You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
Before the twentieth century, chemists recognised acids by their sour taste, their ability to turn litmus red, and their reactions with metals. Bases were substances that neutralised acids. These observations were useful, but they did not explain why these substances behaved the way they did.
In 1923, Johannes Bronsted and Thomas Lowry independently proposed a definition that has dominated chemistry ever since. Their framework is elegant and powerful:
This definition does not require water. It applies to reactions in the gas phase, in organic solvents, and in aqueous solution alike. It is the definition you need for Edexcel A-Level Chemistry.
The older Arrhenius definition (acid = produces H⁺ in water, base = produces OH⁻ in water) is more limited because it only works in aqueous solutions. The Bronsted-Lowry definition is broader and explains reactions that the Arrhenius model cannot, such as the reaction of HCl gas with NH₃ gas to produce NH₄Cl smoke.
Consider the reaction between hydrochloric acid and water:
HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq)
HCl donates a proton to water, so HCl is acting as an acid. Water accepts the proton, so water is acting as a base. The product H₃O⁺ is the hydroxonium ion (sometimes called the hydronium ion), and it is the species responsible for acidic properties in aqueous solution.
Now consider ammonia dissolving in water:
NH₃(g) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq)
Here, ammonia accepts a proton from water, so ammonia is the base. Water donates a proton, so water is acting as an acid. Notice that water can be either an acid or a base depending on the reaction partner — this makes water amphoteric (or amphiprotic).
Why the equilibrium sign matters: The double arrow (⇌) for ammonia tells you the reaction does not go to completion. Most NH₃ molecules remain unreacted at equilibrium. The single arrow (→) for HCl tells you the reaction goes essentially to completion. This distinction between complete and incomplete proton transfer is the foundation of the strong/weak acid classification.
Every Bronsted-Lowry acid-base reaction involves the transfer of a proton from one species to another. When an acid donates a proton, it forms its conjugate base. When a base accepts a proton, it forms its conjugate acid.
For the HCl example:
| Species | Role | Conjugate |
|---|---|---|
| HCl | Acid | Cl⁻ (conjugate base) |
| H₂O | Base | H₃O⁺ (conjugate acid) |
For the ammonia example:
| Species | Role | Conjugate |
|---|---|---|
| H₂O | Acid | OH⁻ (conjugate base) |
| NH₃ | Base | NH₄⁺ (conjugate acid) |
A conjugate acid-base pair differs by exactly one proton. This concept is central to understanding buffer solutions later in this course.
Identifying conjugate pairs in an equation: In any proton-transfer reaction, there are always two conjugate pairs. For the general reaction HA + B ⇌ A⁻ + BH⁺, the pairs are HA/A⁻ and BH⁺/B.
Question: In the reaction HSO₄⁻(aq) + H₂O(l) ⇌ SO₄²⁻(aq) + H₃O⁺(aq), identify the acid, base, and both conjugate pairs.
Solution:
Notice that HSO₄⁻ is acting as an acid here, but in a different reaction it could act as a base (accepting a proton to form H₂SO₄). This makes HSO₄⁻ amphoteric.
A strong acid is one that fully dissociates in aqueous solution. Every molecule donates its proton. Examples include:
The equilibrium lies so far to the right that we write a single forward arrow (→) rather than the equilibrium symbol (⇌).
A weak acid is one that only partially dissociates. At equilibrium, most of the acid molecules remain undissociated. Examples include:
For ethanoic acid:
CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq)
The equilibrium lies well to the left, meaning most molecules remain as CH₃COOH. The extent of dissociation is quantified by the acid dissociation constant Ka, which you will study in detail in Lesson 3.
Why does strength matter? The distinction determines every calculation you will do. For a strong acid, [H⁺] equals the initial acid concentration. For a weak acid, [H⁺] is much less than the initial concentration and must be calculated using Ka.
Similarly, a strong base fully dissociates:
NaOH(s) → Na⁺(aq) + OH⁻(aq)
A weak base only partially accepts protons. Ammonia is the classic example — the equilibrium with water lies to the left, and only a small proportion of NH₃ molecules become NH₄⁺ at any given time.
| Base | Type | Dissociation |
|---|---|---|
| NaOH | Strong | NaOH → Na⁺ + OH⁻ (100%) |
| KOH | Strong | KOH → K⁺ + OH⁻ (100%) |
| Ba(OH)₂ | Strong | Ba(OH)₂ → Ba²⁺ + 2OH⁻ (100%) |
| NH₃ | Weak | NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ (~1%) |
| CH₃NH₂ | Weak | CH₃NH₂ + H₂O ⇌ CH₃NH₃⁺ + OH⁻ (~1%) |
Important: Strong and weak refer to the degree of dissociation, not to the concentration. You can have a dilute solution of a strong acid or a concentrated solution of a weak acid.
