Raising raisins

A catalyst facilitates or increases the rate of a chemical reaction without being transformed itself.

Put a few raisins into a cup with sparkling water.

1. What makes the raisins raise and sink?

2. What chemical reaction happens on the raisins' surface and what role do the raisins play in this reaction?

3. Are the raisins changed by the chemical reaction?

What are these tiny silvery dots that appear on a raisin when it drops into water?
› Small air bubbles.

Where does the gas come from that makes the water effervesce?
› This is CO₂ that was dissolved in the water and now goes back to the gas phase.

In sparkling water, carbon dioxide (CO₂) has been added under pressure and cool temperatures to form carbonic acid (H₂CO₃):

CO₂ + H₂O ⇌ H₂CO₃

When the pressure is reduced (e.g. by opening the bottle), or the water's temperature increased, the chemical equilibrium is changed and CO₂ raises to the surface as small gas bubbles.

When raisins are put into water, tiny air bubbles remain in the pockets of their rough skin. These air bubbles support the formation of CO₂: While in water most of the CO₂ quickly dissolves again, the CO₂ in air bubbles is far more likely to stay in the gas phase. As the gas accumulates in bubbles on the raisin's skin, it provides buoyancy to the raisin. Lifted to the water surface, some of these bubbles burst, reducing the buoyancy, and so the raisin sinks again.

While the raisin is not transformed by the chemical reaction, it facilitates it with the tiny air bubbles bound to its skin. It thus can be taken as a slow-motion illustration of a catalyst.