CAIE A Level Chemistry 9701 · Chemistry of transition elements
When solid is dissolved in water, the solution contains the pink octahedral complex ion . Adding concentrated hydrochloric acid changes the solution to a deep blue colour as the ion forms. This ion has a tetrahedral shape.
Explain why the ion is coloured, and why the colour changes from pink to blue when chloride ions replace water molecules.
4 marks, so the mark scheme is looking for about 4 separate creditworthy points. Making the same point twice in different words earns it once.
Explain means: Give the reasons or the mechanism: why or how something happens. Link each point to the next with because, so or which means. A list of facts with no links usually loses the reasoning marks.
Written by Markquill to follow the Cambridge International syllabus. Markquill is independent and is not affiliated with or endorsed by Cambridge International.