Five aqueous solutions, labelled Q, R, S, T and U, are set out on your bench with a strip of universal indicator paper, a white tile and the colour chart fo...

This question tests two ways of measuring how acidic or alkaline a solution is: matching universal indicator paper against a colour chart (a quick estimate to the nearest whole pH) and using a pH meter (a precise electrical reading to one decimal place). The five solutions Q to U are unlabelled, so the exact colours and pH values depend on the samples on your own bench; the marking is consequential on your own table, so as long as each pH matches the colour you wrote in the same row and each classification follows from that pH, you score. A representative worked set is shown below to show what a correct, self-consistent table looks like.

(a) Record a colour taken from the chart for each of the five pieces of paper (red, orange, green, blue and purple in the example). One mark is given for each two correct chart colours, so all five correct scores the full [3]. The key practical points that make these readings trustworthy are using a fresh piece of paper and a clean teat pipette for every solution, so no drop is carried over, and waiting the full ten seconds before matching, because the colour takes a moment to develop fully.

(b) Read a whole-number pH straight from the chart for the colour you recorded, and it must be self-consistent: a red paper cannot be written as pH 7. In the example Q to U are pH 2, 5, 7, 9 and 13. Marks are for five sensible pH values that each match the colour in the same row [3].

(c) Classify each solution from its own pH using the rule that pH below 7 is acidic, pH exactly 7 is neutral, and pH above 7 is alkaline. In the example Q and R are acidic, S is neutral, and T and U are alkaline. One mark for at least three rows correct, two marks for all five [2].

(d) Ordering from most acidic means starting at the lowest pH and rising: Q, R, S, T, U. All five letters must appear [1] and the sequence must run from your lowest to your highest pH, marked consequentially on your own table [1].

(e) The two acidic solutions are Q and R. Re-measure each with the pH meter and record to one decimal place, for example Q 2.1 and R 4.9. One mark for two readings, one mark for both to one decimal place and close to the whole-number values found with the paper [2].

(f) An advantage of the pH meter is that it gives a value to one decimal place, and it does not rely on judging a colour by eye, so it removes the human error in colour-matching [1].

(g) Between the two meter readings the probe must be rinsed with distilled water and blotted dry [1]. This matters because any of the first solution left clinging to the probe would carry over and mix with the second, shifting its reading away from the true value [1].

(h) A sound plan to test whether an acid becomes less acidic on a tenfold dilution: use a pipette to measure 10.0 cm³ of the acid into a 100 cm³ volumetric flask [1]; make the volume up to the graduation mark with distilled water and mix thoroughly by inverting [1]; then measure the pH of the diluted solution with the same meter and compare it with the pH of the original, keeping the temperature the same so the only variable changed is concentration [1]. A rise in pH would show it has become less acidic.