PAGS Flashcards
Module 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
Monosaccharides and disaccharides dissolve in water and so will not form a precipitate (or emulsion).
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)
• 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.
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
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.
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
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).
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
Proteins are insoluble in ethanol so they will not be extracted.
What are the 2 formulas for serial dilution?
C1xV1 = C2xV2
V2= V1 + volume of distilled water
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]
- 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
What is a dilution series?
A series of dilutions with the dilution factor staying the same
What is the main way to increase accuracy?
Take 3 repeat meanings and calculate mean
What reduces RANDOM error
More sensitive equipment to reduce uncertainty (eg smaller graduations)
What reduces SYSTEMATIC error
Calibrating equipment
How to work out uncertainty: analogue vs digital
Analogue: 1/2 smallest graduation
Digital: the resolution
When measurement gets smaller, does % uncertainty get bigger or smaller?
Bigger
% uncertainty formula
(Uncertainty/measurement) x 100
When you take 2 readings (eg length, temp) what is the % uncertainty formula?
(2 x uncertainty / quantity measured) x 100
