This experiment enables students to distinguish between electrolytes and non-electrolytes.
This experiment enables students to distinguish between electrolytes and non-electrolytes and to verify that covalent substances never conduct electricity even when liquefied, whereas ionic compounds conduct in the molten state.
This works well as a class experiment, with students working in groups of two to three. There will not be time to investigate all the substances, so each group could be assigned three or four of these, and the results pooled at the end.
Each working group requires:
Carbon (graphite) electrodes fitted in a holder (Note 1)
Heat resistant mat
Clamp and stand
Small pieces of emery paper
Connecting leads and crocodile clips
Six-volt DC power pack
Six-volt light bulb in holder (Note 2)
Small pieces of lead (TOXIC), copper and perhaps other metals
Crucibles containing samples of:
Phenylsalicylate (salol) (IRRITANT, DANGEROUS FOR THE ENVIRONMENT)
Zinc chloride (CORROSIVE, DANGEROUS FOR THE ENVIRONMENT)
Refer to Health & Safety and Technical notes section below for additional information.
Health & Safety and Technical notes
Wear eye protection.
Lead, Pb(s), (TOXIC) - see CLEAPSS Hazcard.
Copper, Cu(s) - see CLEAPSS Hazcard.
Phenylsalicylate (salol), C6H4(OH)COOC6H5(s), (IRRITANT, DANGROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard.
Wax - see CLEAPSS Hazcard.
Sugar (sucrose), C12H22O11(s)- see CLEAPSS Hazcard.
Zinc chloride, ZnCl2(s) (CORROSIVE, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard.
Potassium iodide, KI(s) - see CLEAPSS Hazcard.
Sulfur, S8(s) - see CLEAPSS Hazcard. Sulfur is a non-metallic element and is a good substance to have included in the list. But there is a strong likelihood of it catching fire, with sulfur dioxide, SO2(g), (TOXIC), given off. Sulfur fires are hard to extinguish. If it happens, cover the vessel with a damp cloth and leave in place until cool. If there is time, sulfur can be done as a teacher demonstration. Heat a small sample of ‘flowers of sulfur’ very, very slowly. Sulfur is a very poor conductor of heat, and localised heating is likely to cause it to start burning! You must use a fume cupboard.
1 The carbon electrodes need to be fixed in some sort of support – such as a polythene holder or large rubber bung – so that there is no possibility of the electrodes being allowed to short-circuit. The electrodes need to be fixed in such a way as to fit inside the crucible supplied.
2 A light bulb has more visual impact, but an ammeter can be used instead.
a Set up the circuit as shown in the diagram, at this stage do not include the crucible or bunsen burner flame (these are for later).
b Select one of the metals, and by holding the electrodes in contact with it, find out whether or not it conducts electricity then switch the current off.
d Select one of the solids contained in a crucible. Lower the electrodes so that they are well immersed in the solid, and then clamp the electrodes in position.
e Switch on the current and find out whether the solid conducts electricity or not, then switch the current off again.
f Set the crucible over a Bunsen burner on a pipe clay triangle and tripod, and clamp the electrodes in position over the crucible. Gently heat the sample until it just melts, and then turn off the Bunsen flame. If necessary lower the electrodes into the molten substance, before clamping them again.
g Switch on the current again. Does the molten substance conduct electricity now? (then switch the current off again)
h Write up all your observations.
i Raise the electrodes from the crucible, and allow them to cool.
j Clean the electrodes with emery paper.
k Repeat steps d to j with some or all of the other solids.
l Pool your results with other groups so that your table is complete.
The covalent solids only need to be heated for a short time for melting to take place. Under no circumstances should heating be prolonged, otherwise the substances may decompose and/or burn. The students should be warned about what to do if this happens eg cover with a damp cloth. The experiments should be done in a well-ventilated laboratory.
It may be helpful to reserve a crucible for each of the powdered compounds, while having one or two others that can be heated. Once a solid has been liquefied and allowed to cool, the solidified lump is often hard to break up or powder in the crucible.
Zinc chloride melts at about 285 °C, so heating needs to be fairly prolonged in comparison with the covalent solids. It will, however, produce chlorine (TOXIC) so heating should stop as soon as conductivity is detected. Potassium iodide melts at about 675 °C, so very strong and prolonged heating is needed here.
Student questions and answers
Here are some questions for your students, with answers.
1 What do you conclude about the electrical conductivity of metals? (All the metals conduct electricity well. You should explain this conductivity in terms of the ‘free’ electrons within a metallic structure.)
2 Do all of the solid compounds conduct electricity? (No, none of them.)
3 Do any of the molten compounds conduct electricity. If so, which ones? (Yes, zinc chloride and potassium iodide.)
4 Why do some substances conduct only when they have been liquefied? (Some substances are ionic, but electrical conduction is only possible when the ions are free and mobile. This happens once the solid has been melted.)
5 Can you now classify all the compounds as being either ionic or covalent? (Phenylsalicylate, polythene, wax and sugar are covalent. Zinc chloride and potassium iodide are ionic.)
Health & Safety checked, 2016
This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.
© Nuffield Foundation and the Royal Society of Chemistry
Page last updated October 2015
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This is a resource from the Practical Chemistry project, developed by the Nuffield Foundation and the Royal Society of Chemistry. This collection of over 200 practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Practical Chemistry activities accompany Practical Physics and Practical Biology.