Those inner containers of the gas grenades surely are very fragile. So it can happen that they break and spread their devastating effect by burning into whatever surface they get into contact with. In this case Ventis pulled a pair of the reserve containers out of their reserve shelf in a last ditch effort to stop the intruders. Despite the immense pain of the acid burns in his forearms he manages to stay collected enough to not break the next pair and use them properly.
This dpicts how Ventis got his chemical burns on his lower arms. Under stress he got a bit sloppy and crushed two, paying the price for it.