Theory - the electrical conductivity of solutions

You should have made the following observations regarding the brightness of the lamp:

0.1 M solution	lamp brightness	solution conductivity
Hydrochloric acid	very bright	very high
Sodium hydroxide	bright	high
Sodium chloride	dim	low
Magnesium sulfate	dim	low
Aluminium nitrate	bright	high

All of the solutions conduced electricity as they are all ionic. That is, they contain charged particles called ions. These ions are free to move in the solution, and it is this flow of charge which is the electrical current. The exact conductivity of a solution depends on the conductivity of the ions which it contains. This varies as different ions have different mobilities in solution - that is they can move through the solution at different rates. This is too complicated for "A" Level, but we can explain the results above fairly easily.

The aluminium nitrate solution conducted quite well. This is a result of the number of ions it contains. Al(NO₃)₃ will produce 4 ions on solution, whereas all the other materials produce 2.

There is still a difference in the other four to explain, and these all had the same number of ions in solution. This must mean that at least one of the ions in the hydrochloric acid conducts very well, and at least one of the ions in the sodium hydroxide solution conducts fairly well. The ions concerned are the hydrogen ion and the hydroxide ion. These two ions are extremely good conductors as there is a special mechanism by which they "move" through the water. Most ions have to make their way past all the water molecules This is a slow and tortuous process. The diagram shows a "green" chloride ion heading towards the positive electrode:

We have seen that different ions have different mobilities. Depending on factors like size and charge they will find it easier or harder to move through the water. The hydrogen and hydroxide ions transfer indirectly by joining to adjacent water molecules in a similar way to a set of dominoes falling over and knocking down their neighbours. As a result these ions are highly conducting, particularly so in the case of the hydrogen ion. The following diagram shows a hydroxide ion transferring across some water to the positive electrode:

You should have noticed that we used alternating current. The use of a direct current would have caused the ions to move towards the oppositely charged electrode, and electrolysis would have taken place when they arrived. The alternating current used was 50Hz - this means each electrode changed from positive to negative and back again 50 times per second. As a result there is no overall movement of the ions in the solution.