Understanding Acids, bases and salts: OxfordAQA IGCSE Chemistry Explained

Chemistry You Already Half-Know

If you have ever cleaned a wasp sting with vinegar, tasted lemon juice, or noticed how baking soda calms an upset stomach, you already have an intuitive feel for OxfordAQA IGCSE Chemistry acids, bases and salts before opening a single textbook page. This section, acids, bases and salts OxfordAQA IGCSE candidates meet fairly early in the course, takes that everyday intuition and turns it into precise, testable chemistry: the properties of acids and bases, and the preparation of salts. Working through IGCSE 9202 acids, bases and salts content is genuinely one of the more approachable parts of the specification, because so much of it connects to things you can picture happening in a kitchen or a garden as easily as in a lab.

The Properties of Acids and Bases

What Makes Something an Acid or a Base?

Metal oxides and metal hydroxides are bases; when a base is soluble in water, it is specifically called an alkali. Acids react with bases in what is called a neutralisation reaction, producing a salt as one of the products.

Think of it like two opposing teams meeting in the middle: an acid brings hydrogen ions, H⁺(aq), which is what makes a solution acidic, and an alkali brings hydroxide ions, OH⁻(aq), which is what makes a solution alkaline. When the two meet, they combine to form water:

H⁺(aq) + OH⁻(aq) → H₂O(l)

That single equation is the heart of every neutralisation reaction in this specification, no matter which specific acid and base you are dealing with.

The pH Scale

The pH scale runs from 0 to 14 and 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 gives an approximate pH reading through a range of colours, and being able to describe how to use it, dipping in a sample and comparing the resulting colour against a reference chart, is a skill worth practising out loud, not just reading about.

Which Salt Forms?

The specific salt produced by a reaction between an acid and a base depends on two things: which acid is used, and which metal is in the base or alkali.

So sodium hydroxide plus hydrochloric acid gives sodium chloride, while potassium hydroxide plus sulfuric acid gives potassium sulfate. This little table is one of the most useful things you can memorise in this whole topic, because it turns "name the salt formed" questions from guesswork into a lookup.

Worked Example: Naming a Salt

Question: Copper oxide reacts with nitric acid. Name the salt formed.

Answer: Copper nitrate. The metal in the base, copper, combines with the acid family, nitrate, since nitric acid always produces nitrate salts.

Ammonia and Limewater

Ammonia is a slightly unusual base worth knowing on its own terms: it dissolves in water to form an alkaline solution, and it is widely used to produce ammonium salts, which matter enormously as fertilisers. Separately, a solution of calcium hydroxide in water, better known as limewater, reacts with carbon dioxide to produce calcium carbonate, which is exactly the reaction responsible for limewater's characteristic cloudy-white test result used elsewhere in the specification to identify carbon dioxide gas.

Preparation of Salts

There are three main routes to making a soluble salt, and picking the right one depends on what you are starting with.

1. Metal plus acid. Works well for moderately reactive metals; too-reactive metals react dangerously, and too-unreactive metals barely react at all, so not every metal is suitable.
2. Insoluble base plus acid. The base is added to the acid gradually until no more will react, and the excess unreacted solid is then filtered off, leaving a pure salt solution behind.
3. Alkali plus acid. Because both starting materials are soluble, you cannot simply filter off the excess; instead, an indicator is used to show the exact point at which the acid and alkali have completely reacted, producing a salt solution with nothing left over.

Once a soluble salt solution has been made by any of these routes, it can be crystallised: heating to evaporate some of the water concentrates the solution until crystals of the solid salt begin to form as it cools.

Making Insoluble Salts

Insoluble salts are made differently, by precipitation: mixing two solutions whose ions combine to form a solid that will not dissolve. This same idea, deliberately triggering a precipitate, is used practically in treating drinking water and in treating industrial effluent to remove unwanted ions from a solution.

Worked Example: Choosing a Preparation Method

Question: Suggest a method for preparing a pure, dry sample of lead sulfate, which is insoluble in water.

