A2 Unit # 6 Practical Chemistry

Substance A is a green solid which dissolves in water to form a green solution. Substance A is an ionic compound containing one cation and one anion. (a) Give the formulae of two cations which could be responsible for the green colour in the solid. (2)

(b) A student added dilute sodium hydroxide, drop by drop, to an aqueous solution of A. Initially, a green precipitate was formed. The precipitate did not dissolve in excess sodium hydroxide solution. The precipitate darkened on standing to give a brown solid.

(i) Write the formula of the cation in substance A. (1)

(ii) Write a formula for the green precipitate. (1)

(iii) Write a formula for the brown solid. (1)

(iv) State the type of reaction involved when the green precipitate turns brown

Answers: (a) Cr3+ and Fe2+ (b)(i) Fe2+ (ii) Fe(OH)2 (iii) oxidation