When starch reacts with iodine, why does it create the
characteristic blue-black colour?
Answer Posted / mallikarjun
This reaction starts from a solution of hydrogen peroxide with sulfuric acid. To this is added a solution containing potassium iodide, sodium thiosulfate, and starch. There are two reactions occurring in the solution.
In the first, slow reaction, the triiodide ion is produced .
H2O2(aq) + 3 I−(aq) + 2 H+ → I3 + 2 H2O.
In the second, fast reaction, triiodide is reconverted to iodide by the thiosulfate.
I3(aq) + 2 S2O32−(aq) → 3 I−(aq) + S4O62−(aq)
After some time the solution always changes color to a very dark blue, almost black.
When the solutions are mixed, the second reaction causes the triiodide ion to be consumed much faster than it is generated, and only a small amount of triiodide is present in the dynamic equilibrium. Once the thiosulfate ion has been exhausted, this reaction stops and the blue color caused by the triiodide – starch complex appears.
| Is This Answer Correct ? | 2 Yes | 4 No |
Post New Answer View All Answers
What are nucleic acids? What is the historic origin of this name?
Give the formula for baking soda.
Which property among the following generally increases from top to bottom in a group 1a? 1. Electropositive, density, basic nature, classical reactivity
Differentiate between gram +ve and gram -ve bacteria.
why ph is not more than 14?
What are the units of bond length?
Give examples of tautomers.
which fat soluble vitamin has co-enzymic function?
What is the completing sequence of nitrogen-containing bases for an agccgttaac fragment of a dna chain?
Do enzymes act better under acid or basic ph?
Define polar covalent bond?
If a polar molecule has a charge of 4.8 * 10^ (-10) and internuclear distance is 1A then what is its dipole moment?
Give the example for electrophilic substitution reaction?
Which element in 1A group does not form peroxides?
Explain and define standard feat of formation