1. It makes up 0.5 percent of the Earth's crust.
   It makes up 5 percent of the Earth's crust.
2. It makes up 25 percent of the Earth's crust.

The Earth's crust is 5 percent iron, and in some areas it concentrates in ores that contain as much as 70 percent iron.

1. It can be magnetized.
   It is relatively easy to shape after heating it.
2. It is naturally resistant to rust.

Although iron can be magnetized and is easy to shape after being heated, it is susceptible to rust. But you can control rust with paint, galvanizing, chrome plating or sacrificial anodes.

1. hematite
   galena
2. chalcocite

Hematite is a common ore that contains about 70 percent iron.

1. hydrogen
   nitrogen
2. oxygen

If you look at the chemical makeup of the various iron ores, they all contain iron combined with oxygen. To make iron from iron ore, you need to eliminate the oxygen.

1. Steel is stronger than iron.
   Steel contains more impurities than iron.
2. Steel is just polished iron.

Steel is stronger than iron. Steel is iron that has most of the impurities -- such as silica, phosphorous and sulfur -- removed, making it stronger.

1. steel
   wrought iron
2. pig iron

By heating and hammering the bloom, the glassy silicates mix into the iron metal to create wrought iron. Wrought iron is tough and easy to work, making it perfect for creating tools in a blacksmith shop.

1. wrought iron
   steel
2. both of the above

You can make either wrought iron or steel out of pig iron. To create wrought iron, you melt it, mix it with slag and hammer it to eliminate most of the carbon. Steel can be made out of pig iron through the use of an open hearth furnace, a basic oxygen furnace, or using the Bessemer process.

1. slag
   coke
2. rust

Slag forms when the calcium in the limestone combines with the silicates. At the bottom of the blast furnace, liquid iron collects along with a layer of slag on top.

1. chromium
   molybdenum
2. bleach

Adding 10 percent to 30 percent chromium to steel creates stainless-steel, which is very resistant to rust. The addition of chromium and molybdenum creates chrome-moly steel, which is strong and light.

1. the 19th century
   the Middle Ages
2. Around 1000 B.C.

Iron became important around 1000 B.C. It was so important that primitive societies are measured by the point at which they learned how to refine iron and enter the Iron Age.