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TGSU1: Key features of students’ understanding (1)
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.

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.

not applicable

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