Water is the most important amphoteric substance, but it is not the only one. The hydrogen carbonate ion, HCO₃⁻, can act as an acid:
HCO₃⁻(aq) + H₂O(l) ⇌ CO₃²⁻(aq) + H₃O⁺(aq)
or as a base:
HCO₃⁻(aq) + H₂O(l) ⇌ H₂CO₃(aq) + OH⁻(aq)
Amino acids are also amphoteric, which is critical to their behaviour at different pH values. The zwitterionic form of an amino acid has both a protonated amine group (NH₃⁺, acting as a potential acid) and a deprotonated carboxyl group (COO⁻, acting as a potential base).
Other amphoteric species you should recognise: HSO₄⁻, H₂PO₄⁻, HPO₄²⁻, and Al(OH)₃.
| Mistake | Correction |
|---|---|
| Calling a weak acid "dilute" | Weak refers to degree of dissociation, not concentration |
| Saying NaOH is a Bronsted-Lowry base because it releases OH⁻ | The Bronsted-Lowry definition says the OH⁻ ion is the base because it accepts protons |
| Confusing conjugate pairs with reactant/product pairs | A conjugate pair differs by exactly one proton |
| Forgetting water can be an acid | Water donates H⁺ to NH₃, acting as a Bronsted-Lowry acid |
| Stating H₂SO₄/SO₄²⁻ is a conjugate pair | They differ by two protons; the correct pair is H₂SO₄/HSO₄⁻ |
| Substance | Acid or Base | Strong or Weak |
|---|---|---|
| HCl | Acid | Strong |
| HNO₃ | Acid | Strong |
| H₂SO₄ | Acid | Strong (1st dissociation) |
| CH₃COOH | Acid | Weak |
| H₂CO₃ | Acid | Weak |
| HF | Acid | Weak |
| NaOH | Base | Strong |
| KOH | Base | Strong |
| Ba(OH)₂ | Base | Strong |
| NH₃ | Base | Weak |
| CH₃NH₂ | Base | Weak |
The Bronsted-Lowry framework underpins everything that follows. pH calculations depend on knowing whether an acid is strong or weak. Buffer solutions rely on conjugate acid-base pairs. Titration curves look different depending on the strengths of the acid and base involved. Indicators are themselves weak acids with coloured conjugate bases.
Understanding proton transfer is the single most important foundation for this entire topic.
flowchart TD
A[Is the substance donating or accepting H⁺?] --> B{Donating H⁺}
A --> C{Accepting H⁺}
B --> D[It is an ACID]
C --> E[It is a BASE]
D --> F{Does it fully dissociate?}
E --> G{Does it fully accept protons?}
F --> H[Yes = STRONG ACID]
F --> I[No = WEAK ACID]
G --> J[Yes = STRONG BASE]
G --> K[No = WEAK BASE]
Edexcel 9CH0 specification, Topic 12: Acid-base equilibria, sub-strand 12.1 establishes the Bronsted-Lowry framework as the operational definition of acids and bases for the whole topic (refer to the official specification document for exact wording). Sub-strand 12.1 requires you to define an acid as a proton donor and a base as a proton acceptor, identify conjugate acid-base pairs in any proton-transfer equation, distinguish strong and weak acids by the extent of dissociation (not by concentration), and recognise amphoteric species such as water and HSO₄⁻. Although introduced as a Topic 12 idea, the Bronsted-Lowry lens is examined throughout Paper 1 (Topics 1–5, 9, 11–15) and Paper 3 (synoptic + practicals); proton-transfer reasoning underlies organic carboxylic-acid chemistry (Topic 17), nitrogen chemistry (amines, Topic 18), and the analytical interpretation of titration curves (CP2, CP13). The Edexcel data booklet does not list conjugate pairs — they must be reasoned from structures.
