| Date | November 2016 | Marks available | 1 | Reference code | 16N.3.hl.TZ0.16 |
| Level | HL | Paper | 3 | Time zone | TZ0 |
| Command term | Outline | Question number | 16 | Adapted from | N/A |
Question
Anthocyanins are pigments that give colour to many flowers and fruits. The red colour of ripe strawberries is mainly due to the anthocyanin pigment whose structure is shown below.
Outline why this molecule absorbs visible light.
With reference to its chemical structure, outline whether this pigment is found in aqueous solution in the cells or in the lipid-based membranes.
A student investigated the ability of anthocyanins to act as pH indicators. He extracted juice from blackberries and used a UV-vis spectrophotometer to produce absorption spectra at different pH values. His results are shown below.
Deduce the colour of the juice at each pH, giving your reasoning. Use section 17 of the data booklet.
Markscheme
«extensive» conjugation «of double bonds»/delocalization «of electrons»
OR
«many» alternating single/C–C AND double/multiple/C=C bonds
in aqueous solution AND hydroxyl/OH/ionic/oxonium/O+ «groups»
Accept “polar/hydroxy” for “hydroxyl”.
Do not accept “OH- /hydroxide/ oxygen”.
pH 2: «absorption peak 520 nm» red AND pH 11: «absorption peak 620 nm» blue
complementary/opposite colour observed «to wavelength absorbed»
OR
pH 2: «absorption peak 520 nm» green absorbed AND pH 11: «absorption peak 620 nm» orange absorbed
Award [1 max] if colour absorbed and colour observed are correct for either at pH 2 or pH 11.
