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Lesson: Chapter 17 — Rate of Reaction

17.1. Concentration and pressure 1 of 4

The magnesium ribbon experiment

This time what changes is the concentration of the acid. Keeping the total volume the same, though, is the subtle part of this activity.

Activity 17.3

You will need: three well-cleaned pieces of magnesium (Mg) ribbon of equal surface area, three test tubes, dilute hydrochloric (HCl) acid, water.

  • Take three test tubes, add about 15 ml of water to each, and mark the water level with a rubber band.
  • Pour the water away and add 2.5 ml, 5.0 ml and 7.5 ml of dilute HCl acid to the three tubes respectively.
  • Add water up to the rubber band in each tube, so that all three hold the same total volume.
  • Add one piece of Mg ribbon to each tube and observe the rate at which gas bubbles are given off.

Figure to be added

The three test tubes: water + HCl 2.5 ml, water + HCl 5.0 ml and water + HCl 7.5 ml, each with its piece of Mg ribbon (activity 17.3).

Why that step matters

Topping up to the rubber band cannot be skipped. Without it the volume changes too, and we could no longer say whether the result was caused by the concentration or by the volume. Only one thing may be varied at a time.