Experiment 2.3 – 2.4 Electrolysis of Water

Purpose:

To determine the identity of the elements making up the compound water, and to determine the volume of each element produced.

Background:

Electrolysis is a process by which the separation of elements in a compound is carried out by means of the passage of an electric current.

Hypothesis:

What elements do you expect to separate with the electrolysis of water?

In what ratio will these elements be to each other?

Give reasons for your hypotheses.

Materials:

• power source
• matches
• plastic wrap
• wooden splint
• electrical wire (insulated)
• wire with alligator clip on one end and a prong on the other, one red, one black
• 600 ml beaker
• 2 test tubes
• washing soda (sodium carbonate)
• wire cutters/strippers

Procedure:

1. Cut a 30 cm piece of insulated wire. Strip 5 cm of the insulation off one end and 2 cm off the other. Bend to form the electrodes as shown above.

2. Fill beaker with water and add a little washing soda (sodium carbonate). Stir to dissolve.

3. Fill the test tubes to the top with the water/washing soda solution from the beaker. Hold plastic wrap over the ends and invert into the beaker. When the ends are submerged, remove the plastic wrap. You do not want air bubbles in the test tubes.

4. Slide your electrodes into the test tubes. The 5 cm stripped end should be in the test tube.

5. Clip your alligator clips to the other ends of your electrodes and plug in to the power source. Let red be + and black be - . Turn on the power source ( around 20 V).

6. Let your apparatus run for at least 30 min. Observe the gas collecting in the test tubes. After 30 min, compare the amount of gas at the positive electrode (cathode) to that at the negative electrode (anode).

Data

1. Is there a larger volume of gas at the anode or the cathode? How much more is there (twice as much, three times as much, etc)?

2. What gas do you think formed at the anode and what at the cathode? Why?

Testing the gas (read all the way through before doing this)

Test for H₂

1. Light a wooden splint with a match.

2. Remove the test tube that you think contains hydrogen from the water and keep it inverted.

3. Put the flaming splint up inside the test tube. A soft pop will indicate that hydrogen (H₂) is present.

Test for O₂

1. Remove the test tube that you think contains oxygen from the water and quickly cover the end tightly with plastic wrap.

2. Turn the test tube right side up. A small amount of water in the tube is ok.

3. Light a wooden splint and then blow out the flame so the splint is smoldering.

4. Remove the plastic wrap and stick the splint down in the test tube.

5. If the splint glows, oxygen is present.

Conclusions & Extensions:

1. Were your hypotheses correct? Explain.

2. What gas formed at the anode? What gas formed at the cathode? Explain how you know this by the volume of each gas produced and by your flame tests.

3. Is water a substance or a mixture?

4. If you did this test again, would the ratio of hydrogen to oxygen produced be the same? Why or why not?

5. Is electrolysis a physical or a chemical process? How do you know?

6. Look it up on the internet! - Electrolysis is not the only way to separate elements in a compound. Cinnabar (HgS) is a cinnamon to scarlet red mineral containing the element mercury. Write a paragraph describing a method by which mercury can be separated from the sulfur in cinnabar.

7. Can elements in a compound be separated by physical means? Why or why not?