School science labs: storage, quantities and what to ban

The container came out of a cupboard during a stocktake: a 2 kg tub of polyacrylate with a 2014 date on the side, lid cracked, contents set into a single hard block that had to be chiselled out over a bin. Nobody could remember ordering it. Two teachers had used it for a water-absorption demo, and after that it had simply become part of the furniture.

School labs are a reasonable place for this material and an easy place for it to go wrong. The chemistry is benign, the demonstration is genuinely good, and the failure modes are all about storage and supervision rather than toxicity. A department that gets those two right can use it for years out of one small tub.

What the material is, in language a department can use

Sodium polyacrylate is a long carbon chain with carboxylate groups along it, lightly crosslinked into a network. Water enters the network, the charges repel each other, and the whole thing unfolds and holds the water in place. A gram can take up 200 to 300 g of distilled water. Add salt and the network collapses, which is the second-best demonstration in the whole topic and costs one spoon of table salt.

It is not a hazardous substance in the sense that acids and solvents are. It is a nuisance dust, and the sensible controls are the ones you would use for any fine powder: don’t make clouds of it, don’t sweep it dry, wipe with a damp cloth. What it will do is make a floor extraordinarily slippery if a spill is wetted and left, and that is the risk that actually sends someone to hospital.

The quantity question

A secondary school does not need 5 kg. Two hundred grams will run a class set of demonstrations for several years, and the smaller the container, the smaller the problem when it is forgotten. Buy 250 g or 500 g in a sealed screw-top jar, decant into a labelled 100 ml beaker for the lesson, and keep the bulk jar in the chemical store rather than the prep room bench.

Label it with the date received and a review date. Anything still in the cupboard after five years has almost certainly absorbed enough atmospheric moisture to be useless; the granules clump, the swelling is slow, and the demo underwhelms. That is a storage failure, not a product failure, and the fix is a new small tub rather than a bigger order.

What to ban outright

• Drains. A teaspoon of swollen gel in a sink trap becomes a plug that a plumber has to cut out. Every grain goes in the bin, and the bin goes out the same day.
• Dry sweeping. Sweeping a spill raises dust across the whole lab. Wipe, don’t sweep.
• Mixing with acid or bleach “to see what happens”. The answer is nothing good, and the mixture is no longer a demonstration, it is an unknown.
• Open flames and hot plates. Dried gel film on a hot surface smokes and leaves a hard residue that is difficult to remove.
• Take-home samples. Swollen gel in a student’s pocket, bag or mouth is the one scenario a department cannot supervise.

Three demonstrations that hold up in a 40-minute lesson

The classic is the inverted jar: fill a beaker with gel, invert it over a student’s head, and nothing falls. It works every time provided the gel has had ten minutes to set and you used distilled water rather than the hard tap supply that most schools have. With tap water at 300 mg/L you get roughly 15 percent less swelling, which is still enough to hold, but the margin is thinner than the textbook suggests.

The second is the salt reversal. Add a spatula of sodium chloride to a swollen sample and watch it liquefy within a minute as the ionic strength rises and the network collapses.

The third is a mass balance: weigh 1 g dry, add 200 ml of water, leave it overnight, and weigh the gel the next morning. Students calculate the ratio themselves and then compare it with the number on the tub. Most classes get between 180 and 240, and the gap between groups is a useful conversation about measurement rather than about chemistry.

The risk note a head of department will ask for is a single page, and it can be short: substance name and CAS number, quantity held, storage location, who has access, the controls above, and the disposal route. Add a line saying that the material is not classified as hazardous but is a slipping and dust nuisance, and that spills are wiped rather than swept. That note is usually enough for a school’s own insurance review, and it takes twenty minutes to write once.

If a department is buying for the first time, ask for a small sample and the technical data sheet together. A 250 g sample plus the sheet tells you more than any catalogue description, and a supplier who will not send one is telling you something too. The handling notes for super absorbent powder can be pasted into the risk assessment almost unchanged.