Writing Functions 7: Classifying

Academic writing

Rhetorical Functions in Academic Writing: Classifying & categorising

When we classify, we arrange members of a group. For example, if we take the following list:

Physics, Chemistry, Biology, French, German, Spanish.

It is quite clear that we have two different types of word. We have science subjects and languages. So it is simple to divide the list into two:

 Physics, Chemistry, Biology


 French, German, Spanish

When we are classifying, we often need to say what our classification is and how we are making it.


Read the following text.

Lavas may be divided into two contrasting types, acid and basic. Acid or siliceous lavas have a high silica content, about 70 to 75 per cent, and are stiff or viscous. They move slowly over the surface and solidify close to the vent. Basic lavas have a silica content of about 50 per cent. Dark colored and fluid, they flow more easily at lower temperatures and reach a greater distance from the crater than do acid lavas.

This earth of ours by Victor T Allen, p. 3

  • What is the text classifying? Lavas.
  • How many types are there? 2
  • What are the two types? Acid and basic.
  • How do we make the distinction? The amount of silica present.

Look at these examples:


Scientists group rocks into three main types: igneous, sedimentary and metamorphic.

IGNEOUS rocks are produced by white-hot material deep inside the earth which rises towards the surface as a molten mass called magma. If the magma stops before on it reaches the surface, it cools and forms rocks such as granite. If the magma erupts, it forms a red-hot stream called lava. When the lava cools it becomes rock. One of the most common lava rocks is called basalt. Igneous rock is used in the formation of the other two main types of rocks – sedimentary and metamorphic.

SEDIMENTARY rock is formed by small particles or sediments such as sand, mud, dead sea animals and weathered rock. These are deposited in layers and become solid rock over millions of years as they are squeezed by the weight of other deposits above them.

The word metamorphosis means ‘change’. Rocks which have been changed by heat and pressure are called METAMORPHIC rocks. They are formed deep inside the earth. Slate for example is formed from compressed mud or clay. Marble is another type of metamorphic rock. It is produced from limestone which has undergone change through heat and pressure,

The chemical elements in the earth’s crust are classified in two major groups – metals and nonmetals. Elements are classified as metallic or nonmetallic, according to their physical and chemical properties. Metals, for example, are usually good conductors of heat and electricity; nonmetals usually are not. Most metals are malleable; they can be hammered into flat sheets; nonmetals lack this quality. Some metals are also ductile; they can be drawn out into thin wires; nonmetals are not usually ductile. Metals usually have luster, and are able to reflect light. They also have a high density. Nonmetals usually do not exhibit these properties. A few elements, such as arsenic and antimony, exhibit both metallic and nonmetallic properties and are sometimes referred to as metalloids.

Inquiry into earth and space science, by William J Jacobson, p 104-105.

Types of food and their function

Before going further into the question of energy supply and demand, and its relationship to food, the major constituents of man’s diet must be listed. These are: carbohydrates, the major energy-supplying foods; proteins, body building materials; fats, energy-rich food stores; vitamins, vital components of cell chemistry; mineral salts, raw materials of all metabolism; water, an obvious and essential compound required for internal cell pressure, the basic medium of cell chemistry and major constituent of blood and hence the transport system; and roughage, the mass of food, mainly composed of plant cellulose, that gives the solid bulk to food and thus enables the gut muscles to grip it and move it along by peristalsis.

Biology by J. M. Hard, p. 14.


Try this exercise: Exercise 1


The tables below show some of the most common language used in texts which have classification as their purpose.

There are



of lava

: acidic and basic.
. These are acidic and basic.


are acidic and basic.


consists of
can be divided into



. These are acidic and basic.
: acidic and basic.

Acidic and basic are


of lava.

Lava may be classified

according to
on the basis of
depending on

amount of silica present.

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