Which test indicates the presence of starch and causes a color change to dark purple or black?

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Prepare for the PLTW Biomedical Science Test. Study with flashcards and multiple choice questions, each question comes with hints and explanations. Get ready for your exam!

Multiple Choice

Which test indicates the presence of starch and causes a color change to dark purple or black?

Explanation:
Starch detection relies on the starch-iodine reaction. Iodine fits into the helical structure of starch (especially amylose) and forms a starch-iodine complex that has a dark purple to black color. That deep color signals the presence of starch. Other tests detect different biomolecules: Benedict's solution responds to reducing sugars and changes color with heat, Biuret's reagent produces a violet color in the presence of peptide bonds in proteins, and Sudan IV stains lipids by coloring fat droplets red. So the test that indicates starch with a dark purple/black change is the iodine-based Lugol’s test.

Starch detection relies on the starch-iodine reaction. Iodine fits into the helical structure of starch (especially amylose) and forms a starch-iodine complex that has a dark purple to black color. That deep color signals the presence of starch. Other tests detect different biomolecules: Benedict's solution responds to reducing sugars and changes color with heat, Biuret's reagent produces a violet color in the presence of peptide bonds in proteins, and Sudan IV stains lipids by coloring fat droplets red. So the test that indicates starch with a dark purple/black change is the iodine-based Lugol’s test.

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