Time to tackle intermolecular forces with my Year 12 AS-level chemistry students at Eggbuckland Community College, an 11-18 comprehensive technology college in Plymouth. How can I spice up hydrogen bonding? This is a crucial part of chemistry. It is the force that binds water molecules and holds proteins in precise configurations - it the key to life and to shaping forces of the planet such as the oceans. Yet many students find it difficult to understand hydrogen bonds and approach this part of the specification with apprehension.

Putting the kettle on to show the anomalously high boiling point of water will not provide many thrills. Instead I ask my students to imagine a world without hydrogen bonds. We discuss the fact that proteins and DNA would lose the ability to form the 3-D shapes required for life, and that water would boil at approximately - 150oC, leading to six billion explosions of the human bodies that populate the world. Hydrogen bonding is the strongest intermolecular force.

The students need to understand how and why molecules form hydrogen bonds. This will build on their understanding of the shapes of molecules, electronegativity (how good atoms are at attracting electrons) and polarity (which atom in a molecule has the greater share of the electrons). Basically, they need to know that the oxygen atom in H2O has more of the electrons and so is attracted to the electron deficient hydrogen atom in a neighbouring water molecule. I keep it simple - in my experience the most effective way of helping them to understand the theory is to get them to draw diagrams of hydrogen bonding between appropriate molecules, such as water and ammonia.

I find a good demonstration is needed for the final part of the lesson. For example, how to show (as required in the specification) that ice is less dense than water? The Royal Society of Chemistry publication Classic Chemistry Demonstrations describes colouring ice cubes with blue food dye and placing them in a tall glass vessel containing sunflower oil floating on water. The ice floats on the oil, but as it melts the blue droplets of water sink through the oil, proving that their density has increased. I set it up and it looks a bit like a lava lamp, which appeals to my students, who have something of a “retro-Seventies” fixation. I use the Multimedia Science School CD-Rom Structure Viewer with a laptop and projector, and add molecular models, to illustrate the open tetrahedral network of water molecules imposed by hydrogen bonding.

Another intriguing experiment from the RSC book is the “tubeless siphon”. Dissolve poly (ethene oxide) in water and propan-2-ol and pour the solution from one beaker to another. The liquid is so viscous that if you stop tilting the beaker it defies gravity and continues to “climb” up the side and pour itself into the second container. The solution looks really spectacular when coloured luminous green with a little fluorescein - we feel like “Harry Potters”. If I draw a section of the polymer on the board, I can ask my class whether hydrogen bonds should form between polymer chains. Will any of them spot that the water molecules act as hydrogen-bonded bridges?

An experiment from physics involves filling tall transparent tubes with glycerol and dropping a steel ball through the solution. The glycerol is so viscous due to hydrogen bonding that the ball quickly reaches terminal velocity and continues in a sedate fashion down the tube. I then fill another tube with heptane (whose molecules are held together by a much weaker type of force) and the ball plummets to the bottom. The glycerol and heptane have similar molecular masses, so in the absence of hydrogen bonding would have similar strength intermolecular forces. The lesson is a success: the students’ enjoyment of the experiments does enable them to grasp the difficult theory involved.

Dr Chris Hall is head of science at Eggbuckland Community College, Plymouth. Email: chall@eggbuckland.plym.sch.uk For this lesson, he won the Salters’ Prize for Best Chemistry Teacher.

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RESOURCES AND MATERIALS

Classic Chemistry Demonstrations. By Ted Lister. Royal Society of Chemistry Tel: 01223 432300. Price: pound;27.50 (pound;17.50 to members) Structure Viewer a Multimedia Science School CD-RomPrice: pound;99 + VAT + pamp;pTel: 01491 413999 www.new-media.co.uk