Answer: Mix a solution containing lead ions, such as lead nitrate solution, with a solution containing sulfate ions, such as sodium sulfate solution. A precipitate of lead sulfate forms immediately. Filter the mixture to collect the precipitate, wash it with distilled water to remove any remaining soluble impurities, and then dry it.

Common Mistakes Across This Section

• Naming the wrong salt family for a given acid; the hydrochloric-chloride, nitric-nitrate, sulfuric-sulfate pattern is worth over-learning until it is automatic.
• Forgetting that "alkali" specifically means a soluble base, not simply a synonym for "base" in general.
• Describing neutralisation only as "acid plus base makes salt and water" without being able to write or explain the ionic equation involving H⁺ and OH⁻ when asked.
• Confusing the "add excess solid, then filter" method with titration; they solve different problems (removing an insoluble excess versus finding an exact reacting volume) and are not interchangeable.

Self-Check Questions

1. Explain, using ions, what happens during a neutralisation reaction between hydrochloric acid and sodium hydroxide.
2. Name the salt formed when magnesium oxide reacts with sulfuric acid.
3. Describe how you would prepare a pure, dry sample of copper sulfate, a soluble salt, starting from copper oxide and dilute sulfuric acid.
4. Explain why an indicator is needed when preparing a salt from an alkali and an acid, but not when preparing one from an insoluble base and an acid.
5. Describe how limewater can be used to test for carbon dioxide, and name the compound formed.

Exam Strategy for Acids, Bases and Salts

Questions in this section often present a scenario, a specific acid, base and expected product, and ask you to apply the general rules above rather than recall an isolated fact. A few habits pay off:

• Always identify the acid family (chloride, nitrate or sulfate) before attempting to name a salt; get this step right and the rest follows automatically.
• For method questions, decide solubility first: is the target salt soluble or insoluble? That single decision determines whether you describe a titration-style method or a precipitation method.
• When asked to "describe" a preparation method, structure your answer as a sequence of steps, in order, rather than a single dense paragraph; examiners award marks for each correctly sequenced step.

Answering the Self-Check Questions

Compare your own attempts against these model answers before moving on.

• Neutralisation in terms of ions: hydrogen ions from the hydrochloric acid react with hydroxide ions from the sodium hydroxide to form water, H⁺(aq) + OH⁻(aq) → H₂O(l), while the sodium and chloride ions remain in solution as sodium chloride.
• Magnesium oxide plus sulfuric acid: magnesium sulfate, since sulfuric acid always produces a sulfate salt and the metal comes from the base.
• Preparing copper sulfate: warm dilute sulfuric acid, add copper oxide gradually until no more dissolves and excess solid remains, filter to remove the excess copper oxide, then evaporate and cool the filtrate to crystallise pure copper sulfate.
• Why an indicator is needed with an alkali but not an insoluble base: both the acid and the alkali are soluble, so there is no solid excess to filter off at the end; an indicator shows the exact point of complete reaction. With an insoluble base, any leftover solid is simply filtered away instead.
• Limewater test: bubbling carbon dioxide through limewater turns it cloudy white as calcium carbonate forms, which is insoluble and gives the solution its milky appearance.

Building Revision Notes for This Section

Strong OxfordAQA IGCSE Chemistry revision notes for this topic centre on two small tables: the acid-to-salt-family table above, and a comparison of the three salt preparation methods with when each one applies. These OxfordAQA IGCSE Chemistry notes are worth pairing with a couple of fully written-out worked examples in your own handwriting, since salt preparation questions are marked on method sequencing as much as on the final named product.

With OxfordAQA IGCSE Chemistry explained through everyday analogies like the see-saw image above, the underlying chemistry stops feeling abstract. Once you can confidently predict which salt forms from a given acid and base, and choose the right preparation method for a soluble or insoluble target, OxfordAQA IGCSE Chemistry practice questions on this topic become genuinely satisfying to work through, since the logic behind each answer is consistent and predictable every time.