Strong and weak acids and bases
The key misconceptions that students have with acids and alkalis are that:
1. Acids can burn and eat material away:
Students think of acids as active agents that damage skin and other materials. The idea develops in young children, who learn to think of acids as “dangerous”. Acids are not perceived as being particulate, but rather continuous matter with special properties.
2. Neutralisation means an acid breaking down
Rather than considering neutralisation as a reaction between an acid and an alkali, students perceive this as removing acid properties. The alkali may stop the action of an acid, or alternatively the acid may break down.
3. A base/alkali inhibits the burning properties of an acid
Students tend to meet acids in formal education well before alkalis, so ideas about these chemicals are relatively under-developed. Although dilute alkalis are in fact more corrosive than dilute acids, students’ perceptions are that they have no corrosive properties, instead acting to or inhibit acids “eating away” other material.
4. Hydrogen ions are present in acids, but acids remain molecular in solution
That hydrogen ions are responsible for acidic behaviour is relatively well-known amongst students. However, a common model for acid behaviour seems to be that hydrogen ions remain in a molecule and “swap partners” or are “displaced” from this molecule by reaction with an alkali or metal.
Teachers need to be aware of students’ difficulties with these acid/base reactions. Students’ problems may arise because acids and alkalis both look like water. Reacting them together needs precision and some way of knowing that neutralisation is complete, so an indicator is required. Addition of this extra chemical adds and extra layer of “mystery”.
In understanding strong and weak acids and alkalis, students need to consider what is happening at the particle level more deeply. They also learn that over time, different theories have been used to define what acids and alkalis are and what is happening at the particle level. It is not surprising that students find these concepts challenging: it took scientists hundreds of years to work out what acids and bases are and how they work!
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