Chemistry
Properties of metals and alloys Learning objectives: • identify metal elements and metal alloys • describe the purpose of a lead-tin alloy • explain why alloys are harder than pure metals due to the distortion of the layers of atoms.
KEY WORDS alloy distort ductile malleable
Making alloys has been carried out for centuries. That early human history is divided into the Stone Age, the Bronze Age and then the Iron Age shows how important bronze was. Bronze was the first metal alloy – copper and tin were both extracted from their ores in rocks thousands of years ago, and later the mixture of these metals was used to make bronze.
Metals in alloys Pure copper, gold, iron and aluminium are too soft for many uses. These metals are mixed with other metals to make alloys. Most metals we use every day are alloys.
Figure 2.35 A bronze shield from the Bronze Age of the Greek Empire. Objects, tools and weapons were made from bronze as it is harder than either tin or copper.
An alloy is a mixture of a metal element with another element. Alloy
bronze
brass
steel
Elements in alloy
copper and tin
copper and zinc
iron and carbon
Alloys have different properties from the metals that are in them. Most metals have high melting and boiling points. Sometimes a metal is needed that has a lower melting point that can then solidify to make a join. This metal alloy is called solder. Solders are made of lead and tin. Solder has a melting point that is lower than either lead or tin. Metal
lead
tin
solder
Melting point °C
327
232
183
1
What is the difference between the composition of the alloys brass and bronze?
2
Suggest some advantages of using the alloy steel rather than iron.
3
’Alloys never contain non-metals.’ Show whether this statement is true.
Giant structures In pure metals, atoms are arranged in layers. Pure metals are too soft for many uses and so are mixed with other metals to make alloys which are harder.
76
Figure 2.36 Solder is an alloy with a lower melting point than tin or lead. If melted it can be used to make or mend metal objects or wiring.
AQA GCSE Chemistry for Combined Science: Trilogy: Student Book
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5/24/16 8:52 PM