A
substance
Students may use criteria of ‘history’ rather
than ‘properties’ to assign identity to a sample
of material. In their view, rust and iron are the same substance
but in different forms. Charcoals from wood and bread are
seen as different substances. In these cases, it follows that
they do not yet share the scientific concept of a substance
(as a special kind of stuff with invariant properties that
could be in one of three states). Without the concept of a
substance, students cannot recognise a change of substance,
i.e. a chemical change.
States of matter
Students are liable to regard solids, liquids and gases
as three types of substance. In effect, these are seen as
different 'species'. It then becomes impossible to understand
how a ‘solid’ such as copper can react with
a ‘gas’ such as oxygen. This stands in the way
of developing any meaningful understanding of the material
world. Without care, our teaching can inadvertently reinforce
this.
Gases
Students find gases particularly mysterious. For example,
many do not have the idea that oxygen is a substance because
they cannot touch or see oxygen in the same way that they
can touch or see a piece of iron.
Chemical change
Students find the idea of decomposition much more difficult
than composition. Combustion, for example a lighted candle,
is one of the most difficult examples of chemical change
for students to understand.
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