The Department of Chemistry at the University of Illinois

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Electrolysis of water using an electrical current

glass outer and innter tubes of electrolysis apparatus

What are the products when water decomposes electrolytically?

Demonstrating the electrolysis of water requires a specialized piece of glassware, and a source of electrical current. Roll the mouse over the labels on this apparatus to see what that part does.

electrolysis glassware As gases are produced, they displace water which flows into the connecting tube, up the long middle tube and into the reservoir. If the reaction produced a lot of gas, the amount of liquid phsed up the middle tube could overflow the reservoir. In the middle of the connector between the two tubes, there is another tube which connects to the bulbous reservoir at the top. It allows liquid to flow between the two tubes on the sides and reservoir at the top. The ring stand and clamp support the piece of glassware A rubber hose is only a connector and doesn't serve any other function The rubber stoppers hold the water in the inverted tubes. A platinum electrode passes through each rubber stopper. One end of each electrode is immersed in the solution. The other end is connectted to a power source The connecting tube allows ions to migrate between tubes so that the reation remains electrically neutral. Without the connection tube, no reaction would occur. The stopcocks don't help with the demonstration. However, after the reaction is over they are used to went the gases produced during the reaction, and to drain all of the remaining liquid.

bulbous reservoir and stopcocks

The outer glass tubes and the middle tube are filled with water and the reservoir half filled. Food coloring is usually added so that it is easier to see the liquid in the tubes. In this case, a drop or two of red food color was added.

outer and inner tubes, rubber stoppers

Rectangular platinum electrodes attached to a platinum wire are held in position by the rubber stoppers. The platinum wire passes through holes in the rubber stoppers. The platinum electrode are flattened into rectangles to expose as much metal as possible to the reactant (water).

alligator clips attached to wires

The platinum wires extending from the rubber stoppers are attached to the power source with aligator clips. Either a dry cell battery or DC current provides enough current to cause the decomposition of water.

bubbles in tubes

Once the current is applied, you can see that a reaction is occurring in one of two ways. Bubbles of gas rise from the surface of the electrodes as water decomposes to H2 and O2 gas.

three colors of water in tubes

Alternatively, an acid base indicator can indicate that a reaction has occurred. This example contains Universal Indicator, which turns green-blue in base, and pink in acid.

Why are H+ and OH- present since we know that the products of decomposition of water are H2 and O2?

Decompostion of water is a redox reaction. The oxidation reaction occurs at one electrode, and the reduction reaction at the other electrode.

Water is oxidized at the anode. The reaction is...
2H2O ---> O2 + 4H+ + 4e-

Water is reduced at the cathode. The reaction is...
4H2O + 4e- ---> 2H2 + 4OH-

tube on the right has the cathode

We can tell that the tube on the right has the cathode, since at the cathode H2 is produced. In the decompostion reaction, the volume of H2 produced is twice the volume of O2.

2H2O ---> 2H2 + O2

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Chemistry at Illinois University of Illinois at Urbana-Champaign