Salt – Sodium Chloride, NaCl
Do you like a little salt on your food?
Even if you don’t add it yourself, just look at the labels on the packets and tins of food at
home. Salt is added to many foods to bring out the flavour.
It can also help to preserve food. Early explorers, like Captain Cook, took meat rubbed in salt
on their long voyages.
In hot countries salt is extracted from sea-water. The sea-water is left in huge shallow lakes.
Then the Sun’s energy evaporates the water.
Salt can also be extracted from underground, where it is found in solid layers between rock.
The salt and rock are dug up together and then they are separated. You will try a simple
version of this process in a lesson.
Extraction of sodium metal
Reactive metals are extracted from their compounds by electrolysis. Sodium is a very reactive
metal.
We extract sodium from molten sodium chloride. The molten salt is electrolysed. Here are the
products of this process:
At the cathode (-)
At the anode (+)
Sodium metal, Na
Chlorine gas, Cl
As sodium has a low melting point, it is
formed as a liquid (Na(l)).
Uses of sodium
Street lamps – the yellow glow coming from
street lamps is produced by a small amount
of sodium vapour inside the bulbs
Nuclear reactors – liquid sodium metal is
pumped around nuclear reactors to cool them
down.
It is important to keep the chlorine gas away
from the sodium metal in this process,
otherwise they will react together to form
NaCl again!
Uses of chlorine
Chlorine has many uses – here are just 3:
Bleach – to kill bacteria and to whiten paper
Water – killing bacteria in drinking water and
in swimming pools
Solvents – dry-cleaning clothes plus many
other industrial uses
Electrolysis of brine (salt solution)
You have carried out the electrolysis of brine (NaCl(aq)) in a previous lesson. Chlorine gas
(Cl2(g)) is given off at the anode (+) and hydrogen gas (H2(g)) is given off at the cathode (-).
Sodium hydroxide (NaOH), a very strong alkali, is left behind in the water.
In special factories this process is carried out on a massive scale. All of the products are used
for our benefit.
Uses of hydrogen
Making margarine – margarines are made
from vegetable oils. The oils are liquids that
are too runny to be spread onto bread. When
hydrogen is reacted with the oils they
become thicker. Chemists add just the right
amount of hydrogen to make soft, but not
solid, margarine.
Fuel – hydrogen is used to fuel space
rockets. It is also hoped it will provide a safe,
clean and renewable fuel for the future.
Uses of sodium hydroxide
This is the most important of the products
from salt
It is used to make detergents, paper, bleach,
rayon, acetate, extracting aluminium metal,
making many new chemicals