- The amount matters. Nitromethane or ammonium nitrate are nothing in lab quantities but scary in rocket or train amounts.
- Some compounds are used by non-chemists and get nasty then. Bleach is the biggest killer compound in homes through the inadvertent mix with acids. Even under scientists: microelectronics, biology, archaeology... need hazardous compounds, for which the users may have studied the very specific information but have no broader chemical background.
- Stability, for instance of high-test hydrogen peroxide, depends on minute amounts (ppm) of impurities or stabilizers.
- Some substances are not so dangerous alone, but mundane conditions make them treachous. A spillage of liquid oxygen on an asphalt road, even days after the oxygen has evaporated, can let the asphalt detonate.
- The storage conditions changes everything. Acetylene can detonate if gaseous at >2bar or liquid. The concentration too: hydrogen peroxide gives wrong confidence because households use it at % dilution.
- The compound in the old bottle is not the one you poured. Peroxides can form meanwhile for instance.
- The hazard of most substances is not known, even for very common ones like benzene was. It's suspected after a few dozen deaths and believed after some hundreds. Then we replace knowingly dangerous substances by less known ones.