Before you can talk about what a molecule does, you need to know it is there. Four reagents and one extra hydrolysis step let you identify reducing sugars, non-reducing sugars, starch, lipids and proteins, and Benedict's can even tell you roughly how much sugar there is.
Each test pairs a reagent with a colour change. Benedict's (heat) goes from blue to a brick-red precipitate with reducing sugars, through green, yellow and orange as the sugar concentration rises. A non-reducing sugar such as sucrose gives a negative Benedict's until it is hydrolysed by boiling with dilute acid and then neutralised. Iodine in potassium iodide turns blue-black with starch, the emulsion test gives a milky-white emulsion with lipids, and biuret turns lilac with proteins. Standardise the Benedict's test and compare with colour standards, and it becomes a way to estimate concentration.
Pick a sample and a test on the virtual bench, then run it. Try glucose, dilute glucose and sucrose with Benedict's, then work through milk and the two mystery samples with every test.
| Test for | Method | Positive | Negative |
|---|---|---|---|
| reducing sugar | add an equal volume of Benedict's solution, heat in a water bath at about 80 °C or above for a few minutes | green, yellow, orange or brick-red precipitate | stays blue |
| non-reducing sugar | Benedict's negative first; then boil a fresh sample with dilute HCl, cool, neutralise with sodium hydrogencarbonate, repeat Benedict's | brick red after hydrolysis | stays blue |
| starch | add a few drops of iodine in potassium iodide solution | blue-black | stays orange-brown |
| lipid | shake the sample with ethanol, then pour the ethanol into water | milky-white emulsion | stays clear |
| protein | add biuret reagent (sodium hydroxide, then a few drops of dilute copper(II) sulfate) | lilac or purple | stays blue |
Benedict's solution contains blue Cu2+ ions. A reducing sugar donates electrons and reduces them to Cu+, which forms an insoluble brick-red precipitate of copper(I) oxide. Biuret detects peptide bonds, so it works for any protein or polypeptide.
Four quick checks. Each correct answer earns XP and records this skill on your profile.
On the virtual bench, what colour does the dilute glucose sample give with Benedict's?
Milk is tested with all four reagents on the bench. Which result is negative?
Sucrose gives which result with Benedict's before any hydrolysis?
Mystery B stays blue with Benedict's, turns brick red after acid hydrolysis and forms a milky emulsion. What does it contain?
On its own Benedict's is qualitative. Standardise it, and the colour or the speed of the change becomes a measure of how much reducing sugar is present.
Benedict's must be in excess for the semi-quantitative test. If all the Cu2+ is used up, every concentration above a certain value gives the same brick red, and the test can no longer tell them apart.
Sucrose has no free reducing group, because the glycosidic bond joins the reducing ends of glucose and fructose together. Break that bond and the reducing sugars appear.
If the first Benedict's test is already positive, a colour after hydrolysis proves nothing on its own: you would need a deeper colour (more precipitate) after hydrolysis than before to show a non-reducing sugar was there as well.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Why must the solution be neutralised before Benedict's is repeated in the non-reducing sugar test?
In a semi-quantitative Benedict's test, the time to the first colour change is measured for several glucose solutions. Which statement is correct?
An unknown solution gives a colour between the 0.25% and 0.5% glucose standards. What is the best way to improve the precision of the estimate?
Which observation shows that a solution contains protein?
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