Understanding Acids, bases and salts: OxfordAQA IGCSE CORE Chemistry (Short Course) Explained

Acids, bases and salts: the reaction that runs through the whole topic

Acids, bases and salts oxfordaqa igcse builds around a single reaction type: neutralisation. Once you understand what happens when an acid meets a base, most of this topic becomes an exercise in applying that one idea to different starting materials. This deep dive covers the two topics in this section of OxfordAQA IGCSE CORE Chemistry (Short Course): the properties of acids and bases, and the preparation of salts.

The properties of acids and bases

Metal oxides and metal hydroxides are bases. A base that dissolves in water is specifically called an alkali. When an acid reacts with a base, the products are a salt and water, and this type of reaction is called neutralisation.

The particular salt formed depends on two things: the acid used, and the metal present in the base or alkali.

For example, hydrochloric acid reacting with sodium hydroxide produces sodium chloride and water, while sulfuric acid reacting with copper oxide produces copper sulfate and water.

Hydrogen ions, hydroxide ions and pH

Acidic solutions contain hydrogen ions, H⁺(aq), and alkaline solutions contain hydroxide ions, OH^-(aq). The pH scale, running from 0 to 14, measures how acidic or alkaline a solution is; pH 7 is neutral, values below 7 are acidic, and values above 7 are alkaline. Universal indicator is used to estimate the approximate pH of a solution by comparing the colour produced to a reference chart.

Neutralisation can be represented by a single ionic equation that applies to every acid-alkali neutralisation reaction, regardless of which specific acid and alkali are used:

H⁺(aq) + OH^-(aq) → H₂O(l)

This equation is worth memorising exactly, since it appears repeatedly across different contexts and is one of the more commonly tested equations on the paper.

Preparation of salts

Soluble salts can be prepared from acids in three main ways, and the method chosen usually depends on whether the base being used is a metal, an insoluble base, or an alkali.

• Reacting an acid with a metal: not every metal is suitable, since very reactive metals react too violently to be safe, while very unreactive metals do not react with the acid at all.
• Reacting an acid with an insoluble base: the base is added to the acid in small portions until no more reacts, and the excess solid base is then removed by filtration, leaving a salt solution.
• Reacting an acid with an alkali: since both the acid and the alkali are solutions, an indicator is used to show the exact point at which they have completely reacted to form a salt solution, because there is no solid excess to filter off.

Once a salt solution has been prepared, the pure solid salt can be obtained by crystallisation: the solution is gently heated to evaporate some of the water, then left to cool so that crystals of the salt form and can be collected.

Worked example

Question: Suggest a method for preparing a pure, dry sample of copper sulfate crystals, starting from copper oxide and dilute sulfuric acid.

Answer: Add copper oxide, an insoluble base, to warm dilute sulfuric acid a little at a time, stirring after each addition, until the oxide is in excess and no more dissolves. Filter the mixture to remove the excess unreacted copper oxide, leaving a solution of copper sulfate. Heat the filtrate gently to evaporate some of the water, then allow the solution to cool so that crystals form, and finally filter or pat dry the crystals to remove remaining moisture.

Why this topic rewards a methodical approach

oxfordaqa igcse core chemistry (short course) acids, bases and salts is one of the more procedural sections of the specification, which is good news for revision: unlike some topics that hinge on abstract explanation, most of the marks here reward following a clear, memorised sequence of steps correctly. Rather than trying to memorise every possible acid-base combination individually, focus on internalising the three preparation methods and the logic behind choosing between them, since almost every exam question in this topic is really just a variation on that same decision.

It also pays to practise writing full word equations and, where appropriate, balanced symbol equations for a range of acid-base combinations, since these come up as short recall questions in their own right, separate from the longer preparation-method questions.

