Last Drop

Last Drop: the titration required practical, one drop at a time

This free game is part of our AQA GCSE Chemistry (8462) course for exams in EXAM YEARS 2027-2029.

What is a titration?

For AQA GCSE Chemistry (spec point 4.4.2.5, chemistry only), the volumes of acid and alkali solutions that react with each other can be measured by titration using a suitable indicator. Required practical 2 asks you to find the reacting volumes of a strong acid and a strong alkali accurately, and Higher tier students must then use those volumes to work out a concentration in mol/dm³ and in g/dm³.

Last Drop puts you on the night shift at Clearwater Labs with three alkali samples to check. In each one, 25.0 cm³ of alkali goes in a conical flask and an acid of known concentration goes in the burette: hydrochloric acid with sodium hydroxide, nitric acid with potassium hydroxide, and sulfuric acid with a diluted drain cleaner. You run the burette tap yourself. Hold the tap to pour (the longer you hold, the faster it flows), tap for a single drop, swirl the flask, and stop on the last drop: the first colour change that stays.

Getting the kit ready

Every sample starts with the prep, and every wrong move is explained. Rinse the burette with a little of the acid, not water, because water left inside would dilute the acid and make every titre too big. Fill it past 0, run some acid through the tap to clear the air bubble from the tip, and set it to 0.00 cm³. Rinse the pipette with the alkali, and use a pipette filler to measure exactly 25.0 cm³ into the flask: a measuring cylinder is not accurate enough. Add a few drops of an indicator with a sharp colour change, such as methyl orange or phenolphthalein (universal indicator changes gradually, so it has no sharp end point), and stand the flask on a white tile. A flask that is still wet with distilled water is fine, because water does not change the number of moles of alkali the pipette puts in.

The rough titration, the end point and concordant results

First you do a rough titration: run the acid in fairly quickly, swirling, until the colour changes for good. Watch the flask closely. Where the acid lands, a cloud of colour appears and then swirls away, because there is still alkali left to neutralise it. Near the end point the cloud lingers longer and longer, until one drop changes the whole flask. Methyl orange is yellow in the alkali, orange at the end point and red if you add too much acid. Phenolphthalein is pink and turns colourless, so you cannot see an overshoot at all.

Your rough titre becomes an orange line on the magnified burette view. In the accurate titrations you run the acid in quickly until you are about 1 to 2 cm³ before it, then add it drop by drop, swirling after each drop, and stop at the first permanent colour change. Then you read the burette: drag the eye line to the bottom of the meniscus and record the reading to 2 decimal places (the nearest 0.05 cm³). A burette reads 0 at the top, so the numbers get bigger as you go down. You keep going until two titres are concordant (within 0.10 cm³ of each other), then choose which titres go into the mean: concordant ones only, never the rough titre.

The Higher tier calculation

Higher tier students then turn their own mean titre into a concentration, step by step: moles = concentration × volume, with the volume in dm³ (divide cm³ by 1000); use the mole ratio from the balanced equation; then concentration = moles ÷ volume of the alkali (25.0 cm³ = 0.0250 dm³). To give the answer in g/dm³, multiply the concentration in mol/dm³ by the relative formula mass, M_r. The sulfuric acid sample tests the ratio that catches out many students: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O, so one mole of acid reacts with two moles of sodium hydroxide. Foundation tier students can skip the calculation.

How to play: hold OPEN TAP (or the space bar, or the tap itself) to pour, press 1 DROP (D) for one drop, SWIRL (S) or drag circles on the flask, and press END POINT (Enter) when the colour stays. Each wrong move costs points and opens a short explanation with a link to the lesson. A game takes about 6 to 9 minutes, and your best score is saved so you can play again for a perfect end point every time.