Earth’s crust versus the prep. room – why the differences?:

teachers’ notes

Level

This activity is designed for students aged 14-16, although it could be used with 11-14 year-olds.

Topic

The aim of this ‘starter’ activity is to introduce students to the common elements found in the Earth. Many of the elements already familiar

to students may not necessarily be at all common in the Earth’s crust. Students should consider why this might be. The activity can thus be

used to underpin topics on useful materials from the Earth and on the extraction of metals. Some teachers may wish to construct their own

sets of questions based on the data in the tables to adapt the activity to a particular age or ability range.

Description

Students study, compare and answer questions on graphs showing the elemental composition of the Earth’s crust and of the aggregate

composition of the compounds found in a typical school prep. room.

Context

Pupils will need to be familiar with the idea of an element and know that elements are represented by symbols. Ideally they should have

some ideas about the properties of different groups of elements and compounds, for example that virtually all alkali metal compounds are

soluble in water as are nitrates and many sulfates.

Teaching points

The suite of chemicals studied by chemists is different from the suite commonly considered by Earth scientists.

The extent to which students can appreciate the following teaching points will depend on their ability and prior knowledge of chemistry. The

data are easily adaptable to suit a variety of abilities and age ranges.

The chemicals found in the prep. room have, in general, been chosen to illustrate chemical reactivity and the usefulness of substances.

However, the chemicals found in the Earth’s crust are found there because, in general, they are chemically unreactive - silicates, for example.

Many of the elements that are commonly used in chemical reactions (ie in the prep. room) are not common at all in the Earth’s crust.

However, silicon, the second most common element in the Earth’s crust, is very rarely found in chemical reactions and so is not present in

the prep. room.

There are two other factors to consider.

One is solubility in water. In general, very soluble compounds will be found dissolved in the oceans rather than as solids in the Earth’s crust.

Secondly volatility is important. Compounds composed of small molecules tend to be volatile and therefore exist as gases in the atmosphere

rather than as solids in the Earth’s crust.

Timing

This activity could be done in about 10–15 minutes, perhaps at the start of a lesson. Alternatively, it could be given more lesson time if

considered in depth or if students plot the graphs for themselves.

The activity could also be set as a homework exercise or used in case of teacher absence.

Activity

Each group of students will need the Student’s worksheet including a copy of the graphs Composition of the Earth’s crust, Composition’ of

the prep. room, Composition of the Earth’s oceans and Composition of the Earth’s atmosphere.

Alternatively, if the students are to plot the graphs themselves, students will need a copy of the data table (Table 1), either on paper or

electronically (this is available to download as an Excel file). If they are to plot the graphs electronically, they will need access to a computer

with suitable spreadsheet software.

Extensions

Students can also compare the ‘composition of the prep. room’ with the compositions of the oceans and of the atmosphere.

Some teachers may wish to supply the composition data either on paper or electronically and ask the students to plot the graphs

themselves either by hand or using a spreadsheet thus incorporating an ICT exercise into the activity. In this case different types of graphs

or charts could be tried and discussed.

Table 1 Amounts of elements found in compounds in a typical school prep. room, the Earth’s crust, the

oceans and the atmosphere

Answers to questions

Q 1. Oxygen

Q 2. Silicon. Its compounds (allow ‘it’) are unreactive and so they remain in the crust. Also it is not used in the laboratory to illustrate

any common reactions (allow ‘not that many uses to have it in the lab.’).

Q 3. They are present in very low percentages in crust but are quite common in the prep. room. Sodium and potassium compounds are

soluble and will not remain as solids in the crust. They do, however, form compounds that illustrate many chemical reactions in the

laboratory.

Q 4. Aluminium and iron. There should be no problem about using them for very many years to come. Suitable uses include aluminium

in aircraft manufacture, drinks cans, window frames etc, iron in bridges etc.

Q 5. Copper, zinc and lead illustrate many important chemical reactions (allow ‘they are useful metals’). The graph shows that these are

present in the crust in very small amounts and so the long term future for these metals is not secure. (See also question 8.)

Q 6. Nitrates and many sulfates are soluble in water and so will not remain as solids in the crust.

Q 7. The Earth’s crust is almost entirely inorganic, and so only inorganic chemicals were considered in the prep. room survey. The

organic material in the Earth’s crust is connected to the biosphere and, apart from fossil fuels, is mostly confined to the surface.

Q 8. To obtain the chemicals common in the prep. room but which are unusual in the Earth’s crust, there first needs to be a natural

concentration process to give places in the Earth where these substances are reasonably abundant. Then they need to be extracted (by

mining, quarrying, pumping etc) and often then need to be concentrated further by a range of chemical and physical methods (eg. density

methods, smelting, electrolysis, etc). Only then can we begin to think about the extraction processes used to get individual elements such

as metals etc.

Q 9. See the graphs given for the Earth’s atmosphere. The atmosphere is largely nitrogen and very little of anything else but oxygen

(the ‘other’ is argon; the amount of carbon in carbon dioxide and methane doesn’t come on to the scale).

Q 10. See the graphs given for the Earth’s oceans. The data for the oceans does not include the hydrogen and oxygen of the sea water,

as this would overwhelm the other figures. Apart from that, the oceans contain a hugely greater percentage of chlorine than the crust and

much less silicon, aluminium and iron.

Notes

The ‘composition’ of the prep. room was calculated from the recommendations of the Royal Society on the resources needed for

teaching science for 11-16 year olds (Royal Society, 1997). The elements in each inorganic chemical listed in the recommendations were

recorded and summed. The ‘others’ category includes bromine, iodine, bismuth, mercury, silver and tin.

Some teachers might prefer to survey their own prep. room rather than use the Royal Society figures. To do this, note down all the

major inorganic chemicals found there, then record which elements are found in them (eg calcium carbonate contains calcium, carbon

and oxygen) and then add up all the times the element is found. Finally, use these figures to plot the graph of your own prep. room.

The abundance of elements in the crust, atmosphere and ocean were taken from Mason (1966).

For those interested in the figures used to plot the graphs, they are given in Table 1.

References

B Mason, Principles of geochemistry, London: Wiley, 1966

Royal Society Science teaching resources: 11-16 year olds, London: Royal Society, 1997 (Available at

www.royalsoc.ac.uk/files/statfiles/document-131.pdf, accessed January 2003)