Electrolysis is electrical current flow through a liquid
which causes chemical changes.
Yes, you did hear that correctly.
Current flowing through a liquid.
Now, not all liquids can conduct
The liquid can be a molten Ionic compound
Or a aqueous solution
In either case
It must be able to carry current
Lets take a common example...
Table salt.
If you heat sodium chloride to over 801 degrees Celsius
it will melt.
The liquid will contain free flowing positive sodium ions
and negative chloride ions.
This simply means
that sodium and chloride ions are free to move within
the liquid sodium chloride.
Positive ions are called cations.
Negative ions are called anions.
Now consider putting two electrical conductors into the liquid.
Copper would be good, as it has a higher melting point.
Nothing happens to the ions at this point.
But if we connect the two bits of copper
to an electrical cell or a power supply
then things start to get interesting.
We do need to side step for a moment to look at electrical current in a metal
From our lesson "behavior of metallic structures"
we know that electrons in a metal aren't bound to particular atoms.
They form a sea of electrons that can move easily
if a potential difference or a voltage is applied across it.
This could be from an electrical cell
or a battery.
The electrons will begin to drift towards the positive terminal of the battery
and this is what happens when currents flow.
Current is the movement of charge.
In this case, electrons.
Now, let's go back to our "molten sodium chloride".
We've added our copper wires,
let's call them "electrodes",
and now we'll connect these to an electrical cell.
The electrons will start to flow in the wires
and this will cause one electrode to become positively charged,
and the other negatively charged.
This has an immediate knock on effect in the molten sodium chloride,
which, you'll remember, is a mixture of sodium cations
and chloride anions.
Anything positively charged in a liquid, in this case the sodium ions,
is going to be attracted to the negative electrode.
And anything negatively charged,
in this case the chloride ions,
is going to be attracted to the positive electrode.
This is because opposite electrical charges attract.
When the ions meet the electrodes,
electron exchange happens,
and this triggers a chemical reaction.
Solids can form, gases can be given off,
and new solutions can be left behind.
It all depends on the liquid
and what material the electrodes are made from.
There is more detail about this process in our lesson
"Interpreting electrode equations".
There are special names for the electrodes
which are usually metal plates
with a larger surface area than a piece of wire.
The positive electrode is called the anode,
it attracts anions.
And the negative electrode is called the cathode,
it attracts cations.
The liquid is effectively conducting the electrical current.
The only difference is that the charge carries in the liquid.
In other words, the charge particles that are moving are ions.
If a liquid has free moving ions and can conduct like this,
it is called an electrolyte.
Remember that electrolysis can also take place in ionic solutions
as well as molten compounds.
The more concentrated that solution,
the greater the ion flow rate.
Ion flow rate can also be increased by increasing the potential difference
or voltage across the cell.
We'll be studying several example of electrolysis in more detail in other lessons.
For the time being, though,
here is a summary of the key ideas about electrolysis.
Electrolysis is the flow of electrical current
in a conducting solution or molten liquid
that causes chemical changes.
The conducting liquid is called an electrolyte
and has free moving ions.
The conducting plates, bars or wires inserted into an electrolyte,
are called electrodes.
Positive ions are called cations and are attracted to the cathode.
Negative ions are called anions and are attracted to the anode.
When current flows in an electrolyte,
it is the ions that move and carry the electrical charge.
When current flows in the electrodes and wires,
it is the electrones that move and carry the electrical charge.