Alloys (AQA GCSE Chemistry): Revision Notes
Alloys
What are alloys?
The metals we use daily are usually mixtures called alloys. Pure metals are often too soft or weak for everyday use. By mixing metals together, we can make materials that are stronger and more useful.
Pure metals on their own rarely meet the demands of modern applications. For example, pure iron is too soft for construction, and pure gold would bend and scratch easily in jewellery.
Alloys have better properties than pure metals because:
- They are harder and stronger
- They resist corrosion better
- They can be designed for specific jobs
Cast iron
Cast iron is made from iron that comes from a blast furnace. This iron is about 96% pure, but it contains impurities that make it brittle (breaks easily). Even though it's brittle, cast iron is very strong when squeezed.
Uses of cast iron:
- Manhole covers
- Drain pipes
- Pillars in buildings
- Garden furniture
Cast iron is good for these jobs because it's strong in compression (when squeezed) and quite cheap to make. However, its brittleness means it's not suitable for applications where it might be bent or stretched.
Types of steel
Most iron gets turned into steel. Steel is an alloy made from iron mixed with carbon and sometimes other metals. The amount of carbon changes the steel's properties.
Key relationship: The more carbon in steel, the stronger but more brittle it becomes. This is the fundamental principle that determines which type of steel to use for different applications.
Low-carbon steel
- Contains less carbon
- Soft and easy to shape
- Used for things that need to be bent or formed
- Example: car body panels
High-carbon steel
- Contains more carbon
- Stronger but more brittle
- Used where strength is important
- Example: cutting tools
Stainless steel
- Contains chromium and nickel
- Hard and resistant to corrosion (doesn't rust)
- Used for cutlery and kitchen equipment
Gold alloys
Pure gold is very soft and would get damaged easily. That's why gold jewellery is made from gold alloys.
Gold is mixed with:
- Copper
- Silver
- Zinc
Gold purity measurements:
- 18 carat gold = 75% gold (the rest is other metals)
- 24 carat gold = 100% pure gold (very soft)
The carat system tells us how pure the gold is - the higher the number, the more pure gold it contains.
Gold alloys are much harder than pure gold, making them perfect for jewellery that lasts.
Other useful alloys
Brass = copper + zinc
- Used for water taps and door fittings
- Doesn't corrode easily
Bronze = copper + tin
- Used for statues and decorative objects
- Very durable
Duralumin = aluminium + other metals
- Much stronger than pure aluminium
- Low density (lightweight)
- Used in aircraft construction
Each of these alloys demonstrates how combining metals can create materials with exactly the right properties for specific applications - from the corrosion resistance of brass to the strength-to-weight ratio of duralumin in aircraft.
Why use alloys?
Alloys solve the problems of pure metals:
- Pure metals are too soft → Alloys are harder
- Pure metals are too weak → Alloys are stronger
- Pure metals corrode easily → Some alloys resist corrosion
- Pure metals are expensive → Some alloys are cheaper
The other metals in an alloy make it much more useful than the pure metal on its own.
Key Points to Remember:
- Alloys are mixtures of metals that are stronger and more useful than pure metals
- Cast iron is brittle but strong in compression
- Steel types depend on carbon content - more carbon means stronger but more brittle
- Gold alloys are harder than pure gold, making better jewellery
- Alloys can be designed for specific jobs by choosing the right metals to mix