Paper 2 essay guide: Acids, Bases and Salts

This guide shows how Acids, Bases and Salts is examined in Paper 2 and how to structure a full answer. Paper 2 has Section A (structured), Section B (limited-response) and Section C (open-response). This chapter is a favourite for the longer questions because it combines a described experiment, chemical equations and a calculation in one place. Use this guide to plan before you write, so your answer follows the order the marking scheme expects.

How the chapter appears in Paper 2

In Section A the chapter usually appears as a structured question built around data: a titration table, a pH reading, or a salt-preparation procedure with blanks to complete. You interpret the information, write an equation, and carry out a short calculation.

In Section B and Section C the same content becomes a longer answer. A typical question gives an experiment, a titration to find an unknown concentration, or the preparation of a named salt, and asks you to describe the method, write balanced and sometimes ionic equations, explain the chemistry, and finish with a molarity calculation. Section C questions are more open and reward a well-organised, complete response.

What the examiner is really testing. Whether you can move smoothly between three registers: describing a procedure in the correct order, writing correct chemical equations, and setting out a calculation with units. Marks are spread across all three, so a strong essay must do all three, not just one well.

A planning approach

Before writing a long answer, spend a moment planning so nothing is missed:

• Read the whole question first and underline every command word (describe, explain, calculate, state).
• List the chemistry you will need: the correct method for the salt, the balanced equation, the ions involved, and the formula for the calculation.
• Decide the order: apparatus and materials, then the step-by-step method, then the equation, then the calculation, then the observation or conclusion.
• Check the data given: relative atomic masses, volumes and molarities are usually printed, plan which ones go into which formula.

A neat plan of five or six bullet points is enough. It keeps a long answer in a logical order and stops you from repeating yourself.

Model-answer skeleton, a titration question

Use this skeleton (not a memorised essay) and fill it with the values from the question:

• State the aim: to find the unknown concentration of solution X by titration.
• Apparatus: pipette (fixed volume into the conical flask), burette (variable volume of the other solution), conical flask, white tile, indicator.
• Method in order: pipette a fixed volume into the flask; add a few drops of indicator; fill the burette and record the initial reading; run the solution in, swirling, until the indicator just changes colour; record the final reading; repeat for concordant titres.
• Equation: write the balanced equation, for example HCl + NaOH → NaCl + H₂O.
• Calculation: moles of the known = molarity × volume ÷ 1000; use the mole ratio to find moles of the unknown; divide by its volume in dm³ to get molarity; convert to g dm⁻³ if asked.
• Conclusion: state the concentration with its unit.

Model-answer skeleton, a salt-preparation question

• Classify the salt: soluble or insoluble, and (if soluble) whether it is a sodium, potassium or ammonium salt.
• Choose the method: acid + excess insoluble base/metal/carbonate then filter and crystallise (soluble salt of another metal); titration (soluble sodium/potassium/ammonium salt); precipitation then filter, wash and dry (insoluble salt).
• Method in order: the practical steps, including using an excess and filtering it off, evaporating to the point of crystallisation, and drying between filter papers.
• Equation: the balanced equation for the reaction that forms the salt.
• Purity step: washing with a little cold distilled water and drying, with the reason.

Command words and what each demands

• Define, give the exact meaning, in the wording the marking scheme expects (for example, “an acid is a substance that ionises in water to produce hydrogen ions”).
• State, give a short, direct fact or observation with no explanation needed.
• Describe, give the steps or features in order; for a method, enough detail that someone could follow it.
• Explain, give the reason or the chemistry behind something; use the word “because” and refer to ions or particles.
• Compare, give both similarities and differences, and make the point of comparison explicit (for example strong versus weak acids at the same molarity).
• Calculate, show your working line by line, carry the units, and give the final answer with its unit.

Habit that gains marks. For every “explain”, name the species involved: it is the H⁺ ions that make a solution acidic and the OH⁻ ions that make it alkaline. Vague answers that never mention ions rarely score the explanation mark.

Putting it together

The strongest Paper 2 answers on this chapter read in a clear order, contain a correct balanced equation, and finish with a calculation whose units are carried through. Plan first, write in the order above, and check that every command word in the question has been answered. A one-to-one teacher can mark a practice long answer against the skeletons here and show you exactly where a mark was missed. When you have rehearsed both skeletons, move on to the practice questions and time yourself.

Ionic equations earn easy marks

Many Section B and C questions ask for an ionic equation as well as a full one. For neutralisation, the ionic equation is always H⁺ + OH⁻ → H₂O; for precipitation, write only the ions that form the insoluble salt, for example Ba²⁺ + SO₄²⁻ → BaSO₄. Learn to cancel the spectator ions that appear unchanged on both sides. Including the correct ionic equation, with state symbols where asked, is a reliable way to secure marks that many students leave on the table.