Question

Question #68168

Answer 1

Could be anything, like magnesium powder added to dilute acid, or many others.

Explanation:

My bet though, would be on a relatively strong hydrogen peroxide solution `H_2O_2(aq)`, to which manganese dioxide `MnO_2` was added. The `MnO_2` acts as a catalyst for the following reaction:

`2H_2O_2(aq)->2H_2O(l)+O_2(g)`

If you want to make sure you could test for the gas being oxygen (glowing splinter).