How to Master Acids, Bases and Salts | O-Level Chemistry Guide

Acids, bases and salts are foundational topics in Secondary 3 and 4 Chemistry. They connect directly to qualitative analysis, where you use observations such as a gas test or a precipitate to identify substances. Both areas appear in the SEAB O-Level Chemistry syllabus (6092).

If you keep mixing up equations or forgetting which salts dissolve in water, start with three reaction patterns. Then use the solubility rules to choose the right preparation method. Here is how we teach it at Jimmy Tuition.

1. Learn the three common acid reactions

Recognising the reactants helps you predict the products before you balance the equation. This is useful in structured questions and stoichiometry calculations.

Acid + reactive metal → salt + hydrogen

2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g)

Magnesium reacts with dilute hydrochloric acid to give magnesium chloride and hydrogen. Copper and silver do not release hydrogen with typical dilute, non-oxidising acids such as hydrochloric acid.

Acid + carbonate → salt + water + carbon dioxide

H₂SO₄(aq) + CaCO₃(s) → CaSO₄(s) + H₂O(l) + CO₂(g)

Effervescence tells you a gas is being produced. A white precipitate in limewater identifies the gas as carbon dioxide. If the gas formed when an acid was added to the sample, that result points to a carbonate.

Acid + base → salt + water

HNO₃(aq) + NaOH(aq) → NaNO₃(aq) + H₂O(l)

This is neutralisation. In this example, nitric acid reacts with the alkali sodium hydroxide to produce sodium nitrate and water.

2. Use solubility rules to choose a salt preparation method

When a question asks how to prepare a pure sample of a named salt, first decide whether the product is soluble in water. Then choose a method based on the product and the reactants available.

Remember SPAN and BaCaPb

1. SPAN. Sodium, potassium, ammonium and nitrate salts are treated as soluble in school-level questions.
2. Sulfates. Most are soluble. Remember BaCaPb for the common exceptions. Barium sulfate (BaSO₄) and lead(II) sulfate (PbSO₄) are insoluble, and calcium sulfate (CaSO₄) is sparingly soluble.
3. Chlorides. Most are soluble. Common exceptions are silver chloride (AgCl) and lead(II) chloride (PbCl₂), which has low solubility in cold water.

Match the salt to the method

• Titration. Use this when preparing a soluble salt from an acid and an alkali, such as sodium nitrate from nitric acid and sodium hydroxide. Find the volumes needed to neutralise the solutions, then repeat with those volumes and without indicator before crystallising the salt.
• Excess insoluble reactant. Use this to prepare a soluble salt when a suitable insoluble solid reacts with the acid. For copper(II) sulfate, add excess copper(II) oxide to dilute sulfuric acid, filter off the unreacted solid, then concentrate the filtrate and let it cool to form crystals.
• Precipitation. Mix two suitable soluble salt solutions to form an insoluble salt such as barium sulfate. Filter, wash and dry the precipitate.

3. Avoid three common exam mistakes

• Leaving out state symbols in ionic equations. Check for (aq) for aqueous, (s) for solid, (g) for gas and (l) for liquid. A precipitate carries the (s) symbol.
• Confusing a base with an alkali. An alkali is a base that dissolves in water and produces hydroxide ions (OH⁻) in solution. Sodium hydroxide and aqueous ammonia are examples. An insoluble metal oxide such as iron(II) oxide is a base.
• Skipping filtration. When you add an excess insoluble solid to an acid, filter out the unreacted solid before concentrating the filtrate. Otherwise the remaining solid can contaminate the salt crystals.

Learn Secondary Chemistry with Jimmy Tuition

We connect acid reactions, salt preparation and qualitative analysis through visual mind maps and memorable patterns. Students practise explaining why a reaction happens and how to use its observations to answer structured questions.

• In-person small-group classes. Our centre is a three-minute sheltered walk from Admiralty MRT. This makes it convenient for students from nearby schools such as Admiralty Secondary, Woodlands Ring Secondary, Evergreen Secondary, Woodgrove Secondary and Riverside Secondary.
• Interactive live online lessons. Join from anywhere in Singapore and follow the lesson on an interactive whiteboard. Online students can rewatch lessons and contact Mr. Jimmy on WhatsApp for individual help outside class hours.

Read more about class sizes, makeup options and study materials on our Frequently Asked Questions page.