Studyledge

Reversible Reactions and Equilibrium: Question 7

Syllabus 6.3

Multiple choice Core 1 mark

Four students each describe an observation from a school experiment.

Student 1: heats green copper(II) carbonate strongly; it turns black and releases a colourless gas that turns limewater cloudy.

Student 2: burns a short piece of magnesium ribbon in air; a white powder is produced and a bright white light is given out.

Student 3: heats blue hydrated copper(II) sulfate crystals, CuSO4⋅5H2O\text{CuSO}_4 \cdot 5\text{H}_2\text{O}; the crystals turn white, and colourless liquid condenses on the cooler part of the test tube. Adding a few drops of water to the white solid turns it blue again.

Student 4: adds dilute hydrochloric acid to solid sodium carbonate; bubbles of gas are produced and the solid dissolves completely.

Which student's observation is evidence of a reversible reaction?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall what counts as evidence of a reversible reaction

A reaction is reversible if it can be shown to go in both directions: the forward change, and then the reverse change bringing back the original substance(s). A single one-way change (even a dramatic one, or one that produces a gas) is not enough on its own to show reversibility.

Step 2: Check each student’s observation

  • Student 1: heating copper(II) carbonate to give black copper(II) oxide and carbon dioxide is a thermal decomposition. Once the carbon dioxide gas escapes (and is used to test limewater), there is no reverse step described. This is treated as a one-way change.
  • Student 2: burning magnesium in air is combustion, a classic one-way reaction. There is no way to turn the white magnesium oxide powder and light/heat given out back into magnesium ribbon and oxygen gas by simply reversing the conditions.
  • Student 3: heating blue hydrated copper(II) sulfate drives off water of crystallisation, leaving white anhydrous copper(II) sulfate, but the description also states that adding water back turns the white solid blue again. This second, reverse step is exactly the evidence needed to show the change is reversible.
  • Student 4: adding hydrochloric acid to sodium carbonate is a one-way reaction producing carbon dioxide gas, water and a solution of sodium chloride; once the gas has escaped, there is no described way to reverse this back to solid sodium carbonate and hydrochloric acid.

Step 3: Select the correct student

Only Student 3’s observation includes both the forward change (blue →\to white on heating) and the reverse change (white →\to blue on adding water), which together are the defining evidence of a reversible reaction.

Final answer

  • The reversible reaction is described by Student 3\boxed{\text{Student 3}}, option C.