Answer:
Chemical reaction.
Step-by-step explanation:
In the experiment, a catalyst is added to concentrated hydrogen peroxide.
The hydrogen peroxide decomposes rapidly, according to the equation
2H₂O₂(ℓ) ⟶ 2H₂O(ℓ) + O₂(g)
New substances are formed, and old ones disappear, so this is a chemical reaction.
The reaction also releases a large amount of heat.
The rapidly-expanding oxygen gas caries with it droplets of water and hydrogen peroxide (and probably some steam, as well).
The visual effect is like a genie escaping from its bottle.