Strong and weak acids and bases, and the pH scale

IB Chemistry SL· Reactivity 3.1· 12 min read

1. Defining Strong vs Weak Acids and Bases★★☆☆☆⏱ 10 min

📘 Definition

Strong and Weak Electrolytes

Compounds that fully dissociate into ions in water are strong electrolytes, while those that only partially ionise are weak electrolytes. All strong acids and bases are strong electrolytes, all weak acids and bases are weak electrolytes.

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📐 Worked Example

Classify each of the following as strong or weak acid: HCl, CH₃COOH, HNO₃, H₂CO₃

1
Step 1: Recall the 6 common strong acids specified for IB SL exams
2
Step 2: HCl (hydrochloric) and HNO₃ (nitric) are on the strong acid list, fully dissociated in water
3
Step 3: CH₃COOH (ethanoic acid, vinegar) and H₂CO₃ (carbonic acid, soda) are not on the list, so they are weak, partially dissociated at equilibrium

✓ Quick check

Test your understanding of dissociation:

Which of the following 0.1 M solutions has the highest number of free ions?
- Weak ethanoic acid
- Strong hydrochloric acid
- Weak ammonia
- Ethanol (non-acid)
Reveal answer
Strong hydrochloric acid —
Strong HCl fully dissociates into H+ and Cl-, giving twice as many ions as the original undissociated acid molecules.

2. Observable Properties to Distinguish Acid/Base Strength★★★☆☆⏱ 12 min

For equal molar concentrations of acids or bases, you can tell strong and weak variants apart using three simple, measurable observations: electrical conductivity, rate of reaction with active metals or carbonates, and pH value.

Property	Equal [ ] Strong Acid	Equal [ ] Weak Acid
Electrical conductivity	High (high ion concentration)	Low (low ion concentration)
Reaction rate with Mg	Fast, vigorous bubbling	Slow, gentle bubbling
pH value	Low (higher H+ concentration)	Higher (lower H+ concentration)

📐 Worked Example

You have two unlabelled 0.1 mol dm⁻³ solutions of HCl and CH₃COOH. Describe one simple test to tell them apart, no pH meter allowed.

1
Step 1: Take 10 cm³ of each solution in separate identical beakers
2
Step 2: Add a 1 cm strip of magnesium ribbon to each beaker at the same time
3
Step 3: The solution that produces bubbles of hydrogen gas much faster is the strong HCl, the slower reaction is weak CH₃COOH

3. The pH Scale and Calculations for Strong Species★★★☆☆⏱ 15 min

📘 Definition

pH

$p H = - lo g_{10} [H_{(a q)}^{+}]$

A base-10 logarithmic scale that quantifies the hydrogen ion concentration of an aqueous solution, ranging from 0 (very acidic) to 14 (very basic) at 25°C.

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🔬 Derivation

Goal:

Derive pH of 0.1 mol dm⁻³ HCl

Starting from:

HCl is strong, fully dissociates: $H C l \to H^{+} + C l^{-}$

1
1 mole of HCl produces 1 mole of H+ ions, so $[H^{+}] = 0.1$ mol dm⁻³
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Substitute into pH formula: $p H = - lo g_{10} (0.1)$
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$- lo g_{10} (1 0^{- 1}) = 1$

Result:

pH of 0.1 M HCl is exactly 1

📐 Worked Example

Calculate the pH of 0.05 mol dm⁻³ NaOH, a strong monobasic base at 25°C, given $K_{w} = 1.0 \times 1 0^{- 14}$

1
Step 1: NaOH fully dissociates, so $[O H^{-}] = 0.05$ mol dm⁻³
2
Step 2: Use $[H^{+}] = K_{w} / [O H^{-}] = 1.0 \times 1 0^{- 14} /0.05 = 2 \times 1 0^{- 13}$
3
Step 3: $p H = - lo g_{10} (2 \times 1 0^{- 13}) = 12.7$

4. Reference of Common Strong and Weak Species★★☆☆☆⏱ 8 min

Category	Common Strong Examples	Common Weak Examples
Acids	HCl, H₂SO₄, HNO₃	CH₃COOH, H₂CO₃, H₃PO₄
Bases	NaOH, KOH, LiOH	NH₃, CH₃NH₂, Na₂CO₃

✓ Quick check

Confirm you can identify species correctly:

Which of the following will have a pH > 7 at 0.1 M concentration?
- Nitric acid
- Sodium hydroxide
- Hydrochloric acid
- Ethanoic acid
Reveal answer
Sodium hydroxide —
NaOH is a strong base, so it will produce a pH of 13 at 0.1 M concentration.

5. Common Pitfalls

Wrong move:

Confusing acid concentration with acid strength

Why:

Concentration is how many moles you have per litre, strength is how much of those moles dissociate, two independent variables

Correct move:

Always check if the question specifies equal concentration before comparing strength directly to pH

Wrong move:

Stating weak acids have no H+ ions

Why:

Weak acids partially dissociate, they just have lower H+ concentration than equal concentration strong acids

Correct move:

Always write that weak acids form an equilibrium with partial ionisation, not zero ionisation

Wrong move:

Trying to calculate pH of a weak acid at SL exam

Why:

Weak acid pH requires Ka values which are not given or tested at SL level

Correct move:

For SL, if you see a weak acid, you only need to estimate its pH is higher than equal concentration strong acid

Wrong move:

Forgetting pH is a log scale

Why:

A difference of 1 pH unit means 10x difference in H+ concentration, not 2x

Correct move:

When asked how much more acidic pH 2 is than pH 4, the answer is 100x, not 2x

Wrong move:

Including water as a strong acid

Why:

Water is a neutral substance with very low dissociation, not classified as strong or weak acid in IB syllabus

Correct move:

Only use the 5 strong acids (HCl, HBr, HI, HNO₃, H₂SO₄) named in the syllabus and group 1 hydroxides as strong bases for your answers

6. Quick Reference Cheatsheet

Species Type	Dissociation Behaviour	pH at 0.1 M 25°C
Strong monoprotic acid	100% dissociation	1
Weak monoprotic acid	Partial equilibrium dissociation	2-3
Strong monobase	100% dissociation	13
Weak base	Partial equilibrium dissociation	11-12

7. Frequently Asked

Why can two acids of the same concentration have different pH values?

If one acid is strong and the other is weak, the strong acid fully dissociates to produce a higher $[H^{+}]$ , leading to a lower pH, even if their total molar concentration is identical. For example 0.1 M HCl (strong) has pH 1, while 0.1 M ethanoic acid (weak) has pH ~2.9.

Do strong acids always have a lower pH than weak acids?

No. A very dilute strong acid can have a higher pH than a concentrated weak acid. pH depends on both the degree of dissociation (strength) and the total molar concentration of the acid.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2025 · 2
Compare strong and weak acid properties
2024 · 1
Identify highest pH equal concentration solution
2023 · 2
Calculate pH of 0.1 mol/dm³ HCl

Going deeper

practice_questionsIB Chem SL Acids Bases Past Question BankCurated 20+ past paper questions on this topic

What's Next

Now that you have mastered the core differences between strong and weak acids, bases, and the pH scale, you are ready to move on to Ka calculations for weak acids (HL only) and buffer solutions, as well as exploring titration curves which build directly on these pH calculation skills. This topic is a foundational prerequisite for 15-20% of the marks in IB Chemistry SL Paper 2 Section B acid-base questions, so make sure you practice distinguishing strong and weak species in multiple choice questions to avoid easy lost marks on exam day.