Acids, bases and salts / Asid, bes dan garam
Acids, bases and salts tests definitions, pH, strength versus concentration, neutralisation and salt preparation. You must choose the correct preparation route and explain observations.
Core
- An acid releases H⁺ in water; a base accepts H⁺ or releases OH⁻ in water; an alkali is a soluble base.
- pH below 7 is acidic, 7 is neutral, above 7 is alkaline. pH is logarithmic: pH 2 is 100 times more acidic than pH 4.
- A strong acid ionises completely; a weak acid ionises partially. Concentration is amount per volume; strength is degree of ionisation.
- Soluble salts: acid plus metal, carbonate, oxide or hydroxide. For sodium, potassium and ammonium salts, use titration. Insoluble salts: precipitation.
- Solubility: all nitrates are soluble; most chlorides are soluble except AgCl and PbCl₂; most sulfates are soluble except BaSO₄ and PbSO₄, with CaSO₄ slightly soluble; carbonates are insoluble except those of Na, K and NH₄.
Worked example
Prepare pure copper(II) sulfate crystals from copper(II) oxide and dilute sulfuric acid. Add excess CuO to warm H₂SO₄, stir, then filter off unreacted black solid; the blue filtrate is copper(II) sulfate solution. Heat to concentrate, cool to crystallise, filter and dry between filter papers. Do not evaporate to dryness, which can dehydrate or decompose the salt. Reasoning: excess insoluble base ensures all acid reacts, and filtration removes the excess.
Traps
Strong is not the same as concentrated. Diluting an acid raises pH toward 7 but does not make it a weak acid. Do not use titration for insoluble salts. Universal indicator gives approximate pH; methyl orange and phenolphthalein have specific ranges. Use the correct gas test: CO₂ with limewater, NH₃ with damp red litmus, Cl₂ with damp blue litmus that is bleached.