Alloys – SS3 Chemistry Lesson Note (First Term)

An alloy is a mixture of a metal with one or more other metals, or with a non-metal such as carbon, made by melting them together. Bronze, brass and steel are common alloys used for coins, instruments, spoons and machines.

Behavioural objectives

1. State the properties of metals.
2. Define an alloy and give reasons for making alloys.
3. State the components of bronze, brass, steel, stainless steel and solder.
4. List the uses of bronze, steel, brass and solder.
5. Describe aluminium, nickel, stainless steel and gold alloys.

What are metals?

A metal is a shiny substance that conducts heat and electricity and can be beaten into shape.

• Examples: iron, copper, zinc, aluminium, tin and gold.
• Use in alloys: metals are often too soft or too weak when pure, so they are mixed to improve them.

What are the properties of metals?

The main properties of metals are conductivity, malleability, ductility, lustre, fusibility and a characteristic colour.

• Conductivity: most metals let heat and electricity flow through them easily.
• Malleability: metals can be beaten or rolled into sheets without breaking.
• Ductility: metals can be drawn into wires, as with copper.
• Lustre: metals shine when their surface is clean and polished.
• Fusibility: metals can be melted and mixed with other metals.
• Colour: most metals are grey or silvery, but copper is reddish-brown and gold is yellow.
• Melting point and density: most metals have high melting points and are dense. Mercury, which is liquid at room temperature, is an exception.

What is an alloy?

An alloy is a combination of metals, or of a metal with one or more other elements.

• Nature: an alloy is a mixture, so its parts keep their own properties and are not joined in a fixed ratio.
• Composition: the amounts of each part can vary, so the same alloy may have slightly different proportions.

How is an alloy made?

1. Weigh the metals needed in the right proportions.
2. Melt the main metal in a furnace.
3. Add the other metal or element, such as carbon.
4. Stir the molten mixture so that it is even.
5. Allow the mixture to cool and set.

What is the difference between an alloy and a compound?

• Alloy: a mixture of metals, or of a metal and another element, in no fixed ratio.
• Compound: a substance in which elements are chemically joined in a fixed ratio, such as sodium chloride.

Why are alloys made?

• Strength: alloys are usually harder and stronger than the pure metals in them.
• Why harder: atoms of different sizes make it difficult for the layers of atoms to slide over each other.
• Resistance to rust: some alloys resist rusting and tarnishing better than pure metals.
• Appearance: alloys can have a better colour or shine.
• Melting point: some alloys melt at a lower temperature, which suits solder.

What are the common alloys and their components?

The common alloys are bronze, brass, steel, stainless steel and soft solder.

What are the uses of alloys?

Each common alloy is used according to its strength, appearance or melting point.

• Bronze: used for making medals and coins.
• Steel: used for building cars, bridges and machines.
• Stainless steel: used for making spoons and plates.
• Brass: used for making bolts and musical instruments.
• Solder: soft solders are used for joining metals because they melt at a low temperature.

What are some important alloys and their features?

Important alloys are named after their main metal, such as aluminium, nickel, stainless steel and gold alloys.

Aluminium alloys

• Reason for alloying: pure aluminium is soft, so it is mixed with other metals to make it stronger.
• Properties: light, good conductors, and resistant to heat and tarnish.
• Uses: car engines, aeroplane body panels and boat hulls.
• Duralumin: an aluminium alloy containing copper, magnesium and manganese, used in aircraft parts.

Nickel alloys

• Batteries: nickel and other metals react with certain chemicals to produce electricity, so they are used in batteries.
• Stainless steel: nickel is one of the metals added to iron to make stainless steel.

Stainless steel

• Steel: every steel is made from iron and carbon.
• Role of carbon: carbon makes the iron harder. Steel with more carbon is harder and more brittle.
• Role of chromium and nickel: they are added to make the steel resist rusting.
• Uses: refrigerators, sinks, forks, knives and many other household products.

Gold alloys

• Pure gold: it is very soft and malleable, so jewellery made from it bends and wears out easily.
• Gold jewellery: most is made from a gold alloy, which is harder and more durable.
• Carats: pure gold is 24 carat, so 18 carat gold has 18 parts of gold in 24 parts of the alloy.
• Gold in electronics: gold is used because it is malleable and conducts electricity well.

Evaluation

1. Define an alloy.
2. State four properties of metals.
3. Give one use each of bronze, brass and soft solder.
4. Name the metals added to iron and carbon to produce stainless steel.
5. Why is most gold jewellery made from an alloy?

Assignment

List four products made from stainless steel, and name two parts of a car or aeroplane made from aluminium alloys.