AlloysOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is an alloy?
An alloy is a mixture of at least two elements, at least one of which is a metal. Alloys are not compounds - the elements in an alloy are mixed together, not chemically bonded in fixed ratios, so they can still be separated by physical means, like any other mixture.
Section 2
Why are alloys made instead of using pure metals?
Pure metals are often too soft or too weak for many practical uses, because their atoms are arranged in uniform layers that can slide over each other easily. Alloying introduces atoms of a different size into the structure, which distorts the regular layers of metal atoms, making it harder for the layers to slide over one another. As a result, alloys often have properties different from the metals they contain - typically making them harder, stronger or more resistant to corrosion - which is why alloys are frequently more useful than pure metals for everyday and industrial applications.
Think of a pure metal as a stack of identical smooth marbles that can slide over each other easily; adding differently sized atoms is like scattering some larger or smaller balls into the stack, which jams up the sliding.
Section 3
What are steels?
Steels are an important family of alloys formed from iron mixed with carbon, and often with other metals as well. Because the exact composition can be varied, different steels have different properties suited to different uses, such as construction, tools, or cutlery.
Section 4
How does alloying change a metal's properties?
| Property | Pure metal | Typical alloy |
|---|---|---|
| Layer structure | Regular, uniform layers | Layers distorted by different-sized atoms |
| Hardness/strength | Often soft, layers slide easily | Usually harder and stronger |
| Usefulness | Often too soft for many applications | Better suited to demanding applications |
This is a direct consequence of atomic structure: alloying disrupts the neat rows of atoms found in a pure metal, making it much harder for one layer of atoms to slide past another.
Do not describe an alloy as a compound - alloys are mixtures, since the metals and other elements involved are not chemically combined in a fixed ratio.
Must Know
- An alloy is a mixture of at least two elements, at least one of which is a metal.
- Alloys are mixtures, not compounds.
- Alloying often gives properties different from (usually harder/stronger than) the pure metals involved, making alloys more useful.
- This happens because atoms of different sizes distort the regular layers of a pure metal, making it harder for layers to slide.
- Steels are alloys of iron with carbon, and sometimes other metals.
That's the notes covered.
Carry on to the next subtopic.