My Top 5 Chemistry Demos

This post is for the teachers (and tutors, and in fact you can do one of these demos at home) – especially if you’re trying to find an engaging demonstration for open evening, end of term or just to liven up that Friday afternoon period 6.

I love showing off how exciting chemistry can be, and the joy of wowing students with a cool demonstration is one of the things I miss most about not teaching in school any more. So I’m hoping that at least by sharing these, I can encourage more teachers to pack more demonstrations into their lessons.

Value-for-money is a mantra I live by, so I’ve tried to select demos that:

• Don’t require expensive equipment or materials
• Don’t require hours of set up
• Are reliable and unlikely to fail
• Can be linked to the curriculum
• Are (mostly) safe

Best for cheap and safe: hair gel and salt

There really is nothing more to this demo – chuck some cheap hair gel into any receptacle you like, sprinkle over some sodium chloride (or any salt) and mix to watch the hair gel ‘disappear’. The materials are so safe that students of any age can do this themselves, (good for lunchtime clubs) and it can be done at home. The explanation for what’s happening can be a bit tricky for younger students, but I’ve summarised below.

Materials and equipment: cheap hair gel (not wax or putty) – can be found easily in pound shops, plus just some common salt. Any container works and the materials can just be washed down the sink afterwards.

Science behind the demo: Hair gel is a hydrogel – the main ingredient is actually water, with a small amount of a polymer that absorbs water by attracting molecules with interactions called hydrogen bonds. This traps many water molecules within a matric containing a relatively small amount of polymer (so yes, when you buy hair gel you are mostly buying water). Sodium chloride is an ionic compound that dissolves in water. The ions in sodium chloride form strong attractions with water molecules – stronger than hydrogen bonding. These interactions disrupt the hydrogen bonds between the polymer and the water molecules, releasing the water so you can see it moving freely. You can’t really see the polymer molecules from the hair gel as there are so few of them compared to the water.

Curriculum links: solubility, bonding and structure, polymers

Best for suspense-building: solid metal displacement (alternative to thermite)

I’ve had some fun with the thermite reaction, but it’s a pain to set up and technicians can (justifiably) get nervous about the safety – often leading to a disappointingly tame reaction. The need for a fuse can also distract from the chemistry involved, so this is my alternative.

SAFETY NOTE! This reaction is reliable. It takes around 1-2 minutes though, and in that time there is nothing to observe – that’s what makes it so good for suspense-building. You should practice it in an empty lab first to see roughly how long it takes and also to see if your demo is far enough away from students (I usually move the front row of students back). Treat it like a lit firework – do not return to check it! If you really, really think it’s not working, hit the gas cutoff and give it time to cool before returning to the mixture. I usually put the safety screen between the mixture and the Bunsen tap so I can turn off the gas and still be protected by the screen.

Materials and equipment: metal crucible (porcelain does not get hot enough), magnesium powder, copper oxide powder, powder, Bunsen burner, heat mat, at least one safety screen.

Instructions: can be found in the RSC Teach Chemistry resources.

Curriculum links: metal displacement, energy changes (good link to thermodynamic vs kinetic stability for A Level thermodynamics).

Other notes: the most fun in this demo is the fact that students always think you’ve done something wrong, because of the delay before the reaction happens. You need to trust the class to wait – my favourite extra detail is to mix up a second crucible containing magnesium oxide and copper powder and heat that at the same time – students have to choose which mixture to watch based on their knowledge of the reactivity series.

Best for messy and fun: hydrogen peroxide decomposition (elephant’s toothpaste)

You can’t beat an old classic. No special skills involved, just mix the chemicals and watch the fun. You can do this on a small scale to avoid a lot of clean up, but the beauty of this demo is its adaptability. I usually use a 500mL measuring cylinder and add washing up liquid to increase the amount of foam produced. To make it easy to clean up, put it on a sheet of plastic or in a large washing-up bowl on the floor.

Materials and equipment: Saturated potassium iodide solution, 20-30% hydrogen peroxide solution, detergent, large measuring cylinder or flask

Instructions: can be found through the CLEAPSS website.

Curriculum links: decomposition reactions, catalysis,

Best for A Level classes: endothermic solid-solid reaction

This is my favourite introduction to entropy for sixth form classes, but it’s also something you could demonstrate for the energy changes topic at GCSE. You need a fume cupboard and it’s a little more specialised in terms of chemicals than the other examples here, but that’s why it’s good for A Level – it’s completely different to the normal ‘big, exothermic’ demonstrations we do in chemistry. You can add extra wow factor by putting a few drops of water under the beaker and freezing the beaker to a white tile or heat proof mat.

In keeping with the demonstration itself being more specialised, it’s best performed in a fume cupboard. If none available, you can get by next to an open window, in a conical flask with a cotton wool stopper but should remove the mixture and dispose of products quickly to prevent too much ammonia being released.

Materials and equipment: barium hydroxide octahydrate, ammonium chloride, large beaker, white tile. The demonstration is more effective if you also have a digital temperature probe that can read low enough (around -20°C).

Instructions: found on the RSC website.

Curriculum links: this is a great example of a very endothermic reaction that is still feasible at room temperature, thanks to the large positive entropy increase when two solids react to form liquid water and ammonia gas. There are lots of ways you can link it to calculations (waters of hydration, volumes of gases), acid base reactions, test for ions… It’s an all-round good synoptic reaction!

All-time favourite: aluminium and copper sulfate (‘Northern lights’ demo)

Why is this my favourite? It’s synoptic, can be used at GCSE and A Level, is easy to set up and has really pretty colours!

Materials: 2M HCl, copper sulfate, large conical flask (a 500mL flask with a relatively wide neck works best), aluminium foil, heatproof mat, splints.

Instructions: I’ve seen various different instructions for this, some needlessly complicated and some even involving mercury salts! So here’s how I usually set it up:

• Set up the flask where it can be seen. Have a Bunsen on safety flame near (but not right next to the flask!)
• Have a student or colleague by the light switch – this works best with the lights off!
• Add around 100 mL acid to the flask.
• Take a piece of aluminum foil around 10cm x 10cm. At this point, you can demonstrate to students that the aluminium doesn’t react with the acid – scrunch it lightly to form a ball that fits through the neck of the flask but is still providing lots of surface area.
• Add a couple of spatulas of copper (II) sulfate to the flask and swirl to dissolve it in the acid.
• The reactions start quite fast – the aluminium will be both displacing the copper and reacting with the acid to produce hydrogen.
• Use lit splints to ignite the hydrogen at the top of the flask – the flame will be coloured blue-green due to the copper.
• If you want a slightly bigger ‘pop’ from your hydrogen flame, you can put a heat mat over the flask for a few moments to trap more hydrogen. Remember to hold the splint at the top when you light it, not into the flask, as it needs oxygen from the air! Be careful when touching the flask as it is very exothermic – you should be able to see steam as well as the other observations.
• You can usually throw in a few balls of aluminium, and your students will probably want you to! Afterwards, make sure they see the colourless solution formed and big lumps of solid copper.

There’s also a variation that students can carry out here, without the acid.

Curriculum links: acid-metal reactions, reactivity series, displacement reactions, flame tests (or emission spectra), tests for gases, energy changes, transition metals (you can even link to ligands – the blue copper sulfate produces a green solution due to the chloride ions in the acid).