Fruity science

There are lots of experiments that you can do in school using readily available types of fruit. Here are a couple to get you started.

Rates of reaction: rhubarb and oxalic acid

EQUIPMENT:

• Rhubarb stalks (frozen works well)

• Acidified potassium manganate (VII) solution, (approx. 0.1% in 1 moldm-3 sulfuric acid)*

• Beakers (100 mL)

• Measuring cylinder (50 mL)

• White tile

• Stopwatch/timer

• Knives to cut rhubarb

*The exact concentration of potassium manganate solution isn’t critical, but test it before the practical to ensure that it decolourises with rhubarb, you may need to add a little more water to dilute it further if the initial colour is too intense to see a change.

THE SCIENCE BIT…

METHOD:

1. Cut the rhubarb into evenly sized pieces (about 5 cm is good).

2. Add 30 mL potassium manganate solution to a beaker.

3. Place the beaker onto a white tile.

4. Add one piece of rhubarb to the beaker containing the potassium manganate solution.

5. Start the timer.

6. Stop the timer when the potassium manganate solution has turned clear.

You can use this method to look at how surface area affects the rate of reaction (chop a second 5 cm piece of rhubarb in half and put both halves into the same beaker, chop a third piece into quarters etc.); and if you gently heat the rhubarb until it is ‘mush’ and strain it (keeping the filtered liquid) you can look at the effect that changing rhubarb concentration has on reaction rate too, using increasing amounts of rhubarb extract in the manganate solution).

The oxalic acid (C2H2O4) in the rhubarb reacts with the potassium manganate (MnO4) and releases carbon dioxide (CO2) and water (H2O). The pink/purple potassium manganate solution changes to colourless as a consequence of the reaction (more specifically to a very pale pink, but it will look clear in the concentrations that are being used).

2MnO4– + 5C2H2O4 + 6H3O+ → 2Mn2+ + 10CO2 + 14H2O

HAZARDS:

Potassium manganate, solid (CLEAPSS hazcard 81):

Sulfuric acid (CLEAPSS hazcard 98a): Irritant

Acidified potassium manganate solution (~0.1 moldm-3): Irritant

Note: Oxalic acid in the rhubarb is an irritant, which may be an issue if you are looking at rhubarb concentration.

(Adapted from Royal Society of Chemistry)