This free quiz has 7 multiple choice questions on food tests (Benedict's, iodine, biuret and emulsion tests) and colorimetry, all taken from OCR A Level Biology A exam papers. It marks itself, so you get your score as soon as you finish.
What the quiz covers (OCR A spec 2.1.2)
The non-reducing sugar test and why the sample is heated with hydrochloric acid
Using Benedict's, iodine, biuret and emulsion test results to identify molecules
Following an enzyme reaction, such as amylase on starch, with food tests
Using a colorimeter and a calibration curve to find an unknown concentration
Choosing a statistical test to compare two sets of concentration data
Which cells give a blue-black colour with iodine
How to use it
Before you start, write down the positive and negative colour for each test. In the results-table questions, use every column: a negative result tells you as much as a positive one.
Where students lose marks (from OCR examiners' reports and mark schemes)
Each point below is followed by wording that gained marks.
Why the acid step works: Some candidates lost marks by stating that sucrose is a reducing sugar. Few explained that boiling with acid hydrolyses the non-reducing sugar into its monosaccharides.
✔ Gains credit: the non-reducing sugar (for example sucrose) is hydrolysed or broken down by hydrochloric acid into reducing sugars or named monosaccharides, which then give a positive Benedict's result.
Reading negative results correctly: Weaker candidates misread results tables, picking out the positive results for one molecule while ignoring what the other tests showed. Descriptions like dark, light or cloudy did not earn colour marks.
✔ Gains credit: a negative Benedict's result stays blue or turquoise, a negative iodine result stays orange, yellow or brown, and a tissue with a yellow iodine result contains no starch.
Using a calibration curve: Many gave long step-by-step instructions, including serial dilutions, when the command word 'outline' needed only the main steps. Others did not say how readings from known solutions are used to find the unknown concentration.
✔ Gains credit: use solutions of known concentration, plot a calibration curve of absorbance (or transmission) against concentration, measure the absorbance of the unknown sample and read its concentration from the curve.
Measuring the dependent variable: Vague statements such as 'measure the volume of product' or 'test the solution for sucrose' were common and not credited. Examiners also warned that 'amount' is ambiguous in experimental design.
✔ Gains credit: remove samples at set time intervals, add Benedict's to test for glucose, use a colorimeter to obtain absorption or transmission values and convert them to concentration with a calibration curve; use 'concentration' or 'volume' rather than 'amount'.
Spelling scientific terms: The spelling of biuret varied greatly and could be confused with other words such as burette. Writing 'calorimeter' instead of 'colorimeter' was not allowed.
✔ Gains credit: 'add biuret' (or copper sulfate solution) for the protein test, and 'colorimeter' spelt so it cannot be mistaken for another instrument.
Keep practising
Go on to the Biochemistry quiz, the Carbohydrates quiz or the Lipids and Cholesterol quiz, or browse all our OCR A Level Biology quizzes. Our OCR A masterclasses (Year 12 on Wednesdays at 5.15pm, Year 13 on Tuesdays at 6.30pm) work through real exam questions on one topic each week.
Quizzing is really helpful for retention of knowledge