PAGS

Question 1

An effective qualitative test must give a clear positive result whenever the substance being tested for is present. This means it avoids ‘false negatives’. It must also not give a positive result due to the presence of some other substance (this would be a false positive). Explain, in terms of their solubility in different solvents and your knowledge of the emulsion test, why you do not see false positives due to: - Monosaccharides and disaccharides

Answer 1

Monosaccharides and disaccharides dissolve in water and so will not form a precipitate (or emulsion).

Question 1

The emulsion test is qualitative. How could it be made the basis for a semi-quantitative test (giving an indication of whether lipids are present at high, medium or low concentration) or even a fully quantitative test for lipids? (4)

Answer 1

• an equal mass of each solid sample should be taken, to ensure comparability of results
• the height of the emulsion formed could be used to give an indication of the quantity of lipid.
• Students might suggest filtering and weighing the precipitate.
• An alternative could be measuring the density of the emulsion in a well-mixed sample using a colorimeter.

Question 2

n effective qualitative test must give a clear positive result whenever the substance being tested for is present. This means it avoids ‘false negatives’. It must also not give a positive result due to the presence of some other substance (this would be a false positive). Explain, in terms of their solubility in different solvents and your knowledge of the emulsion test, why you do not see false positives due to: - Starch

Answer 2

Starch is not soluble in ethanol, so even though it could form a visible emulsion in water it will not have been extracted by the ethanol.

Question 3

An effective qualitative test must give a clear positive result whenever the substance being tested for is present. This means it avoids ‘false negatives’. It must also not give a positive result due to the presence of some other substance (this would be a false positive). Explain, in terms of their solubility in different solvents and your knowledge of the emulsion test, why you do not see false positives due to: -Nucleic acid

Answer 3

Nucleic acids are insoluble in ethanol (so they will not be extracted) and soluble in water (so they would not precipitate to give an emulsion).

Question 4

n effective qualitative test must give a clear positive result whenever the substance being tested for is present. This means it avoids ‘false negatives’. It must also not give a positive result due to the presence of some other substance (this would be a false positive). Explain, in terms of their solubility in different solvents and your knowledge of the emulsion test, why you do not see false positives due to: -Protein

Answer 4

Proteins are insoluble in ethanol so they will not be extracted.

Question 5

What are the 2 formulas for serial dilution?

Answer 5

C1xV1 = C2xV2
V2= V1 + volume of distilled water

Question 6

Describe a method the student could use to make a 1 in 10 dilution and then how they could use this dilution to make a 1 in 1000 dilution of the original solution. [3]

Answer 6

• Add 1 part stated solution to 9 parts water to make a 10^-1 dilution
• Mix
• Repeat using 9 parts water & 1 part of 10^-1 and 10^-2 dilutions to make a 10^-3 dilution