(a)(i) List two physical properties used as criteria for purity of substances (ii) describe how you would prepare a pure, dry sample of sodium chloride crystals by a neutralization reaction, using bench reagents. (iii) Give two other general methods for preparing soluble salts.
(b) Explain the following observations: (i) a sheet of iron placed in dilute copper (II) tetraoxosulphate (VI) solution reddish brown; (ii) the white gelatinous precipitate formed when a few drops of sodium hydroxide solution are added to a solution of aluminium salt dissolves in excess alkali; (iii) the pale green prepared iron(II) chloride solution changes to brown on bubbling chlorine gas through it. (iv) Write a balanced equation for the reaction of dilute hydrochloric acid with marble. List two industrial process in which limestone is used as a raw material.
Explanation
(a)(i) The boiling point of the substance. The density is a property used to measure the purity of a substance. (ii) The preparation of sodium chloride by neutralization reaction is the reaction between dilute hydrochloric acid and sodium hydroxide i.e NaOH\(_{(aq)}\) + HCI\(_{(aq)}\) \(\to\) NaCI\(_{(aq)}\) + H\(_2\)O\(_{(l)}\) Sodiumchloride The salt is then crystallised out of the water and allowed to dry. The salt is white cubic crystals (iii) A soluble salt can be prepared by the action of dilute acid on the metal. Also by double decomposition.
(b)(i) The reaction of iron in dilute copper (II) tetraoxosulphate (VI) solution produces iron (III) tetraoxosulphate (VI) and copper solid which make the solution become reddish-brown. (ii) Aluminium hydroxide AI(OH)\(_3\) dissolves in excess sodium hydroxide to give sodium aluminate i.e NaAlO\(_2\). (iii) The chloride gas oxidised iron (II) to iron (III) Fe\(^{3+}\) (c)(i) 2HCI\(_{(aq)}\) + CaCO\(_{3(g)}\) —> CaCI\(_{(aq)}\) + CO\(_{2(g)}\) + H\(_2\)O\(_{(l)}\) Marble is used to manufacture cement.
