Common mistakes in acids, bases and salts

Acids, bases and salts is one of the most exam-heavy chapters in Form 4, and it rewards precision. The chemistry itself is not hard, but the marks are lost in small, specific slips, the wrong product in an equation, the wrong salt-preparation method, a titration ratio read carelessly. Here are the mistakes our teachers correct most often, each with the fix, so you can audit your own answers before the exam does.

Mistake 1: writing the wrong products for each acid reaction

There are three reactions of acids to know, and students mix up their products. Fix them firmly:

• Acid + reactive metal → salt + hydrogen gas. (Test: a lighted splint gives a “pop”.)
• Acid + metal carbonate → salt + water + carbon dioxide. (Test: gas turns limewater milky.)
• Acid + base (metal oxide or hydroxide) → salt + water only.

The classic error is producing hydrogen from a carbonate, or forgetting the carbon dioxide. Fix: memorise the reaction by the other reactant, metal gives hydrogen, carbonate gives carbon dioxide, base gives only water.

Mistake 2: ignoring the basicity of the acid

Not every acid gives one hydrogen ion. Hydrochloric acid (HCl) and nitric acid (HNO₃) are monoprotic, one mole gives one mole of H⁺. Sulfuric acid (H₂SO₄) is diprotic, one mole gives two moles of H⁺. This changes the reacting ratio. In 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O the ratio is 2 : 1, not 1 : 1. Students who assume every acid–alkali reaction is 1 : 1 get titration answers that are wrong by a factor of two. Fix: always write and balance the equation before using any ratio.

Mistake 3: choosing the wrong salt-preparation method

The method depends entirely on the salt’s solubility, and students who skip that step pick the wrong route. Use the solubility rules: all sodium, potassium and ammonium salts and all nitrates are soluble; most carbonates are insoluble.

• Soluble salt of sodium, potassium or ammonium → titration of the alkali with an acid, using an indicator.
• Soluble salt of another metal (for example copper(II) sulfate) → add excess insoluble solid (metal, base or carbonate) to the acid, then filter off the excess.
• Insoluble salt (for example lead(II) iodide) → precipitation: mix two soluble solutions, then filter, wash and dry the precipitate.

Fix: before choosing a method, ask “is my salt soluble, and which cation is it?” The answer picks the method for you.

Mistake 4: using an indicator where it does not belong

An indicator is needed only in the titration method, because there is no visible sign that a soluble alkali has been exactly neutralised. In the excess-solid method you can see leftover solid, so no indicator is used, you simply add solid until no more dissolves, then filter. Students who add indicator to a copper(II) oxide preparation, or leave it out of a titration, lose method marks. Fix: indicator with titration only.

Mistake 5: forgetting a salt is not always neutral

“Salt” does not mean “neutral”. A salt of a strong acid and a strong base (such as sodium chloride) is neutral, but the chapter is really about the neutralisation reaction, not the pH of the product. More important is the ionic equation for neutralisation itself: H⁺ + OH⁻ → H₂O. The sodium and chloride ions are spectator ions and do not appear. Students often write the full equation when the question asks specifically for the ionic one. Fix: for an ionic equation, cancel the spectator ions and keep only the species that actually change.

Mistake 6: careless titration arithmetic

Titration calculations combine several earlier traps. Volume must be in dm³, the mole ratio must come from the balanced equation, and the concentration formula is moles ÷ volume. A frequent slip is using the burette volume for the wrong solution, or forgetting to convert cm³. Fix: label every number with what it is (acid or alkali, volume or concentration) before substituting, and rehearse the full method under titration calculation.

Making the chapter reliable

Look back over these six and a theme appears: each is a failure to read the situation, which reactant, which acid, which solubility, which method. The chemistry rewards students who slow down and classify before they calculate. Build a one-page summary, the three reactions, the basicity of common acids, the three preparation routes, the neutralisation ionic equation, and test yourself on it until it is automatic.

If your child keeps losing marks on salt preparation or titration despite revising, it is usually the classification step that has not settled, and a focused lesson fixes it faster than repetition alone. Our teachers teach online one-to-one in English from RM50 an hour, with a paid one-hour trial to begin, working through real Paper 2 questions until the method is second nature.