Question (8 marks):
(a) Ethanoic acid is a weak acid. Hydrochloric acid is a strong acid. Both are described as monoprotic. Explain, with equations, what is meant by the terms Bronsted-Lowry acid, strong acid, and weak acid, and identify the two conjugate acid-base pairs in the dissociation of ethanoic acid in water. (5)
(b) The hydrogen sulfate ion, HSO₄⁻, can behave as an acid or as a base. Write equations for each behaviour in aqueous solution and use them to explain the term amphoteric. (3)
Solution with mark scheme:
(a) Step 1 — define Bronsted-Lowry acid.
A Bronsted-Lowry acid is a proton (H⁺) donor; a Bronsted-Lowry base is a proton acceptor.
M1 — proton-donor wording. Examiners insist on the word proton (or H⁺); writing "H⁺ giver" or "loses a hydrogen" is acceptable but writing "donates electrons" is the Lewis definition and earns nothing.
Step 2 — distinguish strong vs weak by extent of dissociation.
A strong acid fully dissociates in aqueous solution; effectively every molecule donates its proton, so the dissociation is written with a single arrow (→). A weak acid only partially dissociates; the equilibrium lies to the left, so the dissociation is written with the equilibrium symbol (⇌).
M1 — explicitly contrasts full vs partial dissociation; A1 — links strength to extent of dissociation, not to concentration.
Step 3 — write the dissociation of ethanoic acid and identify conjugate pairs.
CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq)
The two conjugate acid-base pairs are:
M1 — correct equation including state symbols and equilibrium arrow. A1 — both conjugate pairs identified, each pair differing by exactly one H⁺.
(b) Step 1 — HSO₄⁻ as an acid.
HSO₄⁻(aq) + H₂O(l) ⇌ SO₄²⁻(aq) + H₃O⁺(aq)
M1 — correct equation with HSO₄⁻ donating a proton.
Step 2 — HSO₄⁻ as a base.
HSO₄⁻(aq) + H₂O(l) ⇌ H₂SO₄(aq) + OH⁻(aq)
(The water acts as the acid here, donating a proton to HSO₄⁻.)
M1 — correct equation with HSO₄⁻ accepting a proton.
Step 3 — define amphoteric.
A species is amphoteric if it can act either as a Bronsted-Lowry acid (donating a proton) or as a Bronsted-Lowry base (accepting a proton).
A1 — explicit linkage to both donor and acceptor roles in the same species.
Total: 8 marks (M5 A3, split as shown).
Question (6 marks): A student is given samples of three colourless solutions of equal concentration: 0.10 mol dm⁻³ HCl, 0.10 mol dm⁻³ CH₃COOH, and 0.10 mol dm⁻³ NH₃.
(a) Identify which of the three is a weak base and explain, in terms of proton transfer and equilibrium position, what is meant by weak. (2)
(b) Compare the [H⁺] of the HCl solution with that of the CH₃COOH solution at the same concentration, and justify the comparison using the Bronsted-Lowry framework. (4)
Mark scheme decomposition by AO:
(a)
(b)
Total: 6 marks split AO1 = 4, AO2 = 2. Edexcel uses the strong/weak distinction repeatedly to test whether candidates can decouple concentration from strength — confusing the two is the single most-penalised idea in Topic 12.
Connects to:
Topic 12.2 — pH calculations: the Bronsted-Lowry definition determines whether you can use [H⁺] = c (strong acid) or must invoke Ka (weak acid). Without classifying the acid first, no pH calculation is possible.
Topic 12.3 — Ka and pKa: the equilibrium constant for the proton-transfer reaction is Ka. Conjugate-pair language carries directly into the Ka expression: stronger acid ⇒ weaker conjugate base ⇒ smaller pKb of the conjugate.
Topic 12.4 — Buffer composition: every buffer is a conjugate acid-base pair (HA / A⁻ or B / BH⁺). Identifying conjugate pairs in this lesson is the prerequisite for buffer chemistry two lessons later.
Topic 17 — Carboxylic acids: RCOOH compounds are weak acids whose acidity depends on the stability of the conjugate base RCOO⁻ (delocalisation of negative charge over both oxygens). The Bronsted-Lowry framework explains why electron-withdrawing R groups (e.g. CCl₃ in trichloroethanoic acid) produce stronger acids.
Topic 18 — Amines: primary, secondary, and tertiary amines are weak bases by the Bronsted-Lowry definition; their basicity correlates with the stability of the conjugate acid RNH₃⁺ (alkyl groups donate electron density, stabilising the positive charge).
