Two beakers, same volume, same concentration, same label on the bottle: 0.100 mol dm−3. Drop identical bits of magnesium in and one fizzes like mad while the other barely bubbles. Nothing has been diluted. The difference is how many of the acid molecules have actually let go of their protons.
📘 What you need to know
A strong acid dissociates almost completely in water. Write it with a single arrow.
A weak acid dissociates only partly and sets up an equilibrium. Write it with a double arrow.
Strong bases: group 1 hydroxides such as NaOH and KOH. Weak bases: NH3 and amines.
Strong is not the same as concentrated. Strength is about the fraction that splits; concentration is about how much you put in the water.
At the same concentration, a strong acid has a lower pH, higher electrical conductivity and faster reactions than a weak one.
Down group 17 the H–X bond gets longer and weaker, so acid strength rises: HF < HCl < HBr < HI.
What “strong” actually means
Strength is about one thing only: what fraction of the molecules have given up their proton.
Dissolve HCl in water and essentially every molecule hands its proton to a water molecule. Almost nothing is left as HCl. The reverse reaction is so tiny we do not even draw it.
Dissolve ethanoic acid instead and only about one molecule in a hundred lets go. The rest float about intact, and an equilibrium settles down with the position well over to the left.
The arrow you draw is part of the answer. A double arrow on HCl or a single arrow on ethanoic acid tells the examiner you have not understood the difference, even if the rest of the equation is perfect.
Count the red circles. Both beakers started with the same number of acid particles, but the left one has released nearly all its protons while the right one has released one.
Strong is not the same as concentrated
This is the single biggest confusion in the whole topic, so it is worth slowing down.
Strength answers: of the acid molecules present, what fraction have split? It is a property of the substance and you cannot change it by adding water.
Concentration answers: how many moles did you put in each cubic decimetre? It is a property of the solution and you change it every time you dilute.
So a very dilute solution of a strong acid can easily have a higher pH than a concentrated solution of a weak one. The right words for concentration are dilute and concentrated. The right words for strength are strong and weak. Never swap them.
Concentrated ethanoic acid (pH 2.23) is more acidic than dilute hydrochloric acid (pH 2.00 becomes 3.00 on a ten-fold dilution). Strength alone never tells you the pH.
Telling them apart in the lab
Take two acids at the same concentration and there are three practical ways to see which is stronger. All three come back to the same cause: the strong acid has more free H+ ions in solution.
What you measure
Strong acid
Weak acid
Why
pH with a pH meter
lower (about 1 for 0.1 mol dm−3 HCl)
higher (about 2.9 for 0.1 mol dm−3 CH3COOH)
more free H+ gives a lower pH
Electrical conductivity
high
low
more ions in solution carry more current
Fizzing with magnesium
fast, vigorous bubbling
slow, gentle bubbling
rate depends on [H+], so more ions means a faster rate
Fizzing with a carbonate
fast CO2 release
slow CO2 release
same reason — both still give off CO2
Temperature rise on neutralising
larger
smaller
some energy is used up splitting the weak acid
Total volume of alkali needed
the same
the same
the number of moles of acid has not changed
The trap in that last row. A weak acid reacts more slowly, but it does not react less. Given time, 25.0 cm3 of 0.100 mol dm−3 ethanoic acid needs exactly as much NaOH as the same volume of 0.100 mol dm−3 HCl. As H+ is used up, the equilibrium simply shifts right and releases more.
Bases work the same way
Nothing new here, just the mirror image. A strong base is fully ionised in solution and a weak base only partly.
Strong base and weak base
NaOH(aq) → Na+(aq) + OH−(aq)
NH3(aq) + H2O(l) ↔ NH4+(aq) + OH−(aq)
Notice that ammonia has no OH− in its formula at all. It makes hydroxide ions by pulling a proton off a water molecule, and because it is a weak base it only manages that with a small fraction of the ammonia present.
Why acid strength changes down group 17
An acid can only donate its proton if the bond holding it can break. So the strength of a hydrogen halide comes down to the H–X bond.
