Density: a test you can measure
Meteorites are usually denser than Earth rocks of the same size. In this lesson you will calculate density, measure volume by displacement and use the results as evidence.
- Use density = mass ÷ volume.
- Measure the volume of an irregular object by displacement.
- Use density to compare different kinds of meteorite with Earth rocks.
Links to: KS3 physics, matter: density and the particle model; working scientifically: measurement, accuracy and evaluating evidence.
What density means
Density is how much mass is packed into each unit of volume. Density = mass ÷ volume. With mass in grams and volume in cubic centimetres, density is in g/cm³.
Water has a density of 1.0 g/cm³. Common Earth rocks such as sandstone and granite are about 2.3 to 2.8 g/cm³. Ordinary chondrites, which contain metal, are about 3.2 to 3.6 g/cm³. Iron meteorites are about 7.5 to 8 g/cm³, close to steel. Density is why meteorites feel heavy for their size.
Measuring volume by displacement
A rock has an irregular shape, so you cannot measure its volume with a ruler. Instead, part-fill a measuring cylinder with water, read the volume, gently lower the rock in on a thread and read the new volume. The rise in water level equals the rock's volume.
For larger objects, use a displacement can: fill it to the spout, lower the object in and collect the water that overflows. Read each volume at eye level, at the bottom of the curved surface.
What density can and cannot tell you
Density is a powerful clue, but it is not proof. Iron ore, magnetite and slag can also be dense. A density measurement rules things out well: a 'meteorite' with a density of 2.5 g/cm³ is almost certainly an ordinary Earth rock. Scientists combine density with other evidence, such as nickel content, minerals and texture, before deciding.
Calculating a density
A suspected meteorite has a mass of 455 g. Lowering it into a measuring cylinder raises the water level from 200 cm³ to 330 cm³. Calculate its density. Could it be an ordinary chondrite?
- Volume = 330 − 200 = 130 cm³.
- Density = mass ÷ volume = 455 ÷ 130 = 3.5 g/cm³.
- Compare: ordinary chondrites are about 3.2 to 3.6 g/cm³; common Earth rocks are about 2.3 to 2.8 g/cm³; iron meteorites are about 7.5 to 8 g/cm³.
Answer: 3.5 g/cm³, which fits an ordinary chondrite and is too dense for most common Earth rocks. It is worth further tests, but not yet proof.
Measure the density of three different objects by displacement, such as a pebble, a steel bolt and a piece of brick. Repeat each measurement three times and find the mean. Present your results in a table, then compare them with the meteorite densities in this lesson. Which reading was hardest to make accurately, and why?
Quiz
5 questions. Choose an answer to see whether you are right and why. Finish the quiz to complete the lesson.
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Question 1What is the formula for density?
Density is mass per unit volume.
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Question 2A rock has a mass of 270 g and a volume of 100 cm³. What is its density?
270 ÷ 100 = 2.7 g/cm³, typical of an Earth rock such as granite.
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Question 3Which has the highest density?
Iron meteorites are about 7.5 to 8 g/cm³, more than twice the density of most rocks.
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Question 4How do you measure the volume of an irregular rock?
The rise in water level, or the water that overflows a displacement can, equals the rock's volume.
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Question 5A 'suspected meteorite' has a density of 2.5 g/cm³. What is the best conclusion?
Most meteorites are denser than about 3 g/cm³ because they contain metal or dense minerals. Only a few rare carbon-rich meteorites are as light as this.