iGCSE Chemistry (0620) Core:6.3 Reversible reactions and equilibrium: Exam Style Questions Paper 1- New Syllabus
▶️ Answer/Explanation
Detailed solution:
The reaction can go forwards (pink to blue) when heated and backwards (blue to pink) when water is added, which is the definition of a reversible reaction. The pink starting material contains water, making it the hydrated form. When it turns blue, it becomes anhydrous. Since adding water brings the pink colour back, the pink cobalt(II) chloride must be the hydrated version.
▶️ Answer/Explanation
Detailed solution:
Blue copper(II) sulfate crystals are hydrated, containing water of crystallisation with the formula $\text{CuSO}_4\cdot5\text{H}_2\text{O}$. Heating drives off this water, leaving white anhydrous copper(II) sulfate. This is a reversible reaction: $\text{CuSO}_4\cdot5\text{H}_2\text{O} \rightleftharpoons \text{CuSO}_4 + 5\text{H}_2\text{O}$. When the white powder is left in air, it absorbs moisture (water vapour) from the air, which reverses the reaction and reforms the blue hydrated crystals. Option D correctly explains that water is absorbed from the air and causes the reaction to reverse.
▶️ Answer/Explanation
Detailed solution:
The symbol $\rightleftharpoons$ has two arrows pointing in opposite directions, one on top of the other. This means the reaction can go both forwards and backwards at the same time. In this case, sulfur dioxide and oxygen can combine to form sulfur trioxide, but simultaneously, sulfur trioxide can break down back into sulfur dioxide and oxygen. This two-way street behaviour is the very definition of a reversible reaction.