Conductivity demonstration
Description
A sample of de mineralised water is shown not to conduct electricity, but tap water is. What is it that might be dissolved in tap water? Alternative substances that do dissolve in tap water are added and it is seen that sugar makes no difference to the conductivity where salt does.
Requires
- A multimeter (digital voltmeter that measures resistance).
- Two electrodes – two bare wires wrapped around a plastic rod for example with a gap in between.
- Demineralised (or distilled) water.
- Tap water.
- Sugar (or glucose or similar soluble molecules).
- Table salt (or other soluble salt).
Method
Connect the digital voltmeter to the electrodes and immerse them in the demineralised water. Use the digital voltmeter to read off the resistance of the set-up. With demineralised water the resistance should be very high, but lower than 18 MW. Next immerse the electrodes in tap water. The resistance should drop considerably. Measure the resistance of the tap water again after some sugar or glucose has been added to it. There should be no significant change. On the other hand the resistance will drop markedly after table salt has been added.
Principle
One might think that because water has an uneven distribution of charge (a permanent dipole moment) that it may be able to conduct electricity. The demineralised water is very pure water and we see that, if it does conduct electricity, it only allows a tiny current to flow. On the other hand tap water is not so pure and has substances dissolved in it. This might lead to a discussion of geology, minerals, and weathering.
Sugar dissolves readily in water because it also has an uneven charge distribution. It adds nothing, however, to the conductivity of the solution as the sugar molecules act much the same as the water molecules. Table salt and other soluble salts, on the other hand, dissolve by breaking up into charged particles known as ions.
Table salt, or sodium chloride, forms crystals in the solid state. In these crystals are the same number of positively charged sodium ions and as there are negatively charged chloride ions arranged in a tightly packed regular array. When the solid dissolves the sodium ions and the chloride ions both become surrounded by water molecules. These charged particles are able to carry electrical current through water just like charged electrons carry current through wires.
Tips
Always wash the electrodes thoroughly after use to help slow corrosion. It is also a good idea to wash them in a little demineralised water prior to use as that will help the demineralised water remain uncontaminated.
Safety
Be aware that spillages should be mopped up promptly to prevent electrical equipment getting wet and to avoid slip hazards.