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Food tests exam questions and mark scheme

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Food tests exam questions and mark scheme

  1. 1. 1. The table below shows six statements that apply to biochemical tests. Complete the table to show which of these statements apply to the biochemical tests carried out on the substances listed. Fill in each box using a tick ( ) to show that the statement applies or a cross ( ) if it does not. The first row has been completed for you. statement substance use heat use biuret reagent use Benedict’s reagent boil with a dilute acid a positive result is a blue-black colour a positive result is an emulsion lipid protein starch reducing sugar non-reducing sugar [Total 4 marks] 1
  2. 2. 2. one mark for each correct row if only ticks, assume that spaces are crosses; if only crosses, assume that spaces are ticks R hybrid ticks statement substance us e he at use biuret reage nt use Benedict ’s reagent boil wit h a dilu te aci d a positi ve result is a blue- black colou r a positiv e result is an emulsi on lipid protein ; starch ; reducing sugar ; non- reducing sugar ; 4 [4] 2
  3. 3. 3. The table below shows information about tests that identify three different types of biological molecule. Complete the table to show the names of the types of molecule that are tested, the reagents used and the results obtained. type of molecule tested reagents used positive result negative result protein ................................ ................................ ................................ ............................... ............................... blue solution ................................. alcohol and water white emulsion clear liquid starch ............................... ................................ ............................... yellow solution [Total 5 marks] 4. (i) Describe the test that is used to indicate the presence of a reducing sugar, such as glucose, and state the observation that would be made if glucose was present. description of test ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... observation if glucose is present ......................................................................................................................... ......................................................................................................................... [3] 3
  4. 4. (ii) No change is observed if sucrose, a non-reducing sugar, is tested for in this way. The bond between the glucose and fructose units must first be broken. The test for a reducing sugar can then be carried out. Describe how this bond can be broken chemically before carrying out the test for a reducing sugar. ......................................................................................................................... ......................................................................................................................... [1] [Total 4 marks] 5. (i) add / use , Benedict’s (reagent); heat; NOT use water bath alone (blue to) green / yellow / orange / brown / red (precipitate); 3 (ii) hydrolysis; boil / heat , with (dilute) , acid / HCl; A (dil) NaOH (add) hydrolytic enzyme / sucrase / invertase; 1 max [4] 6. A student was carrying out tests to determine which biological molecules were present in a food sample. (a) (i) Describe a test that the student could carry out to discover whether this sample contained a lipid. ................................................................................................................ ................................................................................................................ ................................................................................................................ ................................................................................................................ [2] (ii) State what would be seen if a lipid was present. ................................................................................................................ [1] 4
  5. 5. (b) Describe how the structure of a phospholipid differs from that of a triglyceride. You may use the space below for a diagram to help your answer. ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... [3] (c) (i) Describe a test that the student could carry out to discover whether the food sample contained protein. ................................................................................................................ ................................................................................................................ ................................................................................................................ [1] (ii) State what would be seen if protein was present. ................................................................................................................ [1] [Total 8 marks] 7. (a) (i) add / mix with , alcohol / ethanol / propanone / (suitable) organic solvent; then , add to / add / mix with , water; water alone = 0 R heat 2 (ii) emulsion / milky colour / cloudy / AW; R precipitate 1 5
  6. 6. (b) phospholipids have 1 less fatty acid (residue) / 2 fatty acid (residues) not 3; A hydrocarbon 1 less ester bond / 2 ester bonds not 3; phosphate; choline / base / nitrogen; hydrophilic / polar , end / head; max 3 (c) (i) add , copper sulphate (solution) and sodium hydroxide (solution) / biuret (reagent); R Biuret test unqualified R heat 1 (ii) purple / mauve / lilac; R blue 1 [8] 6
  7. 7. (b) phospholipids have 1 less fatty acid (residue) / 2 fatty acid (residues) not 3; A hydrocarbon 1 less ester bond / 2 ester bonds not 3; phosphate; choline / base / nitrogen; hydrophilic / polar , end / head; max 3 (c) (i) add , copper sulphate (solution) and sodium hydroxide (solution) / biuret (reagent); R Biuret test unqualified R heat 1 (ii) purple / mauve / lilac; R blue 1 [8] 6

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