Bronsted-Lowry questions on 9CH0 Paper 1 split AO marks as follows:
| AO | Typical share | Earned by |
|---|---|---|
| AO1 (knowledge / definitions) | 50–60% | Stating the proton-donor/acceptor definition; identifying acid/base/conjugate pairs in a given equation |
| AO2 (application / interpretation) | 30–40% | Linking strong/weak to equilibrium position; predicting [H⁺] from extent of dissociation; explaining amphoteric behaviour |
| AO3 (analysis / evaluation) | 0–10% | Comparing systems at equal concentration but different strength; interpreting amphoteric behaviour in unfamiliar contexts |
Examiner-rewarded phrasing: "fully dissociates / completely dissociates"; "partially dissociates / equilibrium lies to the left"; "differ by one proton (H⁺)"; "weak refers to the extent of dissociation, not the concentration". Phrases that lose marks: "HCl is more concentrated than CH₃COOH" (concentration ≠ strength); writing the dissociation of a strong acid with a reversible arrow (⇌); calling NaOH a Bronsted-Lowry base directly (it is the OH⁻ ion that accepts the proton).
A specific Edexcel pattern: questions phrased "explain the term …" demand a definition and a brief example or context. Bare definitions can score the M1 but rarely the A1 unless they reference the question's substance.
Question: State what is meant by a Bronsted-Lowry acid and identify the conjugate base of HNO₃.
Grade C response (~120 words):
A Bronsted-Lowry acid is a proton donor. The conjugate base of HNO₃ is NO₃⁻ because it differs from HNO₃ by one proton.
Examiner commentary: Full marks (3/3). Concise, technically correct. The candidate names the proton-donor definition, identifies the conjugate base correctly, and explains why (differs by one proton). The wording "differs by one proton" is the standard mark-scheme phrasing, so this answer would clearly hit all three marks. Many candidates over-write here, citing Arrhenius alongside Bronsted-Lowry — that wastes time but does not lose marks.
Grade A response (~180 words):*
A Bronsted-Lowry acid is a species that donates a proton (H⁺) to a base; the donation may occur in aqueous, gaseous, or non-aqueous solution. When HNO₃ donates its proton, the resulting species is its conjugate base, NO₃⁻. The pair HNO₃ / NO₃⁻ differs by exactly one H⁺ and is therefore a conjugate acid-base pair. Because HNO₃ fully dissociates in aqueous solution, NO₃⁻ is a very weak conjugate base — the equilibrium for proton transfer lies essentially completely to the right.
Examiner commentary: Full marks (3/3). Beyond the procedural answer, the candidate signals examiner-aware sophistication: the "(H⁺)" parenthetical removes ambiguity, the "in aqueous, gaseous, or non-aqueous solution" clause shows breadth, and the closing remark on conjugate-base weakness anticipates Lesson 3 (Ka). This is presentation craft, not extra content — and on longer questions exactly this craft converts B/A answers into A*.
Question: Methylamine, CH₃NH₂, dissolves in water to give a weakly basic solution.
(a) Write an equation, including state symbols, for the reaction of methylamine with water and identify both conjugate acid-base pairs. (3) (b) Explain, using the equation, why an aqueous solution of methylamine is described as weakly basic and not dilute. (3)
Grade B response (~210 words):
(a) CH₃NH₂(aq) + H₂O(l) ⇌ CH₃NH₃⁺(aq) + OH⁻(aq). The conjugate pairs are CH₃NH₂/CH₃NH₃⁺ and H₂O/OH⁻.
(b) Methylamine is a weak base because not all the molecules accept a proton from water. The equilibrium is to the left so most CH₃NH₂ stays unreacted. This is different from being dilute, which means the concentration is low.
Examiner commentary: Part (a) earns 3/3 — equation, state symbols, equilibrium arrow, both conjugate pairs correctly assigned. Part (b) is good but loses a mark on precision: the candidate writes "the equilibrium is to the left" without saying why (because the proton-transfer step is unfavourable for a weak base) and does not explicitly link the fraction protonated to the term "weak". They also state "dilute means low concentration" which is correct but trivial. Total: 5/6. The examiner wants the fraction word.
