Earth’s crust versus the prep. room – why the differences?: teachers’ notes
This activity is designed for students aged 14-16, although it could be used with 11-14 year-olds.
English National Curriculum reference 4.3.2g
AQA modular 3468 reference 14.2
AQA linear 3462 reference 11.4
Edexcel modular 1536 reference 9.02
OCR A 1983 reference 3.2.3
OCR B 1977 reference CD3
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.
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.
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.
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.
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.
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.
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.
B Mason, Principles of geochemistry, London: Wiley, 1966
Royal Society Science teaching resources: 11-16 year olds, London: Royal Society, 1997