Common mistakes: Acids, Bases and Salts

Most marks lost in this chapter come from a small set of repeated errors, not from difficult chemistry. For each mistake below we show what students tend to write, why it loses marks, and the correct version. Read them alongside your own past answers and fix the pattern, not just the single question.

6.1 Properties of acids and bases

Mistake 1, Forgetting the role of water.

• What students write: “Hydrogen chloride is an acid” without any condition.
• Why it loses marks: acidic properties appear only when H⁺ ions are free in water; dry hydrogen chloride gas shows no acidic properties.
• Correct version: “Hydrogen chloride ionises in water to produce hydrogen ions, so its aqueous solution is acidic.”

Mistake 2, Confusing strength with concentration.

• What students write: “A strong acid means a concentrated acid.”
• Why it loses marks: strength is about how fully the acid ionises; concentration is about how much acid is in a given volume. They are independent.
• Correct version: “A strong acid ionises completely in water; a concentrated acid simply has many moles of acid per dm³. A dilute acid can still be strong.”

Mistake 3, Over-generalising acid–metal reactions.

• What students write: “All metals react with acid to give hydrogen.”
• Why it loses marks: unreactive metals such as copper do not displace hydrogen from dilute acids.
• Correct version: “Reactive metals above hydrogen in the reactivity series react with dilute acid to give a salt and hydrogen gas.”

6.2 The pH scale

Mistake 4, Getting the pH direction backwards.

• What students write: “A higher pH means more acidic.”
• Why it loses marks: it is the reverse, a lower pH means a higher H⁺ concentration and a more acidic solution.
• Correct version: “As pH decreases below 7 the solution becomes more acidic; as it rises above 7 it becomes more alkaline; pH 7 is neutral.”

Mistake 5, Treating pH as a measure of strength alone.

• What students write: “Acid X has a lower pH, so it must be a stronger acid.”
• Why it loses marks: pH depends on both strength and concentration, so a comparison is only fair at the same molarity.
• Correct version: “At the same molarity, the stronger acid has the lower pH because it ionises more fully to give more H⁺ ions.”

6.3 Concentration and molarity

Mistake 6, Leaving the volume in cm³.

• What students write: molarity = moles ÷ 25 (with the volume still in cm³).
• Why it loses marks: the molarity formula needs the volume in dm³, so the answer comes out 1000 times too small.
• Correct version: “Convert 25.0 cm³ to 0.0250 dm³ first, then divide the moles by 0.0250.”

Mistake 7, Mixing up g dm⁻³ and mol dm⁻³.

• What students write: multiplying by the molar mass to convert g dm⁻³ into mol dm⁻³.
• Why it loses marks: the conversion goes the other way, so the answer is wrong by a factor of the molar mass.
• Correct version: “To get mol dm⁻³, divide the g dm⁻³ value by the molar mass; to get g dm⁻³, multiply the molarity by the molar mass.”

6.4 Standard solution and dilution

Mistake 8, Answering the wrong quantity in a dilution.

• What students write: giving the final total volume when the question asks for the volume of water added.
• Why it loses marks: M₁V₁ = M₂V₂ gives the final volume; the water added is the final volume minus the starting volume.
• Correct version: “Find V₂ from the equation, then subtract the original volume to get the volume of water added.”

Mistake 9, Thinking dilution changes the number of moles.

• What students write: recalculating moles after adding water as if they increased.
• Why it loses marks: adding water changes the concentration, not the number of moles of solute.
• Correct version: “The moles of solute stay the same on dilution; that is exactly why M₁V₁ = M₂V₂ holds.”

6.5 Neutralisation and titration

Mistake 10, Ignoring the mole ratio.

• What students write: using (Mₐ Vₐ) = (M_b V_b) for sulfuric acid and an alkali as if the ratio were 1 : 1.
• Why it loses marks: H₂SO₄ reacts with 2 mol of alkali, so the correct relationship is (Mₐ Vₐ)/(M_b V_b) = 1/2.
• Correct version: “Balance the equation first, then put the mole ratio a : b into (Mₐ Vₐ)/(M_b V_b) = a/b.”

Mistake 11, Using non-concordant titration readings.

• What students write: averaging every burette reading, including a rough first titration.
• Why it loses marks: the rough (trial) value and any outlier make the mean inaccurate; only concordant readings (within about 0.10 cm³) should be averaged.
• Correct version: “Discard the rough titration and any outlier, then average only the concordant titres.”

6.6 Preparation of salts

Mistake 12, Choosing the wrong preparation method.

• What students write: preparing a copper or lead salt by titration, or an insoluble salt by evaporation.
• Why it loses marks: titration is only for soluble sodium, potassium and ammonium salts; insoluble salts are made by precipitation, not evaporation.
• Correct version: “For a soluble salt of a metal other than sodium, potassium or ammonium, react acid with an excess of an insoluble base and filter; for an insoluble salt, mix two soluble solutions and filter off the precipitate.”

A note on qualitative analysis

A recurring slip in 6.7 is giving an observation with no inference, or an inference with no supporting observation, for example writing “carbon dioxide” without “the gas turned limewater milky”, or the reverse. The marking scheme wants both: state what you observe and the conclusion it supports. The same discipline applies to cation tests, where you must say both the colour of the precipitate and whether it dissolves in excess sodium hydroxide or ammonia solution. A one-to-one teacher can check that your answers always pair observation with inference, which is where these easy marks are won.