Experiments to learn - AS level OCR BIology A (Biological Molecules)

Name: Biuret Test

Testing for: Proteins

Experiment: Mixture of potassium (or sodium) hydroxide and copper sulphate added to sample (can be premixed as Biuret solution)

Positive result: Purple/lilac colour – the more peptide bonds, the stronger the lilac colour

Explanation:

KOH (or NaOH) breaks down proteins – makes the solution alkaline

CuSO₄ (blue colour – active ingredient; Copper ions form a lilac coloured               compound with nitrogen ions in peptide bonds

Name: Benedict's Test

Testing for: Reducing sugars (all monosaccharides and some disaccharides e.g. maltose and lactose are reducing sugars)

Experiment: Mix equal volumes of the sample and of Benedict's reagent (an alkaline solution of copper (II) sulphate).

Heat to 90°C in a water bath for five minutes

Positive result: Brick-red colour - the stronger the red, the higher the concentration of reducing sugar in the sample

Explanation: Copper (II) ions are reduced to copper (I) ions and form red copper (I) oxide

Name: Benedict's Test

Testing for: Non-reducing sugars (e.g. sucrose)

Experiment: Mix equal volumes of the sample and of HCl

Heat to 90°C in a water bath for two minutes - leave to cool

Neutralise the solution with sodium hydrogencarbonate (add spatula-wise until there is no effervescence) - test using pH paper

Add an equal volume of Benedict's reagent (remember to add the total volume of HCl and sample)

Heat to 90°C in a water bath for five minutes

Positive result: Brick-red colour - the stronger the red, the stronger the concentration of the sample

Explanation:  Non-reducing sugars are hydrolysed to form reducing sugars that then give a positive result when tested with Benedict's reagent

Filter the coloured precipitate out of the solution/s.

Calibrate a colorimeter using water.

Test the solution/s for perecentage transmission.

 A graph can then be plotted of concentration against transmission.

Testing solutions of known concentrations means that the concentration of an unknown solution can be worked out by reading off the graph plotted.

Reagent test strips can be used to test for reducing sugars: using a colour-coded chart, the concentration of the sugar can be determined

Name: Iodine test

Testing for: Starch

Experiment: 

Solid sample - peel off skin/use a spatula to remove samples of powdered food ---> add a few drops of iodine solution to a piece

Liquid sample - add 2 drops of idonie solution to 2cm3 

Positive result: colour change from yellow-brown ---> purple/blue-black 

Extra: as a control, the test could be repeated with distilled water

Name: Emulsion test

Testing for: Lipds

Experiment:

Solid sample: crush food sample in test tube ---> add ethanol and shake ---> decant ethanol into another test tube ---> add water and shake

Liquid sample: add 2cm3 of ethanol to a few drops of sample ---> add 2cm3 of distilled water and shake

Positive result:layer of cloudy-white suspension forms 

Explanation: the lipids dissolve in the ethanol. Ethanol is miscible with water (dissolved in water) so an emulsion is formed

Name: Chromatography

Testing for: used to separate out mixtures of chemicals

Experiment: concentrated spot of the solution  placed at one corner of a ***** of paper ---> dipped in a solvent which carries the dissolved chemicals up the paper

Dissolved chemicals are deposited: the least soluble first, the most soluble last. Other chemicals may be used to make the spots visible

Explanation: different chemicals have different solubilities in certain solvents

Different chromotograms and the separated components of the mixtures can be identified by calculation the Rf(retention) value:

Rf = distance moved by the compound ÷ distance moved by the solvent