How to get a positive reaction

29th April 2005 at 01:00
Possibly the most common reaction used in science practicals at KS3 is the one between calcium carbonate and hydrochloric acid, where carbon dioxide is bubbled through limewater via a delivery tube. The challenge is to leave a little bit of the novelty aspect to keep the reaction fresh and interesting.

Here are some of the techniques I have adopted:

* Use a 250ml beaker for the reaction. Cover with a latex or rubber glove and seal tight. Vaseline will help. As the reaction proceeds, carbon dioxide will fill the glove and make it stand up.

* Carry out the reaction in a test tube and connect a plastic food bag to the top. Seal it tight and let the bag fill with carbon dioxide. When full, take off the bag and add 2ml of limewater, hold the bag tightly and shake.

The limewater will turn cloudy.

* Create a marble "statue" by cementing marble chips together using plaster of Paris. Cement in a piece of wire. Record the mass of the "statue" once it has dried, and using the wire dunk the "statue" in "acid rain" - acidified water at pH 5. After a week, take out the "statue" and let it thoroughly dry. Re-record the mass and calculate the change. Students should find a small decrease in mass.

Stuart Bennett Head of science, The Aveland High School, Lincolnshire

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