Going down the group the halogen atom gets bigger, so the bonding pair sits further from the nuclei, the bond gets longer and therefore weaker. A weaker bond breaks more easily, so the proton comes off more easily.
Acid strength increases down the group
HF < HCl < HBr < HI
Students often reach for electronegativity here and argue that HF should be the strongest because F pulls electrons hardest. It is a reasonable thought, but bond strength wins: the H–F bond is so short and strong that HF is the only weak acid of the four.
Worked examples
WORKED EXAMPLE
Equal volumes of 0.100 mol dm−3 HCl and 0.100 mol dm−3 CH3COOH are each given an excess of magnesium ribbon. Compare the rate of reaction and the total volume of hydrogen produced.
Step 1: rate depends on [H+]
HCl is strong so it is fully split; ethanoic acid is weak so only a little is split.
[H+] in HCl is much larger → faster rate.Step 2: total volume depends on moles of acid
Both flasks contain the same number of moles of acid, and the magnesium is in excess.
Step 3: explain why the weak acid still finishes the job
As H+ is used up the equilibrium shifts right, releasing the rest of the protons.
HCl reacts faster, but both give the same total volume of H2rate and amount are two different questions — answer both separately
WORKED EXAMPLE
A student says “sulfuric acid is stronger than hydrochloric acid because the bottle is more concentrated”. Explain what is wrong with this.
Step 1: name the two ideas being mixed up
Strength is the fraction of molecules that dissociate. Concentration is moles per cubic decimetre.
Step 2: show they are independentYou can dilute a strong acid as much as you like and it stays strong.Step 3: give the correct wording
The bottle being more concentrated only tells you there is more acid per dm3.
Concentration says nothing about strength — the correct words are dilute and concentratedexam answers get the mark for the phrase “degree of dissociation”
WORKED EXAMPLE
Two 0.100 mol dm−3 solutions have pH 1.00 and pH 2.90. Identify which is the strong acid and estimate how many times more H+ it contains.
Step 1: turn both pH values into concentrationspH 1.00 → [H+] = 10−1.00 = 0.100 mol dm−3pH 2.90 → [H+] = 10−2.90 = 1.26 × 10−3 mol dm−3Step 2: compare with the concentration on the label
The first one has released all 0.100 mol, so it is fully dissociated.
Step 3: find the ratio0.100 ÷ 1.26 × 10−3 = 79.4pH 1.00 is the strong acid, with about 79 times more H+the giveaway is that [H⁺] matches the label concentration exactly
💡 Exam tip
Use → for strong and ↔ for weak every single time you write a dissociation equation.
The mark-scheme phrase is “degree of dissociation” or “extent of ionisation”. Slip it into any strength answer.
For a strong monoprotic acid, [H+] = [acid], so pH follows in one step. That shortcut does not work for weak acids.
When comparing two acids, always state that they are at the same concentration first. Otherwise the comparison means nothing.
“Same rate?” and “same volume of gas?” are different questions. Weak acids are slower but give the same final amount.
For H2SO4, only the first proton comes off completely. HSO4− is a weak acid, so 1 mol dm−3 H2SO4 is not 2 mol dm−3 in H+.
⚠ Common mix-up
Using “strong” to mean “concentrated”. They are unrelated, and mixing them up is the most common error in this topic.
Saying a weak acid only partly reacts with alkali. It reacts completely — it just takes longer, because the equilibrium keeps topping up the H+.
Thinking dilution makes a strong acid weak. Dilution raises the pH but the acid is still fully dissociated.
Claiming a weak acid gives less gas with magnesium. Same moles of acid, same moles of hydrogen.
Assuming HF is the strongest hydrogen halide. The strong H–F bond makes it the weakest acid of the four.
Treating H2SO4 as giving two protons freely. The second one comes off only partly.
Up next: Neutralisation Reactions. Now that you can tell an acid from a base and strong from weak, we can put them together and see what actually comes out.
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