How to identify starch

Safe­ty pre­cau­tions

Per­form this ex­per­i­ment only un­der adult su­per­vi­sion.

Reagents and equip­ment

• cop­per(II) sul­fate;
• potas­si­um io­dide;
• cups;
• lemon;
• pota­to;
• pipette;
• spoon.

Step-by-step in­struc­tions

Mix some cop­per(II) sul­fate and potas­si­um io­dide so­lu­tion in a cup in a 1:2 ra­tio. Cov­er a sliced pota­to and lemon with boil­ing wa­ter and let soak for 5 min­utes. Add some drops of the pre­pared so­lu­tion to each of the cups. The cup of pota­to ac­quires a blue hue. Mean­while, in the cup with lemon, the so­lu­tion be­comes col­or­less. If this didn’t hap­pen, then the lemon didn't con­tain enough vi­ta­min C (ascor­bic acid).

Process de­scrip­tion

Cop­per(II) sul­fate and potas­si­um io­dide re­act to form io­dine, which dis­solves in resid­u­al potas­si­um io­dide to yield a brown so­lu­tion. If this mix­ture is added to the so­lu­tion with pota­to, the con­tained starch will re­act with io­dine and forms a dark blue starch-io­dine com­plex. If the so­lu­tion of io­dine in potas­si­um io­dide is added to the so­lu­tion with lemon, the io­dine re­acts with vi­ta­min C (ascor­bic acid) and is re­duced to col­or­less io­dide ions.

You can find many cool ex­per­i­ments in the MEL Chem­istry sub­scrip­tion!