Topic 7
3 subtopics · Cambridge IGCSE Chemistry 0620
Key Idea
Acids produce H+ ions in solution. Bases are proton acceptors. pH scale: 0-14. pH < 7 = acidic, pH = 7 = neutral, pH > 7 = alkaline.
Explanation
Acids: - Produce H+ ions (protons) in aqueous solution. - Common acids: HCl (hydrochloric), H2SO4 (sulfuric), HNO3 (nitric), CH3COOH (ethanoic). - Strong acids fully ionise (HCl, H2SO4, HNO3). Weak acids partially ionise (CH3COOH). Bases and Alkalis: - Base = proton acceptor. Reacts with acids. - Alkali = soluble base. Produces OH- ions in solution. - Common alkalis: NaOH, KOH, Ca(OH)2, NH3(aq). pH scale: - pH 0-6: acidic (more H+ ions). - pH 7: neutral. - pH 8-14: alkaline (more OH- ions). Indicators: - Litmus: red in acid, blue in alkali. - Phenolphthalein: colourless in acid, pink in alkali. - Methyl orange: red in acid, yellow in alkali.
Practice Questions
A solution has a pH of 2. Which statement about this solution is correct?
Key Idea
Acids react with metals, metal oxides, metal hydroxides, metal carbonates, and ammonia to form salts.
Explanation
Reactions of acids: 1. Acid + Metal to Salt + Hydrogen e.g. Zn + H2SO4 to ZnSO4 + H2 2. Acid + Metal Oxide to Salt + Water e.g. CuO + H2SO4 to CuSO4 + H2O 3. Acid + Metal Hydroxide to Salt + Water (neutralisation) e.g. NaOH + HCl to NaCl + H2O 4. Acid + Metal Carbonate to Salt + Water + Carbon Dioxide e.g. CaCO3 + 2HCl to CaCl2 + H2O + CO2 5. Acid + Ammonia to Ammonium Salt e.g. NH3 + HCl to NH4Cl Naming salts: - HCl gives chloride salt - H2SO4 gives sulfate salt - HNO3 gives nitrate salt
Practice Questions
Write a word equation and a balanced symbol equation for the reaction between magnesium and dilute hydrochloric acid. [3 marks]
Key Idea
Soluble salts are prepared by reacting an acid with excess solid, filtering, then evaporating. Insoluble salts are prepared by precipitation.
Explanation
Preparing SOLUBLE salts (e.g. CuSO4 from CuO + H2SO4): 1. Add excess solid CuO to warm dilute H2SO4 (excess ensures all acid is used up). 2. Filter to remove excess unreacted CuO. 3. Evaporate the filtrate to concentrate the solution. 4. Allow to cool and crystallise. 5. Filter crystals, wash with distilled water, dry. Preparing INSOLUBLE salts by precipitation: 1. Mix two solutions containing the required ions. 2. Insoluble salt precipitates out. 3. Filter, wash, dry. Example: BaSO4 from BaCl2(aq) + Na2SO4(aq) to BaSO4(s) + 2NaCl(aq) Solubility rules: - All nitrates are soluble. - All sodium, potassium, ammonium salts are soluble. - Most chlorides are soluble (except AgCl, PbCl2). - Most sulfates are soluble (except BaSO4, PbSO4, CaSO4). - Most carbonates are insoluble (except Na2CO3, K2CO3).
Practice Questions
Describe how you would prepare pure dry crystals of copper(II) sulfate (CuSO4) starting from copper(II) oxide and dilute sulfuric acid. [5 marks]