Acids and bases (A-level only)
Acids and bases are important in domestic, environmental and industrial contexts.
Full topic guide: the detailed syllabus page with worked examples and common mistakes lives at studyvector.co.uk/a-level/chemistry/physical-chemistry-2/acids-bases-a-level.
Topic preview: Acids and bases (A-level only)
Sample stems from the StudyVector question bank (AQA · Edexcel · OCR) — not generic filler text.
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Coverage and provenance
What this page is based on
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Topic explanation
A-Level Chemistry introduces the Brønsted-Lowry theory of acids and bases, where acids are proton donors and bases are proton acceptors. The concept of conjugate acid-base pairs is central. The topic delves into pH calculations for strong and weak acids and bases, using the acid dissociation constant, Ka, and the ionic product of water, Kw. Buffer solutions, which resist changes in pH, are also a key area of study, vital for biological systems and chemical applications.
Acids and bases (A-level only) is easiest to revise when it is treated as a precise exam behaviour, not a loose note-taking category. In A-Level Chemistry, the goal is to recognise how the topic appears in a question, identify the command word, and decide what evidence, method, or vocabulary earns marks. StudyVector keeps this page tied to AQA · Edexcel · OCR language where coverage is available, then routes practice towards the same topic so revision moves from explanation into retrieval.
A strong revision session starts with a short recall check. Write down the rule, definition, process, or method linked to Acids and bases (A-level only) before looking at any notes. Then answer one exam-style prompt and compare your answer with the mark-scheme logic: did you make a clear point, support it with the right step, and avoid drifting into a nearby topic? This matters because many lost marks come from almost-correct answers that do not match the expected structure.
Use this guide as the first layer: understand the topic, look at the worked examples, complete the mini quiz, then move into full practice. The full StudyVector practice loop is designed to capture whether mistakes are caused by knowledge, method, language, or timing. That distinction is important. If the error is factual, you need reteaching. If the error is method-based, you need a worked retry. If the error is wording, you need command-word calibration. That is how Acids and bases (A-level only) becomes a controlled revision target rather than another page in a folder.
Lost marks → repair task
Why marks are usually lost here
These are the error patterns StudyVector looks for after an attempt. The goal is not a generic explanation; it is one repair move and one follow-up question.
Unit, formula, or method slip
Examiner move: Select the correct method and keep units, substitutions, signs, and rounding visible.
Repair drill: Redo the calculation or method line slowly, naming the formula before substituting values.
Missing chain of reasoning
Examiner move: Show the link between point, method, evidence, and conclusion instead of jumping to the final line.
Repair drill: Write the missing because/therefore step, then retry one isomorphic question.
Command-word miss
Examiner move: Answer the action in the command word before adding extra detail.
Repair drill: 60-second rewrite: start the answer with explain, compare, evaluate, state, or calculate in mind.
Mini quiz
Use these checks before full practice. They test topic recognition, exam technique, and whether you can connect the explanation to a marked response.
1. What should you check first when a Acids and bases (A-level only) question appears in A-Level Chemistry?
- A.The command word and the exact topic focus
- B.The longest paragraph in your notes
- C.A memorised answer from a different topic
2. Which revision action gives the strongest evidence that Acids and bases (A-level only) is improving?
- A.Rereading the explanation twice
- B.Answering a timed exam-style question and reviewing lost marks
- C.Highlighting every key phrase in the topic notes
Sample questions
Topic-specific public question previews are still being reviewed. We keep them off public pages until the topic match is safe.
Exam tips
- Read the command word carefully — "explain" needs reasons; "state" expects a short fact.
- For Acids and bases (A-level only), show structured working even when you are practising multiple choice — it builds accuracy under time pressure.
- Mark yourself against the mark scheme style: one clear point per mark, in logical order.
- Come back to this topic after a day or two; short spaced reviews beat one long cram.
Worked examples
Example 1
Modelled exam response
Calculate the pH of a 0.100 mol dm-3 solution of ethanoic acid (CH3COOH), given that its Ka = 1.74 x 10^-5 mol dm-3. Step 1: Write the Ka expression: Ka = [H+][CH3COO-]/[CH3COOH]. Step 2: Make the approximation that [H+] = [CH3COO-] and that the equilibrium concentration of [CH3COOH] is approximately the initial concentration (0.100 M). Step 3: Rearrange to find [H+]: [H+] = sqrt(Ka * [CH3COOH]) = sqrt(1.74e-5 * 0.100) = 1.32 x 10^-3 mol dm-3. Step 4: Calculate pH: pH = -log10([H+]) = -log10(1.32e-3) = 2.88.
Example 2
Identify the task before answering
Question type: a Acids and bases (A-level only) prompt asks for a clear response in A-Level Chemistry. Step 1: underline the command word. Step 2: name the exact part of Acids and bases (A-level only) being tested. Step 3: decide whether the mark scheme wants a definition, method, explanation, comparison, or calculation. Why it works: most weak answers fail before the content starts because they answer the topic generally rather than the exact exam task.
Example 3
Turn feedback into a repair task
Suppose your answer shows partial understanding but loses marks for precision. First, rewrite the missing mark as a short target: "I need to state the mechanism, unit, reason, or evidence explicitly." Then answer one similar question without notes. Finally, compare the second attempt with the first and check whether the same mark was recovered. Why it works: Acids and bases (A-level only) improves faster when feedback creates a specific retry, not another passive reading session.
Next revision routes from this subject
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Common mistakes
- Confusing strong/weak with concentrated/dilute. A strong acid fully dissociates in water, whereas a weak acid only partially dissociates. Concentration refers to the amount of acid dissolved in a given volume of water.
- Incorrectly setting up Ka expressions. The expression for a weak acid HA is Ka = [H+][A-]/[HA], with concentrations being the equilibrium values, not the initial ones.
- Assuming the pH of a neutral solution is always 7. The pH of a neutral solution is only 7 at 298K (25°C). Since the autoionisation of water is endothermic, Kw increases with temperature, and the pH of a neutral solution decreases (e.g., to 6.77 at 313K).
Exam board notes
All boards require proficiency in pH, Ka, and buffer calculations. AQA often includes questions involving titration curves, asking students to identify buffer regions and choose appropriate indicators. Edexcel may focus on the application of buffers in biological systems, such as the carbonic acid-hydrogencarbonate buffer in blood. OCR frequently tests the calculation of pH for a variety of acidic and basic solutions, including diprotic acids.
FAQs
What is a buffer solution?
A buffer solution is a mixture that resists changes in pH when small amounts of acid or alkali are added. It is typically made from a weak acid and its conjugate base (e.g., ethanoic acid and sodium ethanoate), or a weak base and its conjugate acid.
How does a buffer solution work?
It contains a reservoir of the weak acid (to react with added alkali) and its conjugate base (to react with added acid). These reactions remove the added H+ or OH- ions, thus keeping the pH of the solution relatively stable.
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Full practice set
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