In the laboratory preparation of oxygen using potassium trioxochlorate(V), tetraoxomanganate (VII), usually not recommended in modern laboratory because

Potassium trioxochlorate(V) (KClO₃, potassium chlorate) can decompose on heating to produce oxygen gas. However, it is not recommended for modern laboratory preparation of oxygen because it forms explosive mixtures with carbonaceous materials.

KClO₃ is a powerful oxidising agent. When it comes into contact with organic (carbon-containing) substances such as rubber tubing, cork stoppers, paper, or wood, it can react violently and cause explosions. Even small amounts of organic impurities can trigger a dangerous, rapid exothermic decomposition.

The safer modern alternative for laboratory preparation of oxygen is heating hydrogen peroxide (H₂O₂) with manganese(IV) oxide (MnO₂) as a catalyst, which does not carry the same explosion risk.

While KClO₃ is certainly hazardous, the specific reason it is not recommended is the formation of explosive mixtures with carbonaceous materials, not simply a general statement of being hazardous or explosive on its own.