Cambridge IGCSE Chemistry 7 Acids, Bases and Salts Exam Questions

About Cambridge IGCSE Chemistry 7 Acids, Bases and Salts

Acids, bases and salts in Cambridge IGCSE Chemistry 0620 covers the reactions of acids and bases, indicators and the pH scale, the classification of oxides, and the preparation of soluble and insoluble salts. Extended candidates also define acids as proton donors and bases as proton acceptors, compare strong and weak acids, and recognise amphoteric oxides. The topic draws on ionic formulae and stoichiometry, and links to titrations and ion tests in experimental techniques. Examiners commonly ask for a full method to make pure, dry crystals of a salt, word and symbol equations for acid reactions, the ionic equation for neutralisation and a choice of method based on solubility.

What you need to know

• 7.1 Acids Acids have pH below 7 and turn litmus red, methyl orange red and thymolphthalein colourless. They react with metals to give a salt and hydrogen, with bases to give a salt and water, and with carbonates to give a salt, water and carbon dioxide.
• 7.1 Bases and alkalis Bases are oxides or hydroxides of metals; alkalis are soluble bases with pH above 7, turning litmus blue, methyl orange yellow and thymolphthalein blue. Neutralisation: H+ + OH- → H2O. Universal indicator shows the pH.
• 7.1 Strong and weak acids (Supplement) Acids are proton (H+) donors and bases are proton acceptors. A strong acid such as hydrochloric acid is completely dissociated in water; a weak acid such as ethanoic acid is partially dissociated. Lower pH means a higher concentration of H+ ions.
• 7.2 Oxides Metal oxides are basic and non-metal oxides such as CO2 and SO2 are acidic. Extended candidates learn that amphoteric oxides, aluminium oxide and zinc oxide, react with both acids and bases.
• 7.3 Solubility rules Sodium, potassium and ammonium salts and all nitrates are soluble. Chlorides are soluble except silver and lead; sulfates are soluble except barium, calcium and lead. Carbonates are insoluble except sodium, potassium and ammonium.
• 7.3 Preparing soluble salts Add excess insoluble metal, base or carbonate to warm acid, filter off the excess, heat the filtrate to the point of crystallisation, cool, then filter and dry the crystals. For a soluble base, use a titration to find the exact volumes, then repeat without indicator.
• 7.3 Insoluble salts and water of crystallisation Insoluble salts are made by precipitation: mix two solutions, filter, wash the precipitate with distilled water and dry. Hydrated salts contain water of crystallisation, for example CuSO4.5H2O; anhydrous salts contain none.

Common mistakes in 7 Acids, Bases and Salts

• Heating the salt solution to dryness. Heat only until crystals start to form, then leave to cool so that well-formed crystals appear.
• Confusing strong with concentrated. Strong refers to complete dissociation; concentrated refers to how much acid is dissolved per dm3.
• Forgetting to add the solid in excess. Excess ensures all the acid reacts, and the unreacted solid is then removed by filtration.
• Using the excess-solid method with an alkali such as sodium hydroxide. Soluble bases need a titration because excess cannot be filtered off.
• Mixing up indicator colours. Thymolphthalein is colourless in acid and blue in alkali; methyl orange is red in acid and yellow in alkali.

Try three 7 Acids, Bases and Salts exam-style questions