Grade A response (~290 words):*
(a) CH₃NH₂(aq) + H₂O(l) ⇌ CH₃NH₃⁺(aq) + OH⁻(aq).
The two conjugate acid-base pairs are:
Both pairs differ by exactly one H⁺.
(b) The equilibrium lies far to the left because the proton transfer from H₂O to CH₃NH₂ is energetically unfavourable; only a small fraction of methylamine molecules are protonated at any instant. Weak refers to this small fraction (the extent of dissociation/protonation), independent of how much methylamine was initially dissolved. Dilute, by contrast, refers to concentration — the number of moles of methylamine per dm³ of solution. A 0.1 mol dm⁻³ solution of CH₃NH₂ and a 1.0 mol dm⁻³ solution are both weak bases (same fraction protonated), but the latter is more concentrated. The two descriptors are independent variables.
Examiner commentary: Full marks (6/6). The candidate writes the equation cleanly, identifies pairs with role labels (acid/base), and in (b) makes the orthogonality of strength and concentration completely explicit with a numerical example. The phrase "energetically unfavourable" hints at thermodynamic reasoning (Topic 16) without overshooting the question. This is the answer Edexcel rewards with the final A1 — the explicit "two independent variables" framing.
Question: Discuss the Bronsted-Lowry framework for acids and bases. Your answer should: (i) define the framework and contrast it with the Arrhenius definition; (ii) explain conjugate acid-base pairs using two worked examples; (iii) discuss the difference between strong/weak and concentrated/dilute. (9)
Grade A response (~430 words):*
The Bronsted-Lowry framework defines an acid as a proton (H⁺) donor and a base as a proton acceptor. This is broader than the Arrhenius definition (acid produces H⁺ in water; base produces OH⁻ in water), because Bronsted-Lowry applies to any solvent (or none) — the gas-phase reaction HCl(g) + NH₃(g) → NH₄Cl(s) is a Bronsted-Lowry acid-base reaction even though no water is involved.
Every proton-transfer reaction creates two conjugate acid-base pairs differing by exactly one H⁺. Example 1: HCl(aq) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq). HCl donates a proton to H₂O, giving the pairs HCl/Cl⁻ and H₃O⁺/H₂O. Example 2: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq). Now H₂O acts as the acid (donating H⁺) and NH₃ as the base, giving pairs NH₃/NH₄⁺ and H₂O/OH⁻. Water appears in both examples, in opposite roles — water is amphoteric.
Strong and weak describe the extent of dissociation. A strong acid fully dissociates: every HCl molecule transfers its proton to water, so [H⁺] equals the initial acid concentration. A weak acid only partially dissociates: in 0.1 mol dm⁻³ ethanoic acid, only roughly 1% of molecules transfer their proton at equilibrium, so [H⁺] ≈ 1.3 × 10⁻³ mol dm⁻³ rather than 0.1 mol dm⁻³.
Concentrated and dilute describe the amount of solute per unit volume. These two descriptors are independent. A 0.001 mol dm⁻³ HCl solution is strong but dilute; a 5 mol dm⁻³ ethanoic acid solution is weak but concentrated. Confusing the two is the most-penalised error in Topic 12 — examiners look for explicit acknowledgement that strength reflects the fraction of molecules dissociated, not the concentration of acid in solution.
Examiner commentary: Full marks (9/9). The candidate covers all three required parts (i)–(iii) with worked examples, clean equations, and a quantitative illustration of the strong-but-dilute / weak-but-concentrated distinction. The closing reference to "the most-penalised error in Topic 12" is editorial framing that signals the candidate has internalised the assessor's priorities. The use of "amphoteric" in passing (without overclaiming the term as a separate idea) shows synthesis. This is the kind of answer that earns synoptic-band marks and suggests A* across the paper.
The errors that distinguish A from A* on Bronsted-Lowry questions:
Confusing "Lewis acid" with "Bronsted-Lowry acid". A Lewis acid is an electron-pair acceptor; a Bronsted-Lowry acid is a proton donor. Every Bronsted-Lowry acid is a Lewis acid (the proton accepts an electron pair from the base), but not every Lewis acid is Bronsted-Lowry (e.g. AlCl₃, BF₃ have no proton). A-Level uses Bronsted-Lowry exclusively.
Calling NaOH a Bronsted-Lowry base. NaOH is an ionic compound that dissociates to give Na⁺ and OH⁻. It is the OH⁻ ion that accepts the proton. Strictly, NaOH is an Arrhenius base; the Bronsted-Lowry base is OH⁻.
