Stuffs with common characteristics, stuffs that are different (in which properties?)
Different strokes for different folks
Preamble: Is chemistry about stuffs?
Or about people doing and thinking about stuffs?
During your time learning Chemistry for understanding, you may have read something like: “Matter is classified in the following way: …[description here]…...”. Prof Bob has two concerns about this manner of speaking.
First, this sounds almost as though each stuff inherently “belongs” in a certain category - as though categorised by Mother Nature. No, things don’t exist in categories independently of people: people assign them into categories.
Secondly, … Into which categories, according to which criteria? There may be many different ways that people can categorise the same stuffs – depending on the usefulness of allocating according to particular characteristics of the stuffs.
So, let’s try again, showing classification to be an active human process: “People can classify stuffs in a number of ways, depending on which characteristics are useful for a particular purpose. One of the possible ways of classifying stuffs is the following: .................... ”.
Watch Prof Bob and Aussie talk about it ....
Prof Bob discusses with Aussie ways in which people can, and do, classify stuffs.
KEY IDEAS - People classifying stuffs: Why?
Can we have some order, please!
Why do we classify?
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 stuffs 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 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”.
Classification in chemistry
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, such as in the following example:
Another way of showing these relationships among classes, sub-classes and sub-sub-classes is the following flow diagram:
There are other obvious characteristics (properties) that we can use to classify stuffs. These include, for example:
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. ……..
Solutes which are electrolytes vs. those that are non-electrolytes (See Module 0909)
and many, many more.
Classification - a human construct
Different purposes. Shades of grey.
Remember, when you are learning Chemistry, that classifications are human constructions: they are simply devices decided by people in order to assist understanding of patterns of behaviour - in some way that suits a purpose. In particular, they help our learning chemistry by allowing us to focus on commonalities, rather than on the specific details of each stuff - although the more expert you might become, the more important are the details. Remember too that classification can help to demonstrate the differences (not just the similarities) between characteristics of stuffs.
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.
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).
SELF CHECK: Some thinking tasks
Are you all sorted? Check ....
1. Into a copy of the Venn diagram shown above, put the following materials into their appropriate classes:
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.
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.
Answers (No peeking until you have made a commitment to an answer, in each case.
2. Chlorine gas, Cl2(g)
Finding your way around .....
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