Question

A few drops of iodine solution were added to rice water. The solution turned blue-black in colour. This indicates that rice water contains
A. Complex proteins
B. Simple proteins
C. Fats
D. Starch

Answer 1

Hint:-Iodine solution also known as Lugol’s iodine or aqueous iodine solution. Iodine solution is a mixture of metallic Iodine and potassium iodine in distilled water. Lugol’s iodine solution is also used as disinfectant and antiseptic for small wounds and also for gram staining etc.

Complete step-by-step solution:-As we know that rice water contains amino acids, fats, minerals, starch, proteins, vitamins etc. when a few drops of iodine solution were added to rice water then the solution turned blue - black in colour due to formation of starch - iodine complex. Starch is a carbohydrate which found in plants and is made up of two types of polysaccharides (made up of glucose units) chains that are amylose (linear) and amylopectin (branched). Amylose in starch is the main factor for colour formation as iodine comes in contact with beta coils structure of amylose and gives blue - black colour with iodine. Thus, the reaction of iodine with rice water indicates that rice water contains starch.
So, the correct answer is option D – Starch.

Note:- While doing some inorganic reaction of oxidation and reduction, Iodine can be used as an indicator which results in changes in iodine element and Iodine ion. When there is iodine ion present in the iodine element then only the iodine element will give the characteristic blue – black colour. Neither alone can give this colour result. Also, amylose in the starch is the main reason behind the deep blue colour in presence of iodine. It is also used as a reagent in laboratories for detection of starch in other substances and also in medical tests.