Forgetting that water can be either acid or base. In HCl + H₂O, water is the base; in NH₃ + H₂O, water is the acid. Students often default to "water is a base" because they remember H₃O⁺ first.
Mis-identifying conjugate pairs that differ by two protons. H₂SO₄/SO₄²⁻ is not a conjugate pair because they differ by two H⁺. The correct pairs are H₂SO₄/HSO₄⁻ and HSO₄⁻/SO₄²⁻.
Writing reversible arrows for strong acid dissociation. HCl + H₂O → H₃O⁺ + Cl⁻ uses a single arrow because dissociation is essentially complete. Using ⇌ here implies an equilibrium that does not exist in any practical sense.
Conflating "strong" with "corrosive". Concentrated weak acids (e.g. 5 mol dm⁻³ ethanoic acid) can be highly corrosive; dilute strong acids (e.g. 0.001 mol dm⁻³ HCl) are not. Strength refers to dissociation, not hazard.
Forgetting amphoteric species in equilibrium contexts. HCO₃⁻ acts as both acid and base in physiological systems; ignoring this when discussing blood-buffer chemistry costs synoptic marks in Lesson 5.
Three patterns repeatedly cost candidates marks on Paper 1 Bronsted-Lowry questions. They are about precision of language.
This pattern is endemic to Paper 1 Topic 12 questions: candidates know the chemistry, lose marks on the language.
The Bronsted-Lowry framework opens onto several undergraduate trajectories:
Oxbridge interview prompt: "Why is HCl a strong acid in water but a weak acid in pure ethanoic acid solvent? What does this tell you about the role of the solvent in Bronsted-Lowry chemistry?"
Although the Bronsted-Lowry framework is conceptual rather than practical, it underpins Core Practical 2 (Measurement of an enthalpy change, when applied to neutralisation reactions) and Core Practical 13 (Finding the Ka of a weak acid by titration). In CP2, the reaction HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) is a Bronsted-Lowry acid-base reaction; the enthalpy of neutralisation is essentially constant (≈ −57 kJ mol⁻¹) for strong acid + strong base combinations because the only effective reaction is H⁺(aq) + OH⁻(aq) → H₂O(l). Pairings involving a weak acid or base give numerically smaller (less negative) enthalpies of neutralisation because some of the released energy goes into completing the dissociation of the weak species — a Bronsted-Lowry interpretation. In CP13, the choice of indicator and the shape of the titration curve depend on the strong/weak classification of both titrant and analyte, which is the central pedagogic point of this lesson. Examiners often link conceptual knowledge of Bronsted-Lowry directly to practical interpretation in 6-mark CP-linked questions on Paper 3.
This content is aligned with the Pearson Edexcel GCE A Level Chemistry (9CH0) specification, Topic 12: Acid-base equilibria (sub-strand 12.1). The Bronsted-Lowry framework is examined in Paper 1 (Topics 1–5 and 9 inclusive of acid-base concepts) and Paper 3 (synoptic and practical-skills paper). For the most accurate and up-to-date information, please refer to the official Pearson Edexcel specification document.
graph TD
A["Proton transfer:<br/>HA + B ⇌ A⁻ + BH⁺"] --> B{"Which species<br/>donates H⁺?"}
B -->|"HA donates"| C["HA is the ACID<br/>A⁻ is conjugate base"]
B -->|"B accepts"| D["B is the BASE<br/>BH⁺ is conjugate acid"]
C --> E{"Extent of<br/>dissociation?"}
D --> F{"Extent of<br/>protonation?"}
E -->|"Complete (→)"| G["STRONG ACID<br/>e.g. HCl, HNO₃"]
E -->|"Partial (⇌)"| H["WEAK ACID<br/>e.g. CH₃COOH"]
F -->|"Complete (→)"| I["STRONG BASE<br/>e.g. NaOH (via OH⁻)"]
F -->|"Partial (⇌)"| J["WEAK BASE<br/>e.g. NH₃, RNH₂"]
G --> K["[H⁺] = c"]
H --> L["[H⁺] from Ka<br/>(Lesson 3)"]
I --> K
J --> L
style C fill:#27ae60,color:#fff
style D fill:#27ae60,color:#fff
style L fill:#3498db,color:#fff