Practical Skills

Question 1

Why does Benedict’s solution turn brick red when a reducing sugar is present?

Answer 1

Reducing sugars contain an aldehyde group (CHO) so they can reduce the copper 2+ ions to copper 1+ ions. This forms a brick-red precipitate

Question 2

Which sugars are reducing sugars?

Answer 2

All monosaccharides and some disaccharides such as lactose and maltose

Question 3

What is the test for non-reducing sugars?

Answer 3

• Add dilute HCl to the non-reducing sugar and boil for 5 minutes and then neutralise the non-reducing sugar with NaHCO3. Boiling with dilute HCl breaks the glycosidic bond between the two monosaccharides meaning that reducing sugars will now be present.
• Add Benedict’s solution and heat for 3 minutes at 80 degrees

Question 4

Which colours of Benedict’s solution indicate the presence of reducing sugars?

Answer 4

Green, yellow, orange, red. Brick-red precipitate indicates a high concentration of reducing sugar whereas green indicates a low concentration

Question 5

What is the test for reducing sugars?

Answer 5

• Add benedict’s solution to the test sample, and heat at 80 degrees for 3 minutes
• Observe any colour changes and decide if reducing sugars were present

Question 6

What is the test for protein?

Answer 6

Add a few drops of biuret reagent to the sample, if protein is present, the sample will turn lilac. A blue colour indicates that there is no protein present. The sample will change to lilac when protein is present due to copper ions reacting with nitrogen atoms in the peptide bond

Question 7

What is the emulsion test for lipids?

Answer 7

• Add ethanol to the sample and mix thoroughly and tip the solution into a clean test tube containing water
• If lipids are present a white emulsion will form. This is because lipids are soluble in alcohol but insoluble in water so they come out of solution when added to water

Question 8

What is the iodine test for starch?

Answer 8

Add a few drops of iodine to the sample, if starch is present then the sample will change colour to blue/black, if there is no starch present then the iodine solution will stay yellow/brown

Question 9

What is a colorimeter?

Answer 9

A machine that measures the absorbance/transmission of filtered light through a coloured solution, the higher the absorbance or the lower the transmission, the higher the concentration of the solution. You can use a colorimeter to plot a calibration curve to determine concentrations of unknown solutions

Question 10

How do you carry out colorimetry?

Answer 10

• Place a sample of distilled water into the colorimeter to set it to 0.00
• The filter in the colorimeter is set to the colour that would be best absorbed by the samples, the samples of known concentrations are added to the colorimeter and the absorbance of each is measured and recorded
• The absorbances of samples of unknown concentration are then measured and their concentration is calculated using a calibration curve

Question 11

What is a biosensor?

Answer 11

A device used to measure the amount of compound in solution, the compound is converted into an electrical signal that is proportional to the amount of compound

Question 12

What is the method for paper chromatography?

Answer 12

• Add a small amount of mixture to the bottom of the paper that is just touching a solvent
• The solvent moves up the paper taking each component of the mixture with it, each component of the mixture will bind to the paper with a different affinity
• The components with the highest affinity will be at the bottom of the paper
• The components with the lowest affinity will be at the top of the paper
• Therefore the components are separated due to their differential affinities

Question 13

How do you calculate retention values?

Answer 13

Rf = Solute front / solvent front

Question 14

What is paper chromatography carried out in?

Answer 14

A glass jar with a piece of chromatography paper clipped to a support rod in the jar

Question 15

What are the main applications of chromatography?

Answer 15

• Pharmaceutical - testing the purity of a product
• Environmental science - testing for pollutants
• Medicine - blood processing and purification
• Forensic science - identifying compounds

Question 16

How is DNA extracted from fruit?

Answer 16

• Crush a tissue sample of the fruit using a pestle and mortar and add the crushed tissue sample to a test tube
• Add detergent and salt to the test tube
• Filter the mixture leaving DNA in solution
• Add ice cold ethanol, DNA should form a white precipitate