Molecules - Practical Work

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Biochemical tests to detect the presence of carbohydrate and protein


1. Benedict's test - used to detect reducing sugars.

A reducing sugar is a sugar that can donate electrons to (or reduce) Benedict's reagent. When a reducing sugar is mixed with Benedict's reagent and heated :

blue - green - yellow - orange - brick red precipitate

The Benedict's test allows you to estimate the quantity of sugar in the solution being tested. If only small amount present, reaction will stop at early stage (green or yellow).

All monosaccharide sugars and some disaccharide sugars (maltose) are reducing sugars. Some disaccharide sugars (sucrose) are non-reducing sugars. They can be identified through giving a negative Benedict's test result initially, but if hydrolysed into its monosaccharides with dilute HCl, it gives a positive result.

2. Clinistix - used to detect glucose.

Clinistix strips are impregnated with chemicals that change colour in the presence of glucose. Clinistix is specific (unlike Benedict's test).

strips turn purple/blue if glucose is present

Can be a way for doctors to detect the presence of glucose in the urine (symptom of diabetes)

3. The iodine test - used to detect starch.

yellow/brown - blue/black


1. The Biuret test - detects the presence of peptide links.

Standard test for protein. Test carried out by adding equal volumes of KOH solution to test sample, followed by a few drops of dilute…


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