0202 Classification of matter

Categorisation of stuffs with common characteristics

Why do we classify stuffs?
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KEY IDEAS

Classification is the allocation of different objects to different groups (called classes) for the purposes of generalisation about the properties of members in each class, or to distinguish between the properties of members in different classes.

Although the properties of every member of a population (like people, plants, animals, transport systems, ways of thinking, ……….) are unique and important, to try to understand, or explain (or influence) the behaviour of every individual can be an impossible task.

We try to reduce the complexity by allocating the objects to various classes such that the commonalities among members of any one class are sufficiently similar for us to make generalisations about their properties.

Members may belong to more than one class: for example, a person may belong to the class “Female”, and to the class “Blonde”, as well as to the class “Aged 15 to 25”.
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In the field of chemistry, classification of materials (stuffs) is used to clarify the meaning of some basic concepts related to matter (anything which has mass and occupies volume of space). This can be done by using a Venn diagram to show relationships:

​Another way of showing these relationships among classes, sub-classes and sub- sub-classes is the following flow diagram:

There are more obvious characteristics (properties) that we can use to classify stuffs. These include, for example:

• Solids vs. liquids. vs. gases (at room T and p)
• White vs. red vs. green vs. ……
• Soluble in water vs. insoluble in water
• Ionic compounds vs. covalent molecular compounds vs. covalent network compounds
• Organic compounds vs. inorganic compounds
• Oxidising agents vs. reducing agents
• Acidic substances vs. alkaline substances vs. neutral substances
• Group 1 elements of the periodic table vs. Group 2 elements vs. ……..
• Electrical conductors vs. non-conductors (insulators)

and many, many more.
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Remember that classifications are human constructions: they are simply devices decided by people in order to assist description or definition of patterns of behaviours in some way that suits a purpose.

The dividing line between classes (Which materials are this class, and which are in that class?) is sometimes not definite, and can be allocated differently by different people. This is the case with the classifications metal vs. non-metal vs. semi-metal (metalloids). Depending upon the criteria used to classify materials, the dividing lines (that govern which materials “belong” in each class) can vary.

External reference
For further discussion of this subject, and clarification of the meanings of the concepts referred to in the Venn diagram above, see …….
​Mahaffy, P.G., Bucat, B., Tasker, R., and others. “CHEMISTRY: Human Activity, Chemical Reactivity” (Nelson Education), Section 2.2 Classifying Matter, pages 20-23 (both International and Canadian editions).


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SELF CHECK

1. Into a copy of the Venn diagram shown above, put the following materials into their appropriate classes:

• vinegar
• sodium chloride
• chlorine
• concrete
• water
• sea water
• milk
• gold
• mud
• air

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2. Which stuff am I?

• I am in the class of materials called pure substances.
• I am in the class of materials called gases.
• I am in the class of materials considered to be composed of molecules
• I am in the class of materials called elements.
• I am in the class of materials that are green.
• I am in the class of materials called the Group 17 elements of the periodic table.

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3. Which stuff am I?

• I am in the class of materials called mixtures.
• I am in the class of materials that are homogeneous. Therefore, I am a solution.
• I am in the class of materials called gases.
• I am in the class of materials essential for life.
• The main component of the mixture is nitrogen molecules, and the next most is oxygen molecules.

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Answers (No peeking until you have made a commitment to an answer, in each case.

​1.

2.  Chlorine gas, Cl2(g)

​3.  Air