Elements, compounds and mixtures in meteorites
Meteorites are made of the same chemical elements as Earth, combined in the same ways. This lesson uses them to practise elements, compounds and mixtures, and how to separate them.
- Identify elements, compounds and mixtures in meteorites.
- Explain why iron sulfide behaves differently from a mixture of iron and sulfur.
- Calculate the mass of each metal in an iron meteorite from percentages.
Links to: KS3 chemistry, matter: elements, compounds and mixtures, chemical symbols and formulae, metals, and separating mixtures; maths: percentages.
The ingredients
The most common elements in stony meteorites are oxygen, iron, silicon and magnesium, followed by sulfur, calcium, aluminium and nickel. These are the same elements that make up Earth as a whole. They combine into minerals, naturally occurring compounds with a definite composition and crystal structure.
Olivine, the most common mineral in many meteorites, is a compound of magnesium, iron, silicon and oxygen. Its magnesium-rich form, forsterite, has the formula Mg₂SiO₄. Troilite is iron sulfide, FeS, a compound of iron and sulfur.
An alloy is a mixture
The metal in meteorites is not pure iron. It is an alloy of iron and nickel, usually with about 5 to 30 per cent nickel. An alloy is a mixture of a metal with other elements, not a compound: the proportions can vary and the atoms are not bonded in a fixed ratio.
The nickel is a vital clue. Natural iron metal is extremely rare on Earth's surface, and iron made by people usually contains far less nickel. High nickel is one of the tests that shows a piece of metal came from space.
Mixtures can be separated
A meteorite as a whole is a mixture of minerals and metal, so it can be separated by physical means. If a chondrite is crushed, a magnet pulls out the iron-nickel grains and leaves the silicate minerals behind.
Compare this with the classic experiment of heating iron filings with sulfur. Before heating, a magnet separates the iron from the yellow sulfur. After heating, they have reacted to make iron sulfide, a new compound that is not attracted to a magnet in the same way. Troilite in meteorites is that same compound.
How much nickel?
An iron meteorite has a mass of 2.4 kg. It is 91.5% iron, 8.0% nickel and 0.5% other elements by mass. What mass of nickel does it contain, in grams?
- Convert to grams: 2.4 kg = 2,400 g.
- 8.0% = 8.0 ÷ 100 = 0.080.
- Mass of nickel = 0.080 × 2,400 = 192 g.
- Check: iron = 0.915 × 2,400 = 2,196 g; other = 0.005 × 2,400 = 12 g; 2,196 + 192 + 12 = 2,400 g.
Answer: 192 g of nickel. The percentages add to 100%, and the masses add back to the total.
Look up iron (Fe), nickel (Ni), magnesium (Mg), silicon (Si), oxygen (O) and sulfur (S) in a periodic table. Sort them into metals and non-metals. Then classify each of these as an element, compound or mixture, with a reason: olivine, iron-nickel metal, troilite, a whole chondrite, nickel.
Quiz
5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.
-
Question 1Troilite has the formula FeS. What is it?
Iron and sulfur are chemically bonded in a fixed ratio, so troilite is a compound, iron sulfide.
-
Question 2Why is meteorite iron-nickel metal described as a mixture?
An alloy is a mixture of a metal with other elements. Meteoritic metal has varying amounts of nickel.
-
Question 3How could you separate the metal grains from a crushed chondrite?
The iron-nickel grains are magnetic and the silicate minerals are much less so.
-
Question 4Why is a high nickel content good evidence that metal came from space?
Meteoritic metal typically contains about 5 to 30% nickel, which is unusual for metal found or made on Earth.
-
Question 5A 500 g iron meteorite is 10% nickel by mass. What mass of nickel does it contain?
10% of 500 g = 0.10 × 500 = 50 g.