Oxygen gas is commonly prepared in the laboratory by the decomposition of

Hydrogen peroxide breaks down easily to produce oxygen gas and water. Adding a catalyst like manganese dioxide speeds up this decomposition reaction.

The equation is: 2H₂O₂ → 2H₂O + O₂. The oxygen gas bubbles off and can be collected over water or in a container.

This method is safer and easier than electrolyzing water, which requires high voltage. Hydrogen peroxide naturally decomposes, so the reaction proceeds quickly with just a catalyst.