Higher Tier: acid strength differs from acid concentration
| English | Français |
|---|---|
| acid strength | acid strength |
| partial ionisation | partial ionisation |
What would explain this observation?
- Higher Tier: A dilute strong acid and a concentrated weak acid describe two different comparisons. Strong is about ionisation; concentrated is about how much acid is present per solution volume.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- A strong acid is completely ionised in aqueous solution in this GCSE account. Hydrochloric, nitric and sulfuric acids are the named strong examples. A weak acid is only partially ionised; ethanoic, citric and carbonic acids are named examples. Dilute and concentrated describe acid amount per volume, whereas weak and strong describe degree of ionisation. One term cannot replace the other.
- acid strength 酸强度: The extent of ionisation of an acid in aqueous solution; partial ionisation 部分电离: Ionisation of only some acid particles in the stated aqueous equilibrium.
Which statement correctly distinguishes strength and concentration?
At a given concentration under comparable conditions, the stronger acid has lower pH because it produces greater hydrogen-ion concentration. Each decrease of one pH unit corresponds to a tenfold increase in H⁺ concentration. A decrease of two units means a hundredfold increase. Use whole-number pH comparisons in this source scope; do not treat pH as a linear concentration scale.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- At a given concentration under comparable conditions, the stronger acid has lower pH because it produces greater hydrogen-ion concentration. Each decrease of one pH unit corresponds to a tenfold increase in H⁺ concentration. A decrease of two units means a hundredfold increase. Use whole-number pH comparisons in this source scope; do not treat pH as a linear concentration scale.
- Compare labelled solutions of known identity and concentration using supplied data or teacher-approved dilute samples and a calibrated pH probe. Separate a strength comparison at fixed concentration from a concentration comparison using the same acid. The GCSE strong-acid wording is a model; do not silently turn it into advanced assumptions about every dissociation step of concentrated sulfuric acid.
Which two habits make the investigation or model in this case more defensible?
Compare labelled solutions of known identity and concentration using supplied data or teacher-approved dilute samples and a calibrated pH probe. Separate a strength comparison at fixed concentration from a concentration comparison using the same acid. The GCSE strong-acid wording is a model; do not silently turn it into advanced assumptions about every dissociation step of concentrated sulfuric acid.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: solution A has pH 2 and solution B pH 5 under the stated conditions. A has 10³=1,000 times the H⁺ concentration of B. The pH values alone do not identify the acids or their total acid concentrations. A low pH can reflect both ionisation behaviour and the amount present.
Compare pH 3 with pH 6. How many times greater is H⁺ concentration at pH 3? Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
Compare pH 3 with pH 6. How many times greater is H⁺ concentration at pH 3?
The result is 1000 times. Known: solution A has pH 2 and solution B pH 5 under the stated conditions. A has 10³=1,000 times the H⁺ concentration of B. The pH values alone do not identify the acids or their total acid concentrations. A low pH can reflect both ionisation behaviour and the amount present.
Check the conclusion and its limits
- Weak does not mean harmless and strong does not mean concentrated. Dilution does not change an acid’s named strong/weak classification. A neutral pH 7 solution is not ion-free; it has balanced acid–alkali character in the school account. Avoid predicting pH from a label alone without concentration and conditions.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
A weak acid is necessarily safe to handle at any concentration. This claim is false: Weak does not mean harmless and strong does not mean concentrated. Dilution does not change an acid’s named strong/weak classification. A neutral pH 7 solution is not ion-free; it has balanced acid–alkali character in the school account. Avoid predicting pH from a label alone without concentration and conditions.
Higher Tier: acid strength differs from acid concentration: At a given concentration under comparable conditions, the stronger acid has lower pH because it produces greater hydrogen-ion concentration. Each decrease of one pH unit corresponds to a tenfold increase in H⁺ concentration. A decrease of two units means a hundredfold increase. Use whole-number pH comparisons in this source scope; do not treat pH as a linear concentration scale.
A weak acid is necessarily safe to handle at any concentration.
Weak does not mean harmless and strong does not mean concentrated. Dilution does not change an acid’s named strong/weak classification. A neutral pH 7 solution is not ion-free; it has balanced acid–alkali character in the school account. Avoid predicting pH from a label alone without concentration and conditions.
The extent of ionisation of an acid in aqueous solution: write the technical term.
acid strength means The extent of ionisation of an acid in aqueous solution.