(L11) benedicts tests

Common examples of reducing sugars

Glucose, fructose, maltose, and lactose.

Define: Reducing sugar

A sugar that can donate electrons to another chemical substance.

Method for the Benedict's test for reducing sugars

Add an equal volume of Benedict's reagent to the liquid sample, heat in a boiling water bath for approximately 5 minutes, and observe the colour change.

Negative result for the Benedict's test

The solution remains blue, indicating that no reducing sugar is detected.

Sequence of colour changes indicating increasing sugar concentration in Benedict's test

Blue (none) -> Green (low) -> Yellow -> Orange -> Brick-red precipitate (high).

Define: Semi-quantitative test

A test that gives an approximate indication of the quantity or concentration of a substance rather than an exact numerical value.

Common example of a non-reducing sugar

Sucrose.

Initial step required before testing for non-reducing sugars

Test a sample directly with Benedict's reagent to confirm it remains blue (no reducing sugars present initially).

Role of dilute hydrochloric acid in the non-reducing sugar test

Hydrolyses glycosidic bonds in non-reducing sugars, breaking them down into their constituent reducing monosaccharides.

Role of sodium hydrogencarbonate in the non-reducing sugar test

Neutralises the acid because Benedict's reagent requires an alkaline environment to function.

Define: Hydrolysis

The breaking of a chemical bond through the addition of water.

Define: Precipitate

An insoluble solid that forms and separates from a liquid solution during a chemical reaction.

Define: Qualitative test

A test that identifies the presence or absence of a substance without indicating the amount.