Common mistakes in acids, bases and salts

• Using an indicator when preparing a salt with an insoluble base; indicators are only needed when both reactants are in solution, as with an alkali, because there is no solid to filter with an insoluble base method.
• Forgetting to filter out excess solid before crystallising the salt solution, which leaves the final product impure.
• Confusing a base with an alkali; every alkali is a base, but not every base is soluble enough to count as an alkali.
• Writing the wrong salt name for a given acid, for example naming a "sulfic salt" instead of a sulfate when sulfuric acid is used.
• Assuming pH 7 always means water; any neutral solution has pH 7, not only pure water.

Practising the salt-preparation decision tree

A useful revision exercise is to build a simple decision tree in your notes: start by asking whether the base is a metal, an insoluble solid, or a soluble alkali, then follow the branch that matches to the correct preparation method. Test yourself with a list of ten different acid-and-base combinations, some involving reactive metals, some involving insoluble oxides such as copper oxide, and some involving alkalis such as sodium hydroxide, and see how quickly you can name the correct method for each without hesitating. This kind of rapid-fire practice mirrors the way these questions are usually phrased on the actual paper, where you are rarely told directly which method to use and are instead expected to work it out from the starting materials given.

Pay close attention, too, to safety reasoning. Examiners sometimes ask why a particular metal would not be suitable for making a salt by direct reaction with acid, and the expected answer usually distinguishes between metals that are too reactive, which react dangerously fast, and metals that are not reactive enough, which simply fail to react with the acid at all. Being able to explain the safety reasoning behind a method choice, not just recite the method itself, is what separates a full-mark answer from a partial one.

Self-check questions

1. Name the salt formed when nitric acid reacts with potassium hydroxide.
2. Suggest a suitable method for preparing zinc sulfate, starting from zinc metal and dilute sulfuric acid, and explain why this metal is suitable.
3. Write the ionic equation for neutralisation and explain what each symbol represents.
4. Explain why an indicator is needed when preparing a salt from an alkali but not when preparing a salt from an insoluble base.
5. A solution has a pH of 3. State whether it is acidic, neutral or alkaline, and explain what this tells you about the concentration of hydrogen ions compared with a solution of pH 6.

One more habit worth building: whenever you write a preparation method in full sentences, read it back and check whether a classmate with no chemistry background could follow it step by step without needing to guess an omitted detail. Preparation-method questions are marked on completeness as much as accuracy, and a method that skips a step, such as forgetting to mention filtration, loses marks even if every other part of the answer is correct.

How this topic is examined

Questions on acids, bases and salts oxfordaqa igcse frequently ask you to suggest an appropriate preparation method for a named salt, so practise matching the acid, metal and method together quickly. Marks are also commonly available for describing the practical steps in the correct order, particularly filtration before crystallisation, so sequencing your answer correctly matters as much as including the right steps. For extra igcse 9222 acids, bases and salts practice, work through past-paper questions that ask you to plan a preparation method from scratch, since that style of question rewards a clear, ordered method above all else.

These oxfordaqa igcse core chemistry (short course) revision notes connect directly to the chemical analysis topic, since indicators and pH testing are themselves analytical techniques, and to the quantitative chemistry topic that follows, where you will calculate the exact mass of salt produced from a given quantity of acid.

Once the three preparation methods and the ionic equation for neutralisation are second nature, this oxfordaqa igcse core chemistry (short course) notes topic becomes one of the more predictable sources of marks on the paper, and this oxfordaqa igcse core chemistry (short course) explained page is worth revisiting whenever you need a quick refresher on which method fits which starting material. Further oxfordaqa igcse core chemistry (short course) practice questions on salt preparation are the most efficient way to lock in the correct sequencing before exam day.

Self-Check Questions

1. Define the key terms introduced in this section using your own words.
2. Sketch a labelled diagram that illustrates one of the processes described above.
3. Write a balanced symbol equation for one reaction covered in this topic.
4. Explain why understanding this area is useful for the OxfordAQA IGCSE CORE Chemistry exam.
5. Describe one practical application of the chemistry discussed in this section.

Revision tip: After completing these questions, check your answers against the specification objectives listed at the start of this topic. If any objective is not covered by your answers, revisit that part of your notes and write a brief summary.