Alloys
In ancient history people realised that mixtures of metals, called alloys, were better to use than individual metals alone. For example, the alloy of copper and tin, called bronze, was found to be harder than copper and so it was more suitable for making weapons.
Alloys are the most common way in which metals are used today. They can be made of any amount of a metal mixed with one or more other metals, or even non-metals such as carbon. This means that alloys do not have a constant composition or chemical formula so they are classified as mixtures.
We make alloys by melting different metals together and allowing the melted mixture to cool to give the solid alloy. The importance and usefulness of alloys lies in the fact that their properties, e.g. hardness, differ from those of the individual metals in them.
The composition and uses of some common alloys is given below
Table 1: Common Alloys
Name of alloy / Composition / Uses / Special PropertiesStainless Steel / 80% iron
18% chromium
2% nickel / Cutlery, sinks, machinery / Strong and resists corrosion
Carbon Steel / 99% iron
1% carbon / Bridges, scissors, car bodies / Hard
Brass / 65% copper
35% zinc / Musical instruments, decorative plating / Hard, resists corrosion, attractive appearance
Solder / 63% tin
37% lead / Joining metal joints / Low melting point, easily worked
Bronze / 85% copper
15% tin / Statues, medals (e.g. Olympic games, Victoria Cross) / Hard, resists corrosion, attractive appearance
Note: The percentages of metals in the alloys in the table above are simple examples. In fact, a wider range of compositions is used. For example an alloy can consist of anything between 80 – 90% copper and 10 – 20% tin and still be called bronze.
Structure of alloys
The properties of metals and alloys are based on observations. To understand the high melting points, hardness, strength and electrical conductivity of metals and alloys their structure at the atomic level must be considered:
· The model that chemists have of metals is of giant lattice structures of metal ions.
· These atoms are surrounded by a “sea” of mobile electrons.
· The atoms are held together by strong metallic bonds.
· Alloys are believed to have a similar structure, since they have similar properties to pure metals. Of course in an alloy there are different atoms through the lattice as shown below:
Figure 1 – Structure of Alloys
Note: Because alloys are a mixture, they are difficult to recycle, collect and reuse. Since the metals in an alloy are completely mixed together, it is not easy to separate the metals for individual reuse. So, unless the alloy itself is recycled, it is expensive to obtain pure metals for reuse.
Figure 2: Bronze Statue of The Little Mermaid[1]
Activities
1. Identify a reason why alloys are classified as mixtures.
2. Explain why ancient peoples began to use alloys such as bronze rather than pure metals such as copper.
3. Outline how alloys are made.
4. Use Table 1 to answer the following questions:
a. Identify the alloy made of tine and lead.
b. Identify the alloy used to make statues and medals.
c. Identify the alloy which contains a non-metal.
d. Outline why solder is so useful.
5. Suggest why some music groups are called “Brass Bands”.
6. Describe the model that chemists have for the structure of alloys at an atomic level.
7. Identify the difficulty involved in recycling pure metals form alloys.
8. Answer true or false to the following statements, justifying your answers.
a. Most alloys contain at lest two metals.
b. Alloys are chemical compounds.
c. Bronze is made of copper and iron.
d. Some alloys contain a non-metal.
e. All steels contain iron.
f. Solder has a low melting point.
g. Brass is made of copper and zinc.
h. Statues are often made of bronze.
9. Answer true or false to the following statements, justifying your answers.
a. Carbon steel is mainly carbon with a little bit of iron.
b. The forces holding metals together are called ionic bonds.
c. Alloys are classified as mixtures.
d. Alloys have chemical formulas.
e. Bronze is harder than copper.
f. Knives, forks and spoons are usually made from stainless steel.
g. Alloys have only been known and used for the last 200 years.
h. Trumpets are in an orchestra made of brass.
10. Answer true or false to the following statements, justifying your answers.
a. Alloys occur in rocks.
b. Metals can be joined by soldering.
c. Solder is another name for tin.
d. Metallic bonds are usually very weak.
e. In an alloy there is only one kind of atom.
f. An alloy can consist of only one type of element.
g. An alloy can be a mixture of only two non-metals.
h. Most alloys have low melting points.
11. An alloy mad from lead and tin is:
a. brass
b. bronze
c. solder
d. stainless steel.
12. Carbon steel is used to make:
a. musical instruments
b. medals
c. statues
d. scissors
13. The alloy which contains a non-metal is:
a. stainless steel
b. carbon steel
c. bronze
d. brass
14. The alloy which usually contains three or more metals is:
a. bronze
b. brass
c. stainless steel
d. carbon steel
15. The alloy used to make the third place medals in the Olympic games is:
a. brass
b. bronze
c. solder
d. stainless steel
16. Which of the following is most likely to be made of an alloy?
a. wedding ring
b. electrical wiring
c. coin
d. wood
17. Which of the following alloys has the lowest melting point?
a. bronze
b. stainless steel
c. brass
d. solder
18. What are the forces called which hold alloys together?
a. ionic bonds
b. metallic bonds
c. covalent bonds
d. dispersion forces
19. Copy and complete the following:
a. An alloy is usually a ______of metals.
b. The main metal in steel alloys is ______.
c. The bonds that hold elements like copper together are called ______bonds.
d. Alloys are useful because they have different ______from the metals in them.
e. The first alloy believed to be used by ancient people was ______.
f. Alloys and metals usually have (high/low) melting points.
[1] http://www.nd.edu/~ndband/photos/scandinavia/page_09.htm