To test the presence of Starch and Metanil yellow - Lab Work

To test the presence of Starch in Food sample and metanil yellow in pigeon pea - Class 9th Science Lab Work


To test the presence of starch in potato.

The presence of starch in a given food sample can be determined using iodine solution. Starch is a carbohydrate that produces blue colour when brought in contact with the iodine solution.

Materials Required
Potato, iodine solution, petridish, knife and a dropper.

Step 1: Potato is washed along with its skin.
Step 2: A thin slice of potato is taken in a petridish.
Step 3: Few drops of iodine solution is added on the surface of the thin potato slice.
Step 4: Change in colour in the area of slice is observed where iodine soluton was added.

The colour of potato slice changes into dark blue-black colour on addition of iodine solution.

Results and Discussion
Appearence of dark blue-black colour shows the presence of starch in potato slice.

1. Handle the apparatus with care.
2. Petridish must be cleaned properly.
3. Potato is washed clearly and slice of potato should be thin.
4. Only few drops of iodine solution should be added with the help of dropper.


To test the presence of metanil yellow in pigeon pea.

Food products are often adulterated for economic gains. Metanil yellow is used to adulterate pigeon pea. Metanil yellow turns into pink colour when it reacts with the concentrated hydrochloric acid. Whereas the pieces of unadultrated sample does not exhibit such a change.

Materials Required
Pieces of pigeon pea, concentrated hydrochloric acid, test tube holder, test tube and a dropper.

Step 1: Few dry pieces of pigeon pea sample is taken in a dry test tube.
Step 2: Few drops of conc. HCl is added to these dry pieces and shaked well.
Step 3: The change in the colour of the reaction mixture is observed.

The colour of reaction mixture of pigeon pea sample when reacted with conc. HCl changes into pink colour.

Results and Discussion
The change in the colour from yellow to pink shows the presence of metanil yellow in the pieces of pigeon pea. The pigeon pea is adulterated.

1. HCl must be handled with care and must not be added in excess.
2.Test-tube holder should be used at the time of adding conc. HCI and the mouth of the test tube is kept away from ourself.
3. Test tubes should be clean and dried properly.

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