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Next exam on November 14 – 15, 2026:
All concepts

Acidity, Basicity and pH

溶液的酸碱性与 pH

The ionic product of water and the definition of pH, pH calculations, why salt solutions can be acidic or basic, and comparing strong with weak acids.

9 real-exam questions · 4 sittings13 in mock papers23 practice

Formulas

  • Definition of pH

    Definition
    pH=−lg⁡c(H+)\mathrm{pH}=-\lg c(\mathrm{H}^+)

    Applies when：For dilute solutions, with c in mol/L. Each unit drop in pH is a tenfold rise in c(H⁺)

  • Ionic product of water

    Law
    Kw=c(H+)⋅c(OH−)K_w=c(\mathrm{H}^+)\cdot c(\mathrm{OH}^-)

    Applies when：The ionic product is 1×10⁻¹⁴ at 25 °C; it varies with temperature only — adding acid or base does not change it

How the real exam tests it

Questions from real CSCA papers that test this concept. Work each one out before you open the answer.

  1. CSCA Chemistry April 2026 Past Paper
    At room temperature, which of the following aqueous solutions has a pH greater than 7? ( )
    • A. CH3COONa\mathrm{CH_{3}COONa}
    • B. NaCl
    • C. NH4Cl\mathrm{NH_{4}Cl}
    • D. NH4NO3\mathrm{NH_{4}NO_{3}}
    Show answer & solution

    Answer: A

    Answer: A CH3COONa\mathrm{CH_{3}COONa} is the salt of a strong base and a weak acid; it hydrolyzes to give a basic solution, pH > 7.
    Take the full CSCA Chemistry April 2026 Past Paper — timed & graded
  2. CSCA Chemistry March 2026 Past Paper
    Which aqueous solution has pH>7\mathrm { pH } > 7 at room temperature?
    • A．KCl
    • B．Na2CO3\mathrm { Na } _ { 2 } \mathrm { CO } _ { 3 }
    • C．NH4Cl\mathrm { NH } _ { 4 } \mathrm { Cl }
    • D．Al2(SO4)3\mathrm { Al } _ { 2 } \left( \mathrm { SO } _ { 4 } \right) _ { 3 }
    Show answer & solution

    Answer: B

    Na2CO3:CO32−\mathrm { Na } _ { 2 } \mathrm { CO } _ { 3 } : \mathrm { CO } _ { 3 } { } ^ { 2 - } hydrolysis → basic ( pH>7\mathrm { pH } > 7 ). A: neutral. C/D: acidic. Answer: B.
    Take the full CSCA Chemistry March 2026 Past Paper — timed & graded

Real-exam evidence

Recorded past-paper appearances are listed below; online verification is labelled separately.

9 questions across 4 sittings1 of 4 sittings available to verify free online

13 questions across 5 mock papers — our own papers, so a coverage signal rather than real-exam evidence

23 related practice questions availablePractise free: Solution Concentration and pH

Sitting evidence comes from the CSCA past papers on record; questions whose original paper is missing are excluded, and sprint papers are deliberately not counted.

Glossary

acidbasesalt

Related concepts

Electrolytes and Ionisation