AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Carat being 100% (pure gold), and 18 carat being 75% gold.
Full specification wording
Alloys as useful materials: 24 carat being 100% (pure gold), and 18 carat being 75% gold.
Carat is a measure of the purity of gold in an alloy, expressed as parts in 24.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 4 of 10
Study notes
From the checked teaching notes for this specification point.
The idea
Carat is a measure of the purity of gold in an alloy, expressed as parts in 24. Pure gold is 24 carat, so it is 100% gold. An 18 carat gold alloy is 18/24 gold, which simplifies to 3/4, or 75% gold; the remaining 25% is other metals added to harden it. To convert carat to percentage, divide the carat value by 24 and multiply by 100. For example, 9 carat gold is 9 ÷ 24 × 100 = 37.5% gold. Pure gold is too soft for most jewellery, so alloying it with metals such as copper, silver or zinc makes it harder and more durable while the carat value records how much gold remains.
What earns the marks
- State that carat compares the mass of gold present with the total mass of the alloy, using a scale where 24 carat is pure gold.
- Calculate the percentage of gold from a carat value using carat ÷ 24 × 100, for example 18 ÷ 24 × 100 = 75%.
- Explain that 18 carat gold contains 75% gold and 25% other metals by mass.
- Recognise that pure 24 carat gold is too soft for many uses, so other metals are added to harden it.
- Interpret carat values as a purity scale rather than a measure of mass or volume of the whole piece.
Common mistakes and how to put them right
Treating carat as a direct percentage, so that 18 carat is read as 18% gold.
Put it right by dividing by 24 first, giving 75%.
Assuming all the non-gold mass in 18 carat gold is a single metal.
Put it right by stating that the remaining 25% may be one or more other metals.
Confusing carat with the mass of a jewellery item, for example thinking a heavier ring is purer.
Put it right by explaining that carat describes the proportion of gold, not the total mass.
Exam tips
- Show the division by 24 and the multiplication by 100 when converting carat to percentage, so the method is visible.
- Link the percentage of gold to a property, such as hardness or resistance to wear, when explaining why alloys are used.
- Check whether a question asks for a percentage, a fraction or a mass before writing the final answer.
Try the lesson
The start of the Alloys as useful materials: 24 carat being 100% (pure gold), and 18 carat being 75% gold. lesson, made from checked teaching notes for this specification point.
Have a go first
A gold ring is described as 18 carat. What percentage of the ring is pure gold?
Listen
Carat is a measure of the purity of gold in an alloy, expressed as parts in 24. Pure gold is 24 carat, so it is 100% gold. An 18 carat gold alloy is 18/24 gold, which simplifies to 3/4, or 75% gold; the remaining 25% is other metals added to harden it. To convert carat to percentage, divide the carat value by 24 and multiply by 100. For example, 9 carat gold is 9 ÷ 24 × 100 = 37.5% gold. Pure gold is too soft for most jewellery, so alloying it with metals such as copper, silver or zinc makes it harder and more durable while the carat value records how much gold remains. A strong answer earns marks for these points. State that carat compares the mass of gold present with the total mass of the alloy, using a scale where 24 carat is pure gold. Calculate the percentage of gold from a carat value using carat ÷ 24 × 100, for example 18 ÷ 24 × 100 = 75%. Explain that 18 carat gold contains 75% gold and 25% other metals by mass. Recognise that pure 24 carat gold is too soft for many uses, so other metals are added to harden it. A common slip: Treating carat as a direct percentage, so that 18 carat is read as 18% gold. Instead, dividing by 24 first, giving 75%. Exam tip: Show the division by 24 and the multiplication by 100 when converting carat to percentage, so the method is visible.
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