Stored chemistry experiment in Horowhenua College science lab became potentially explosive substance, principal says

The Defence Force were called to Horowhenua College after a stored chemistry experiment in a science lab became a potentially explosive substance, the principal says.Emergency services were called to the school about 10.20am.Principal Grant Congdon said a stored solution made up for a senior chemistry experiment "precipitated" over time into a potentially explosive substance in a storage cupboard at the school.A bomb disposal robot removed the flask, he said and it was detonated by Defence Force ordinance disposal staff in the school's back field."A big bang and the chemical is gone," Congdon said.Congdon said the solution, known as Tollens' reagent, which contained silver ions and ammonia, had been stored for a number of years before it was discovered by a staff member who raised the alarm."It had been there for such a long time that there's been a transformation within the flask to the point where a precipitate had formed and that any sort of movement, friction or heat can potentially make it explode."On the side of the flask, you could see it there as a black powder, that was the part of what was in the flask which was potentially dangerous," Congdon said.Congdon said police, Fire and Emergency, ambulance and two members of the Defence Force Bomb squad equipped with protective suits and a bomb disposal robot attended.He said about up to 130 students were evacuated.The school did not have enough time to alert neighbours on the school's boundaries before the chemical was destroyed, he said."It was a big enough bang for someone several hundred metres away to feel the blast impact. So it was a fairly decent noise," Congdon said.Congdon said the days events were "a first" in his years as an educator.He was unable to say exactly how long the chemical had been stored in the school but it would have amounted to years.The school would be reviewing its storage and safety procedures in response to the incident, Congdon said."And obviously it raises questions for us to just check and see how is our storage procedures for chemicals and gases and all those sorts of things around the college, just to check that we've got procedures in place that are fit for purpose," Congdon said.Congdon said he was impressed with the performance of staff and emergency services responding to the discovery and he was grateful the situation had come to a safe ending."The oversight from FENZ, the bomb disposal guys were just so professional and the communication to everybody was on point."We had to be pretty flexible with looking after students and removing them from rooms as things as that chemical was being uplifted and then taken out to the back field. Staff were amazing. Everyone just worked very well together."A police spokesperson said the science block was evacuated and there were no reports of injuries stemming from the incident.