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Revise Extracting Substances with Electrolysis

Worksheet Overview

QUESTION 1 of 10

Aluminium is an incredibly useful metal, because it is light and strong. In nature, we find it as aluminium oxide in rocks, and for a long time it was hard to extract aluminium metal from the oxide. Electrolysis of molten aluminium oxide is the way we can do this.

We cannot do electrolysis on solids, and aluminium oxide does not dissolve in water. This means that we have to melt aluminium oxide for electrolysis to happen. The melting point of pure aluminium oxide is very high (over 2000 ºC), but adding another mineral, called cryolite, reduces the melting temperature to about 1000 ºC. This is helpful, because it reduces the cost of the process.

Graphite electrodes are used, made of pure carbon. These are placed in the molten aluminium oxide/cryolite mixture. Then electrolysis starts.

Aluminium ions are attracted to the cathode, where they form aluminium atoms. The molten metal sinks to the bottom of the vessel, where it falls out and is collected.

Oxide anions are attracted to the anode, where they form oxygen. The oxygen reacts with the carbon electrodes, making carbon dioxide. This gradually destroys the electrodes, so they have to be regularly replaced.

Electrolysis of aluminium oxide is expensive, for several reasons.The main ones are

the cost of heating the aluminium oxide

the cost of electricity for the electrolysis

Because of this, aluminium is expensive, which is why it is so important to recycle aluminium.

Which of these are useful properties of aluminium?

light

magnetic

strong

powerful

How is aluminium found in nature?

As lumps of metal

As aluminium nitrate

As aluminium oxide

How do we extract aluminium from its oxide?

electrolysis

heating with carbon

filtration

What is the state of aluminium oxide while it undergoes electrolysis?

solid

dissolved in water

molten

Why do we add cryolite to the aluminium oxide? 

solid

dissolved in water

molten

What element are the electrodes made from?

What happens at the cathode? Use these words to complete the gaps.

aluminium

atoms

attracts

cations

electrons

What happens at the anode? Use these words to complete the gaps.

anions

atoms

attracts

from

oxide

Why do the electrodes need to be replaced frequetly?

The break off at high temperatures.

They slowly melt at high temperatures.

They react with oxygen to make carbon dioxide.

Why is aluminium expensive?

Aluminium oxide is very rare.

Electrolysis uses a lot of energy.

Aluminium is expensive to transport

  • Question 1

Which of these are useful properties of aluminium?

CORRECT ANSWER
light
strong
EDDIE SAYS
Aluminium is quite strong, but the really useful property is how light it is. Airplanes are made of aluminium, because heavier metals might not get off the ground at all.
  • Question 2

How is aluminium found in nature?

CORRECT ANSWER
As aluminium oxide
EDDIE SAYS
Aluminium is quite a reactive metal, so we don't find aluminium metal in nature; it's usually aluminium oxide. That means we need to separate the aluminium from the oxygen to get the useful metal.
  • Question 3

How do we extract aluminium from its oxide?

CORRECT ANSWER
electrolysis
EDDIE SAYS
We can extract iron from iron oxide by heating the oxide with carbon, but this doesn't work for aluminium. Filtration doesn't work, because nothing dissolves.
  • Question 4

What is the state of aluminium oxide while it undergoes electrolysis?

CORRECT ANSWER
molten
EDDIE SAYS
We can't do electrolysis on a solid substance; the ions need to be able to move. Aluminium oxide doesn't dissolve in water, so we have to melt it. This is expensive, but aluminium is so useful that it's worth doing.
  • Question 5

Why do we add cryolite to the aluminium oxide? 

CORRECT ANSWER
EDDIE SAYS
Heating large amounts of rock to 2000 ºC is expensive, so anything we can do to reduce this temperature is worthwhile. Mixtures tend to have lower melting temperatures than pure substances; it's the same reason we put salt on roads to stop water turning into ice.
  • Question 6

What element are the electrodes made from?

CORRECT ANSWER
carbon
C
EDDIE SAYS
The electrodes are made of graphite, which is a form of the element carbon. The important thing is that the electrodes need a very high melting point, so they don't melt in the molten aluminium oxide.
  • Question 7

What happens at the cathode? Use these words to complete the gaps.

aluminium

atoms

attracts

cations

electrons

CORRECT ANSWER
EDDIE SAYS
The basic idea is that cations go to the cathode, as for any other electrolysis. They then get converted back into atoms by adding electrons.
  • Question 8

What happens at the anode? Use these words to complete the gaps.

anions

atoms

attracts

from

oxide

CORRECT ANSWER
EDDIE SAYS
What happens at the anode is also pretty standard for electrolysis. The anions are attracted to the anode, where they lose electrons and are turned back into atoms.
  • Question 9

Why do the electrodes need to be replaced frequetly?

CORRECT ANSWER
They react with oxygen to make carbon dioxide.
EDDIE SAYS
Graphite has a high melting temperature. The big problem is that electrolysis of aluminium oxide releases lots of oxygen, which then reacts with the carbon in the graphite electrodes.
  • Question 10

Why is aluminium expensive?

CORRECT ANSWER
Electrolysis uses a lot of energy.
EDDIE SAYS
We need quite a lot of electrical energy to make electrolysis work. The biggest problem is the need to melt aluminium oxide, which needs huge amounts of energy. It\'s much better to recycle things like aluminium cans than extract more aluminium from rocks.
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