Paper chromatography and Rf values applied to identifying substances, the advantages of instrumental methods of analysis, and using flame emission spectroscopy to identify and measure metal ions.

A C4.2 calculation and explanation. Reward: the Rf value is the distance moved by the dye divided by the distance moved by the solvent: $R_f = \dfrac{6.0}{8.0} = 0.75$. An Rf value has no units. Chromatography shows whether the colouring is pure or a mixture by the number of spots: a pure substance produces a single spot, while a mixture separates into several spots as the different substances travel different distances. So if the food colouring gives more than one spot, it is a mixture. Markers credit the correct Rf calculation of 0.75, the point that Rf has no units, and the link between number of spots and purity (one spot pure, several spots a mixture).

A Higher tier question on instrumental analysis. Reward two advantages from: instrumental methods are more accurate, more sensitive (can detect very small amounts or low concentrations), and faster, and they can analyse very small samples or distinguish ions that look similar by eye. Flame emission spectroscopy identifies a metal ion because each metal ion produces a line spectrum with lines at characteristic wavelengths (a unique pattern), so the pattern of lines is matched to a known element to identify it; the intensity of the lines can also be used to find the concentration. Markers credit two valid advantages (such as more sensitive and faster), and the explanation that each ion gives a characteristic line spectrum used to identify it (with intensity giving concentration). A common error is to give vague advantages like "better" without saying why.