Classifying Materials 19 Activity Find out what elements make up the compound, water
Water is a compound. It can be broken down into its elements by chemical means. A piece of apparatus called a Hoffman voltameter can be used to split water into its elements. It does this using electricity. Note: These instructions are for the mini Hoffman voltameter shown.
Apparatus Hoffman voltameter, power pack (or battery), dilute sulfuric acid
Method 1. Three quarters fill the voltameter with water. 2. Fill each of the test tubes with water and turn them upside down over the electrodes.
3. Carefully add a little sulfuric acid to the water in the voltameter as directed by your teacher. The sulfuric acid helps the electric current flow through the water.
4. Connect the leads to the voltameter and turn on the electric current.
5. Observe how gas builds up in the test tubes above the electrodes.
6. Carefully stopper the test tubes under water. 7. Repeat until you have four tubes of gas, two from each electrode.
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Fig. 4 A simplified version of the Hoffman voltameter.
Result Discuss with your lab partner what gases you think may be in the tubes. Do you think both tubes contain the same gas? Test one tube of gas from each electrode for oxygen and for hydrogen. (a) To test for oxygen • Light a wooden splint. • Allow it to burn for a few seconds. • Blow out the splint. You will see that it still glows red. • Remove the stopper from the tube of gas and immediately insert the glowing splint into the tube. • If the splint relights the gas is oxygen.
Glowing splint
Test tube of oxygen
(b) To test for hydrogen • Light a wooden splint. • Remove the stopper from the tube of gas and immediately place the lighted splint in the top of the tube. • If you hear a squeaky pop the gas is hydrogen.
