The pH scale and hydrogen–hydroxide neutralisation
| English | Français |
|---|---|
| pH scale/piː eɪtʃ skeɪl/ | échelle de pH |
| neutralisation/ˌnjuːtrəlaɪˈzeɪʃn/ | neutralisation |
What would explain this observation?
- Universal indicator gives a range of colours as an acid is added to an alkali. A pH probe can record a numerical change, but it still needs an appropriate measurement method.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- In this GCSE aqueous account, acids produce hydrogen ions H⁺ and alkalis contain hydroxide ions OH⁻. On the usual 0–14 school scale, pH below 7 is acidic, pH 7 neutral and pH above 7 alkaline. Universal or wide-range indicator gives approximate pH by matching its colour with the supplied chart; a suitable calibrated probe provides a numerical reading.
- pH scale 酸碱度标度: The scale used to express solution acidity or alkalinity; neutralisation 中和: Reaction of an acid with a base; H⁺ and OH⁻ form water for an acid–alkali reaction.
Which supplied pH identifies an alkaline solution on the stated school scale?
Neutralisation between acid and alkali forms water: H⁺(aq) + OH⁻(aq) → H₂O(l). The positive and negative charges cancel, and atoms are conserved. Excess acid after mixing leaves the solution acidic; excess alkali leaves it alkaline. Neutralisation does not automatically mean that equal volumes of every pair of solutions will produce pH 7.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Neutralisation between acid and alkali forms water: H⁺(aq) + OH⁻(aq) → H₂O(l). The positive and negative charges cancel, and atoms are conserved. Excess acid after mixing leaves the solution acidic; excess alkali leaves it alkaline. Neutralisation does not automatically mean that equal volumes of every pair of solutions will produce pH 7.
- Use clean samples and small consistent indicator quantities or a correctly calibrated, rinsed pH probe. Match colours under suitable lighting and read the chart for the indicator actually used. During a supervised strong-acid/strong-alkali comparison record the measured pH against added volume. Indicator colour estimates and probe readings have different uncertainties; do not invent precision from a broad colour band.
Which two habits make the investigation or model in this case more defensible?
Use clean samples and small consistent indicator quantities or a correctly calibrated, rinsed pH probe. Match colours under suitable lighting and read the chart for the indicator actually used. During a supervised strong-acid/strong-alkali comparison record the measured pH against added volume. Indicator colour estimates and probe readings have different uncertainties; do not invent precision from a broad colour band.
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: supplied solutions P, Q and R have pH 3, 7 and 11. P is acidic, Q neutral and R alkaline under this school model. If ten represented H⁺ ions react with seven represented OH⁻ ions, seven water molecules form and three H⁺ ions remain. This particle illustration explains excess acid without requiring a Higher-only logarithmic concentration calculation.
A particle model has 15 H⁺ and 9 OH⁻ ions. After complete neutralisation, how many excess H⁺ ions remain? Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A particle model has 15 H⁺ and 9 OH⁻ ions. After complete neutralisation, how many excess H⁺ ions remain?
The result is 6 ions. Known: supplied solutions P, Q and R have pH 3, 7 and 11. P is acidic, Q neutral and R alkaline under this school model. If ten represented H⁺ ions react with seven represented OH⁻ ions, seven water molecules form and three H⁺ ions remain. This particle illustration explains excess acid without requiring a Higher-only logarithmic concentration calculation.
Check the conclusion and its limits
- A neutral solution contains ions; neutral does not mean ion-free. Alkali is not the same word as any insoluble base. A pH difference is not a simple gram-mass difference, and the tenfold hydrogen-concentration interpretation is in the separate HT strong/weak-acid lesson.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
A pH 7 solution contains no ions of any kind. This claim is false: A neutral solution contains ions; neutral does not mean ion-free. Alkali is not the same word as any insoluble base. A pH difference is not a simple gram-mass difference, and the tenfold hydrogen-concentration interpretation is in the separate HT strong/weak-acid lesson.
The pH scale and hydrogen–hydroxide neutralisation: Neutralisation between acid and alkali forms water: H⁺(aq) + OH⁻(aq) → H₂O(l). The positive and negative charges cancel, and atoms are conserved. Excess acid after mixing leaves the solution acidic; excess alkali leaves it alkaline. Neutralisation does not automatically mean that equal volumes of every pair of solutions will produce pH 7.
A pH 7 solution contains no ions of any kind.
A neutral solution contains ions; neutral does not mean ion-free. Alkali is not the same word as any insoluble base. A pH difference is not a simple gram-mass difference, and the tenfold hydrogen-concentration interpretation is in the separate HT strong/weak-acid lesson.
The scale used to express solution acidity or alkalinity: write the technical term.
pH scale means The scale used to express solution acidity or alkalinity.