JSSS
Student
To investigate the decomposition of water
OC34
Introduction
Electrolysis means splitting something up using electricity. Compounds are pure substances, but
can be broken down chemically (e.g. using heat or an electric current) to simpler substances. In
this investigation, electricity is used to split the compound water into its constituent parts.
Apparatus
Student gas voltameter, power pack, water to which sulfuric acid has been added (‘acidified
water’)
Safety
Wear gloves and safety glasses.
Do not exceed 10 Volts.
Fact Pure water does not readily conduct electricity. Adding acid to it enables it to conduct
electricity more easily.
Procedure
Bath
Acidified water
Positive
terminal
Negative
terminal
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JSSS
Student
1. Half fill the bath of the voltameter with acidified water.
2. Fill the two small test tubes with the acidified water, and set up the apparatus as shown in
the diagram.
3. Apply a voltage of 2 V d.c. to the terminals.
4. Observe and describe what happens in the test tubes.
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5. Increase the voltage in 2 V steps as far as 10 V d.c. and describe what happens.
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6. Compare the volumes of gases in the two test tubes. Write down what you notice?
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7. Test each of the gases formed with a glowing splint, and write down your results.
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8. What do these results tell you about each of the gases?
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9. Repeat the investigation using a voltage of 10 V d.c.
10. Test each of the gases formed with a lighted splint and write down your results.
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11. What do these results tell you about each of the gases?
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12. What gas is formed at the positive electrode?
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13. What gas is formed at the negative electrode?
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14. What does the experiment indicate about the composition of water?
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15. What is the chemical formula of water?
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