Acidity comes from hydrogen ions, H⁺(aq), and alkalinity comes from hydroxide ions, OH⁻(aq). A question that asks “why is this solution acidic?” expects you to name the ion, not only the colour. The same idea explains why some acids react faster than others even at the same concentration.
This lesson follows using an indicator result to classify a solution and belongs to the acids, bases and salts module.
What makes an acid an acid?
An acid is a substance that donates hydrogen ions (protons). In water, hydrogen chloride ionises:
HCl → H⁺ + Cl⁻
An alkali is a soluble base that releases hydroxide ions in water, for example sodium hydroxide:
NaOH → Na⁺ + OH⁻
A base is any substance that neutralises an acid, and it accepts hydrogen ions. An alkali is a base that dissolves in water. All alkalis are bases, but not all bases are alkalis, because copper(II) oxide is a base that does not dissolve.
Check both equations with atoms and charge. On the left of the first equation there is 1 H and 1 Cl, and on the right there is 1 H⁺ and 1 Cl⁻. The charges cancel (+1 and −1), so it balances.
What is the difference between strong and weak?
A strong acid ionises completely in water, and a weak acid ionises only partly. Ethanoic acid is weak, and its ionisation is reversible:
CH₃COOH ⇌ CH₃COO⁻ + H⁺
In a solution of ethanoic acid, most of the molecules stay as CH₃COOH. Hydrochloric acid has almost no HCl molecules left.
Strength is about the fraction ionised. Concentration is about how much acid is dissolved per dm³. These are independent properties.
Worked example
The values below are invented for teaching, but they follow the model you use in class. Two solutions each have volume 25.0 cm³ and concentration 0.100 mol/dm³.
| Solution | Acid | Approximate pH |
|---|---|---|
| X | Hydrochloric acid | 1 |
| Y | Ethanoic acid | 3 |
Question: X and Y each react with excess magnesium. Compare the rate at the start and the total volume of hydrogen made.
Step 1, amount of acid in each: 0.0250 dm³ × 0.100 mol/dm³ = 0.00250 mol of acid, for both.
Step 2, rate at the start: X has a much higher concentration of H⁺ because it is fully ionised (pH 1 means 0.1 mol/dm³ of H⁺). Y has a much lower H⁺ concentration. So X reacts faster at the start.
Step 3, total gas: both hold 0.00250 mol of acid. Weak acids keep ionising as H⁺ is used up, so all of it eventually reacts. The equation is Mg + 2HCl → MgCl₂ + H₂ and Mg + 2CH₃COOH → Mg(CH₃COO)₂ + H₂. Each needs the same 2:1 ratio, so the same amount of hydrogen forms (0.00125 mol). The final volume is the same.
Verify by ratio: 0.00250 mol acid ÷ 2 = 0.00125 mol H₂ in each case.
The mistake to watch for
Mistaken answer: “Y is a weaker acid, so it makes less hydrogen.”
The student linked weak with less product.
Strength changes how fast the acid reacts. The amount of hydrogen depends on the amount of acid available (moles), not on how many ions are free at the start. The correction is to ask two separate questions: “how fast?” (ion concentration) and “how much?” (moles of acid).
Check yourself
1. Write the ionisation of nitric acid, HNO₃, in water as a strong acid.
Show answer
HNO₃ → H⁺ + NO₃⁻. The charges balance (+1 and −1) and each atom appears once on both sides.
2. Solution P is 0.50 mol/dm³ of a weak acid. Solution Q is 0.010 mol/dm³ of a strong acid. Which is the weaker acid, and which is the more dilute?
Show answer
P is the weaker acid (strength is about ionisation) and Q is the more dilute (concentration is 0.010 mol/dm³ compared with 0.50 mol/dm³).
3. Sulfuric acid is treated in the simple model as fully ionised into 2H⁺ and SO₄²⁻. What is the H⁺ concentration in 0.050 mol/dm³ H₂SO₄?
Show answer
Each formula unit gives 2 H⁺, so [H⁺] = 2 × 0.050 = 0.10 mol/dm³. This is the simplified model used for school calculations.
Where this leads next
The next step is to use ion ideas to separate two changes that both lower acidity, in distinguishing neutralisation from dilution. The mole and equation-ratio tutor and the equation balance reasoning trainer let you check ratios and atom counts. Try the practice set when you are ready.
If particle explanations stay vague in your written answers, see how individual teaching works in online one-to-one Chemistry tuition.