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!
Whilst this list provides a source of information and ideas for experimental work, it is important to note that recommendations can date very quickly. Do NOT follow suggestions which conflict with current advice from CLEAPSS, SSERC or recent safety guides. eLibrary users are responsible for ensuring that any activity, including practical work, which they carry out is consistent with current regulations related to Health and Safety and that they carry an appropriate risk assessment. Further information is provided in our Health and Safety guidance.
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Acids and Alkalis
One way to help students understand the difference between “strong” and “weak” and dilute and concentrated is to get them to carry out sequential 1:10 dilutions. Strong acid, strong base, weak acid and weak base will show that pH changes by 1 whole value for every dilution. A strong acid or base will require more dilutions to reach the same value as a weak base. This suggests that there are more particles responsible for acidity/ alkalinity present in strong acids and bases than in weak ones. This can be contrasted with concentrated solutions in which a large amount of any substance may be dissolved in water. This is independent of whether the acid/ base is “strong” or “weak”, but applies equally to non-acidic substances such as sodium chloride or sugar.
Activity 5 in this Learn Chemistry resource (pages 15-20) can be used to help students to understand what is happening at the molecular level. This is Activity 5 in the Acids and Alkali's Student Pack.
The concentration of an acid or alkali is a measure of how much acid or alkali is dissolved in a known volume of liquid, e.g. water. The molarity of a solution is a measure of concentration: the number of moles dissolved in 1000cm3 of water. Concentration of solutions is usually expressed in moles per litre of solution. (A litre is the same as 1dm3 or 1000cm3.). This way of stating concentration is sometimes called ‘molarity’.
In a concentrated acid or alkali there is a relatively large amount of solute dissolved, compared with a dilute acid or alkali. However, this does not tell us what the concentration of H+ ions is in the acid or OH- ions is in the alkali. This depends on the strength of the acid or alkali. Strength refers to the degree of ionisation in water.
A strong acid or alkali is one that is nearly or completely ionised in water. Examples are: hydrochloric acid, nitric acid and sulphuric acid. Sodium and potassium hydroxide are examples of strong bases.
A weak acid or alkali, on the other hand, is only partially ionised in water. Ethanoic, citric and carbonic acids are all weak acids. The ethanoic acid molecule and the ions reach a dynamic equilibrium with the equilibrium normally well to the left, so there is little H+ present.