | Fehling's solution (or Benedict's) | Reducing sugars | Blue changes to brick-red precipitate | | Million's reagent | Proteins (the tyrosine in them) | Red/pink coloration on heating | | Anhydrous copper (II) sulphate | Water | White changes to blue | | Iron(III) oxide | Not a standard food test reagent | – |

Step 3: Solution

• A. Anhydrous copper (II) sulphate detects water, not starch.
• B. Fehling's solution detects reducing sugars like glucose. Starch is not a reducing sugar, so it gives no reaction.
• C. Iodine solution reacts with starch, forming a starch-iodine complex that gives a blue-black color. This is the correct test.
• D. Iron(III) oxide is not used for food tests.
• E. Million's reagent detects protein.

Step 4: Answer The correct option is C. iodine solution.

Tip: Remember "Iodine = Is starch". Starch gives blue-black with iodine. For the others, think: Fehling's = sugar, Million's = protein, copper sulphate = water, and Sudan III or the ethanol emulsion test = fats and oils.