Electrolysis

Passing an electric current through a liquid is called ELECTROLYSIS. We are going to look carefully at what happens with two liquids, water and copper sulphate, but first the general ideas.

The electric current enters the liquid at the positive plate (called the anode) and leaves it at the negative plate (called the cathode).

In liquids the current is carried by ions. Ions are charged particles (atoms or groups of atoms). Ions can either be positive or negative, the positive ions being attracted to the negative plate and the negative ions to the positive plate. A positive ion is a particle with some negative charge taken away and a negative ion is a particle with some extra negative charge added.

Some liquids conduct electricity – these contain ions and some do not – these do not contain ions. Some conduct well, they have many ions per cubic metre, others do not – they have fewer ions per cubic metre. The following list separates some common liquids into conductors and non-conductors.

Conducting liquids

Dilute sulphuric acid, lemon juice, vinegar, copper sulphate solution, tap water

Non-conducting liquids

Paraffin, cooking oil, meths, distilled water (de-ionised),
(You can find an experiment to investigate this at: Conduction of electricity in liquids experiment)

Copper sulphate solution

If an electric current is passed through pure copper sulphate solution the following things can be seen:
(a) the cathode (negative electrode) has copper deposited on it;
(b) the anode (positive electrode) slowly dissolves away;
(c) the bigger the current the more copper is deposited;
(d) the longer the experiment goes on the more copper is deposited.

You can easily explain these effects: copper is dissolving from the anode and positive copper ions are being carried through the liquid (called the electrolyte) to the cathode.

Since the amount of copper deposited depends on the current and the time, it is likely that it is the CHARGE that is important and this is actually the fact. The more charge that moves through the liquid the more copper you will get.

If you did an experiment passing a current of 0.5 A after 1200 s (20 minutes) you are likely to get about 0.25 g of copper deposited. In fact to get 1 kg of copper you would need a current of about 5 A flowing for nearly nine months

If you got in 0.25 gm in 20 minutes you would get 0.5 gm in 40 minutes and so on. Also if you double the current you will double the amount of copper in a certain time. So to get a lot of copper you need a large current flowing for a long time.

Electrolysis of water

If an electric current is passed through water to which a few drops of concentrated acid have been added, bubbles of gas are given off at both electrodes. If the gases are collected and tested it is found that:

This shows you the polarity of the ions of hydrogen and oxygen in solution. Twice as much hydrogen is given off as there is oxygen, this agrees with the chemical formula for water H₂O.

Uses of electrolysis

Electrolysis has many uses in industry and we can only look at a few here rather briefly.
1. Electroplating - this means coating one metal with another by electrolysis, e.g. silver plating of cutlery, jewelry and sports cups; chromium plating of car bumpers, kettles and taps; musical instruments such as flutes are silver plated to prevent rusting by saliva; iron is zinc plated (galvanised) for use as corrugated sheeting or nails; the stamper used in making CDs is made of nickel, plated on an aluminium former; sweet wrappers and bottle tops are plated for appearance and hygiene.

2. Extraction and purification of some metals:
Copper is refined by electrolysis and aluminium is extracted from its ore by this method.

Changing the conductivity of a liquid

The conductivity of a liquid can be changed by adding some ions. This is usually done by adding an impurity to the liquid. Distilled water contains no ions and so will not conduct electricity but if you slowly add salt to it the salt dissolves and the charged sodium and chloride ions in the salt will move through the water so conducting electricity. The size of the current can be used to measure the concentration of the salt solution. (See the Salt meter in the Foundation experiments section).

Even rubbing your hands together in deionised water is sufficient to produce a small amount of impurity and so allowing the water to conduct electricity.