A practical lab in the browser, built to let students get it wrong

Kenyan schools sit exams that assess practical skill directly — CBC, Cambridge IGCSE, Cambridge International A-Level. All three examine whether a candidate can actually do the work: pick the right apparatus, follow a method, take a reading, judge a result.

Meanwhile the lab is shared between streams, half the reagents are out, and running forty students through a titration with glass burettes and 0.1 M sodium hydroxide is a supervision problem before it's a teaching one. So the practical gets described at the board instead of done at the bench, and students meet the apparatus for the first time in the exam.

What the exam actually tests is the decisions. Which indicator, and why that one. What to rinse each piece of glassware with, and what happens to your titre if you get it wrong. What order to build the setup in. How to read a meniscus. When the pink is permanent rather than a flash that fades.

None of that is trained by watching someone else do it correctly. You learn it by making the mistake and finding out what it cost — which is exactly the thing a shared lab with limited reagents can't afford to let you do.

A simulator that walks you through the correct steps in the correct order teaches you to recognise a titration. It doesn't teach you to run one. The whole value is in the space between the two, so the design brief became: let the student be wrong, then tell them precisely why — and make being wrong cheap enough to do repeatedly.

That one decision shaped everything downstream. It's why the apparatus stage is a puzzle rather than a checklist, why the setup has to be sequenced rather than played, and why every rejection carries a reason instead of a red cross.

The briefing carries the method, the safety notes, what you should be able to do by the end, and how long it takes at the bench. Then the storeroom shelf: fifteen items, ten of which belong to this experiment. Some of the rest belong to a different practical, and you aren't told which until you commit.

Take your tray to the bench and the check tells you how many items are wrong and how many are missing — never which ones. Get it wrong and the shelf reshuffles for the next attempt, so you can't pattern-match your way through. Methyl orange sits on that shelf every time, which is the point: it's a perfectly good indicator, just not for this titration.

Then the setup. Eight steps, one correct order, and a specific reason every time you pick the wrong one — “there's nothing in the flask yet to add indicator to”, “get the acid and indicator into the flask first”. The counter at the side keeps a tally of steps tried out of order, because the reason each time is the useful part.

The tap opens at a steady stream, a slow trickle, or a single drop, and the flask colours as you go. The task isn't to reach a number: it's to titrate to a permanent pale pink, record the titre, get a second one concordant within 0.10 cm³, and calculate the concentration from what you measured.

Overshoot and you take a fresh flask and start that run again — which costs you nothing but time, which is the entire argument for doing it here first. A pH meter can be clipped on for students who want to see the curve behind the colour change, and short explainers sit under the bench for the questions that come up mid-titration: why the pink flashes and fades, why phenolphthalein and not methyl orange, what to do if you overshoot.

A Cambridge practical exam isn't one document. There's the confidential instructions telling the technician what to set out, the question paper the candidate sees, and the mark scheme the examiner works from. Schools normally only ever see the middle one.

Sayansia holds the set — IGCSE chemistry, biology and physics, and the A-Level advanced practical skills papers — parsed down to the question and the marks available. A student can sit the paper and be marked against the scheme the examiner actually uses, rather than against an approximation of it.

Teachers create classes against a curriculum and subject, and set work from one place: any experiment on the bench, or any past paper marked against the real scheme. Assignments can be published straight away or held as drafts, so a term's worth can be lined up and released one at a time.

Everything set lands on a calendar by week, and results come back per attempt — including the attempts that failed, which for a practical is the interesting half. The library stays open alongside it: students can run any experiment on the bench outside of what's been set, as often as they want.