The pH scale and hydrogen–hydroxide neutralisation
Diperkenalkan| English | Makna |
|---|---|
| pH scale/piː eɪtʃ skeɪl/ | skala pH |
| neutralisation/ˌnjuːtrəlaɪˈzeɪʃn/ | penetralan |
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?
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On the school scale, above 7 is alkaline, so pH 11 is the alkaline solution. 7 is neutral and 3 is acidic.
Match each technical term to its precise meaning.
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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?
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Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
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?
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The result is 6 ions. Known: 15 H+ and 9 OH- ions; they pair one-to-one forming water. Excess H+ = 15 - 9 = 6 ions left over, so the mixture stays acidic. Neutralisation consumes equal numbers of each ion.
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: Pure water itself contains H+ and OH- ions in equal balance - that balance is exactly what pH 7 means. "Neutral" is a balance of ions, never an absence of them.
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.
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Pure water itself contains H+ and OH- ions in equal balance - that balance is exactly what pH 7 means. "Neutral" is a balance of ions, never an absence of them.
The scale used to express solution acidity or alkalinity: write the technical term.
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pH scale means The scale used to express solution acidity or alkalinity.