Collapsing chemistry

16th January 2004 at 00:00
Capturing my pupils' imaginations at the start of a lesson has a big impact on how they engage with the subject. A chemistry demonstration at the beginning can stimulate excitement and discussion throughout the lesson.

For this experiment, you need a 2-litre plastic bottle with a lid, safety goggles, 100ml of 2 Molar sodium hydroxide (NaOH) and a CO2 generator.

Before the lesson fill the bottle with CO2 gas.

I start by asking whether pupils believe in the ability of the mind to affect objects. Show the bottle to the class and put on the safety goggles.

Slowly pour the NaOH into the bottle and tighten the lid. Tell pupils to focus on the bottle and try to affect its shape with their minds.

The bottle will start to collapse and make cracking sounds. Pick up the bottle and gently swirl the contents. As you shake the bottle the reaction gets faster. Tell pupils to concentrate harder and, as if by magic, the bottle will start to collapse entirely during the next few minutes. Remove the lid and dispose of the contents with plenty of water.

I have shown this to GCSE science classes in the context of acid-base chemistry. I have also discussed with pupils how air pressure can cause the bottle to collapse. The chemistry behind it is a two-step process. First, water-soluble sodium hydrogen carbonate is generated, followed by sodium carbonate. As the CO2 reacts with the NaOH, it reduces the pressure inside the bottle and slowly collapses. Later, I challenge pupils to come up with possible chemical explanations for this effect. With weaker pupils, I tell them that the bottle has CO2 in it.

Richard Waller, head of KS3 science, Comberton Village College, Cambridge

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