Biology AS · Biological molecules. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Four simple tests show which biological molecules are in a sample. For each test you must know the reagent, the method and the colour change from negative to positive.
Reducing sugars reduce the blue copper(II) ions in Benedict's solution to a brick-red precipitate of copper(I) oxide. Sucrose is a non-reducing sugar, so it gives no change until it has been hydrolysed with acid into glucose and fructose.
The Benedict's test can be made semi-quantitative. If every tube is treated in exactly the same way, with Benedict's solution in excess, then a higher sugar concentration gives a colour further towards red and a shorter time to the first colour change.
Forgetting to neutralise after boiling with acid. Benedict's solution only works in alkaline conditions, so without sodium hydrogencarbonate the test stays blue even when sugar is present.
Answer: The concentration is between 1.0% and 2.0% glucose.
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