Puzzle this one out

28th April 2006 at 01:00
Covalent compounds can be hard to make sense of, but Peter Hollamby has made them fun for key stages 3 and 4

Teaching formulae and equations can be a challenge, and a topic which students can find rather boring. Lessons improve when practical work exemplifies a chemical reaction, but paper exercises remain a turn-off for most students. Finding ways to show ratios of atoms or ions involves scissors and cardboard, and the exercise, albeit a relatively successful one, is hardly inspiring.

When I was awarded a secondment to work for the Science Enhancement Programme with the Royal Society of Chemistry, it gave me time to develop a number of ideas to enhance chemistry teaching.

One idea was to develop "chemistry jigsaws". Working with Middlesex University teaching resources department, we produced two jigsaw kits that could provide opportunities for students to use their kinaesthetic intelligence to learn how to construct chemical formulae and equations.

A key stage 4 Ionic Compounds kit allowed pupils to use a traditional method of formula construction which relies on making squares or rectangles with ions and cations. They can then write down the ratio of the pieces making up the rectangle and quote a formula. This method has been used by many teachers before, but because the kit was made from good-quality colour-coded plastic, I felt students would be more motivated. And since the plastic was transparent, the pieces could be used with an overhead projector.

I was even happier with the KS34 Covalent Compounds kit that we developed because it allowed students to model molecules and balance equations in two dimensions. The "atoms" were colour-coded and students soon adapted to the method, which could be used to model relatively complicated formulae. The bonds, plus signs and equation arrows were also transparent so I could demonstrate modelling using the OHP.

Pupils can easily model an equation and balance it correctly because they only have to consider size, shape and colour in a two-dimensional array on the laboratory bench, instead of having to work with chemical symbols. Once the equation was balanced, they could write down the formulae corresponding to the shapes and learn how ratios and balancing are represented using formulae.

I gave a number of Year 7 students a pile of covalent jigsaw pieces which I dumped in the middle of a laboratory bench along with the challenge: "Have a mess-about with these."

"What are they, sir?"

"What do you think?"


"Yes, that's right. Each atom has a different colour."

"What's the red one?"

"Oxygen. And if I told you that the white ones are hydrogen atoms, what could you make for me?"


The key for the jigsaws was written on the whiteboard. Several molecules were constructed and each student in turn asked for the name of the molecules they had just made. They were then pleased to be able to offer the formula, which was always correct.

One student, who had commandeered a lot of the pieces, proudly revealed his effort: "I've made a snake, sir."

"Interesting! What atoms does it contain?"

"Carbon and hydrogen, sir."

"That's right - it's called a hydrocarbon. Can you tell me its formula?"

Counting began and the student replied... "C8H18"

A conversation about long stringy molecules and candle wax ensued and after some coaching the realisation developed that hydrocarbons could be represented by a general formula of n carbons and 2n+2 hydrogens.

"What are the plus signs and arrows for?"

"They help you to make chemical reactions."

"What's a chemical reaction?"

"A permanent change."

"Yes, that's right. In your lessons, you know about frying an egg or making toast as examples. What can we do with these jigsaws?"

"What can we add to this sir?" (This question related to the snake.) "Well, it's a hydrocarbon, a bit like petrol; or if it were a little more than twice as long, it could be candle-wax. What's needed for these compounds to burn?"


"Where is the oxygen molecule? Add it to the snake."

At this point C8H18 was placed alongside a "plus sign" and one oxygen molecule. The arrow was placed next to oxygen and students were asked:

"What does this make?"

The question did not produce an immediate response, but the idea that atoms would have to "swap" developed. The problem with C8H18 was that it offered a huge range of possibilities because students could make dozens of different molecules with C, H and O atoms, and still maintain a balanced equation, although they did not realise that was necessary at this stage.

"Chemistry can be complicated. Suppose I told you that the only products really are carbon dioxide and water. I can prove this by doing tests; we'll talk about it later."

They were asked to make the extra molecules and add them into the equation.C8H18 + O2 ' H2O + CO2 was quickly constructed.

"This looks wrong, how many oxygens do I need?"

"I think we'll have to do a simpler equation than this; even students in the GCSE class would have a few problems with this. What do you think is wrong?"

A discussion about balancing ensued and the need to have the same number of particles, red, white and black was offered.

"We don't have enough jigsaw pieces."

"Sorry, we'll do some more later; I just wanted to observe your responses to these pieces."

"Can we buy sets of jigsaws?"

"We'll see."

l For more details about chemistry jigsaws go to: www.chemsoc.orgnetworkslearnnetjigsaws.htm

Jigsaw kits are available from Middlesex University Teaching Resources Ltd, Unit 10, The IO Centre, Lea Road, Waltham Cross, Hertfordshire EN9 1AS.

Prices are from pound;3.25.

Tel: 01992 716052 www.mutr.co.uk

* The Royal Society of Chemistry: www.rsc.org

Science Enhancement Programmewww.sep.org.uk

Peter Hollamby is a consultant for the Royal Society of Chemistry and the Science Enhancement Programme, formerly head of chemistry at St Cyres School, Penarth

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