Fire stopper

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Watch the video and download the technician notes from the Education in Chemistry website: rsc.li/2SDmhF2

On 22 August 1986, as the sun rose over Lake Nyos in Cameroon, West Africa, the forest and surrounding villages were eerily quiet. Almost 1800 people had died overnight. 3000 cattle were also wiped out; household pets and even the local bird population were mysteriously affected. In Nyos village, only six people survived. They told of rumbles from the lake in an ancient volcanic crater above the village, followed by a 50-metre-high cloud of white gas sweeping through.

The tragedy – caused by an outgassing of trapped volcanic carbon dioxide emissions which had been building up under pressure deep beneath the lake – emphasises how a gas that we easily take for granted can be lethal. In this case, a landslide disturbed the lower waters, nucleating gas bubbles and releasing the carbon dioxide like a fizzy pop bottle being opened.

This demonstration shows the density and extinguishing properties of carbon dioxide and helps to emphasise that, from a chemist’s perspective, an ‘empty’ beaker is in fact full of substance.

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Kit

10 g calcium carbonate powder
100 cm³ 2 M hydrogen chloride*
Large beaker or jug with volume of at least 2 dm³
Piece of card or plastic to cover beaker and act as a loose barrier
250 cm³ beaker
1 m section of guttering
Tea light candles
Clamp stand with clamp
Sticky tack or plasticine
Indicator (optional)

*Acid notes

Alternatively, you can use 200 cm³ of 1 M hydrogen chloride. However, finding beakers of proportions that will accommodate the liquid and fit inside the larger beaker is challenging. The demonstration can be simplified by adding the carbonate and acid into the base of the larger beaker – but sometimes liquid can drip out as you pour, reducing the visual impact of the demonstration.

Preparation

Weigh out 10 g (0.1 M) of calcium carbonate powder into a 250 cm³ beaker. Measure out the acid and (optionally) add a few drops of indicator. Use a clamp stand or pile of books to raise one end of the guttering approximately 25 cm off the desk. Place tea lights along the length of the guttering, using sticky tack or plasticine on the downslope side to secure them in position and hold them level.

In front of the class

Wear eye protection. Place the 250 cm³ beaker containing the calcium carbonate into the large beaker. Add approximately 50 cm³ of the acid to demonstrate to the class the acid neutralisation and production of the gas. Immediately place a piece of card or plastic over the top of the beaker as a loose barrier to minimise the release of aerosolised acid droplets and reduce losses to diffusion.

Wait for the bubbles to subside before adding the remaining acid and replacing the screen. Don’t be tempted to add all the acid together – the foam tends to spill out into the larger beaker. Remove the smaller beaker and ask the class what they think is in the larger beaker. It will appear ‘empty’ to them, but of course the relatively high density of the carbon dioxide keeps it in the beaker.

Light the tea lights on the guttering and pour the gas from the beaker down from the top of the guttering. The wave of gas will displace the air around each candle and extinguish them in sequence.

Alternative approaches

Demonstration #56 from Classic Chemistry Demonstrations describes using dry ice or a CO₂ cylinder as an alternative to producing the gas by neutralisation. On the small scale, this can be poured over a single tea light in a 100 cm³ beaker to illustrate trapping the gas within. Attempts to relight the candle with a splint will fail due to the gas that pools in the beaker. This can be scaled up with a 5–10 L transparent storage box. These commonly have a base of approximately 20 x 30 cm, so a large number of tea lights can be placed in the base and extinguished by the 2 L beaker of carbon dioxide, which will displace air to a height of around 3 cm.

Teaching goal

This demonstration is useful for exploring aspects of the fire triangle, properties of carbon dioxide, or introducing chemical reactions and neutralisation – as the addition of indicator can illustrate a colour change, as well as the production of a gas.

Equation: CaCO₃ (s) + 2HCl (aq) → Ca(Cl)₂ (aq) + CO₂ (g) + H₂ O (l)

The neutralisation of 0.1 M of calcium carbonate evolves approximately 2.4 L of gas, enough to fill the beaker. At 21°C and 100 kPa the density of carbon dioxide is 1.8 kg/m³, compared to air’s density of 1.2 kg/m³. This means that the gas displaces the air and remains in the beaker rather than immediately escaping. The gas will flow down the guttering and displace the air from around the candles to extinguish them.

Those who survived the tragedy at Nyos were more than 50 metres above the valley floor, where the gas was pooling below the elevated crater lake. You could model this by using a taller candle part way down the pipe; its flame would be elevated above the flow of the gas. The white gas described by the survivors was likely aerosolised water droplets (mist) from the lake carried down along with the transparent and colourless carbon dioxide. Since the event in Nyos, levels of gas in the lake are routinely monitored and regularly vented to prevent such an incident occurring again.