To investigate the electrical conductivities of different substances and solutions.
Apparatus:
Solid salt (
) crystals; different liquids such as distilled water, tap water, seawater, benzene and alcohol; solutions of salts e.g.
,
; a solution of an acid (e.g.
) and a solution of a base (e.g.
); torch cells; ammeter; conducting wire, crocodile clips and 2 carbon rods.
Always use benzene in a fume cupboard as it is very toxic. If you don't have access to a fume cupboard then ensure that the work area is well ventilated.
Method:
Set up the experiment by connecting the circuit as shown in the diagram below. In the diagram, 'X' represents the substance or solution that you will be testing. When you are using the solid crystals, the crocodile clips can be attached directly to each end of the crystal. When you are using solutions, two carbon rods are placed into the liquid and the clips are attached to each of the rods. In each case, complete the circuit and allow the current to flow for about 30 seconds. Observe whether the ammeter shows a reading.
Results:
Record your observations in a table similar to the one below:
Test substance
Ammeter reading
What do you notice? Can you explain these observations?
Remember that for electricity to flow, there needs to be a movement of charged particles e.g. ions. With the solid
crystals, there was no flow of electricity recorded on the ammeter. Although the solid is made up of ions, they are held together very tightly within the crystal lattice and therefore no current will flow. Distilled water, benzene and alcohol also don't conduct a current because they are
covalent compounds and therefore do not contain ions.
The ammeter should have recorded a current when the salt solutions and the acid and base solutions were connected in the circuit. In solution, salts
dissociate into their ions, so that these are free to move in the solution. Acids and bases behave in a similar way and dissociate to form hydronium and oxonium ions. Look at the following examples:
Conclusions:
Solutions that contain free-moving ions are able to conduct electricity because of the movement of charged particles. Solutions that do not contain free-moving ions do not conduct electricity.
Interesting fact
Conductivity in streams and rivers is affected by the geology of the area where the water is flowing through. Streams that run through areas with granite bedrock tend to have lower conductivity because granite is made of materials that do not ionise when washed into the water. On the other hand, streams that run through areas with clay soils tend to have higher conductivity because the materials ionise when they are washed into the water. Pollution can also affect conductivity. A failing sewage system or an inflow of fertiliser runoff would raise the conductivity because of the presence of chloride, phosphate, and nitrate ions, while an oil spill (non-ionic) would lower the conductivity. It is very important that conductivity is kept within a certain acceptable range so that the organisms living in these water systems are able to survive.
