Is it permissible to take solution (X) that was purchased for the hospital and replace it with solution (Y), and what should be done with the solutions that were taken in the same manner?

These solutions are a trust (amana), and it is not permissible to take from them without the permission of the official in charge. Workplace regulations allow for this. There is no difference between replacing what is taken from them or not, due to the Almighty's saying: "O you who have believed, do not betray Allah and the Messenger or betray your trusts while you know [the consequence]." And the Prophet, peace and blessings be upon him, said: "The signs of a hypocrite are three," and among them he mentioned: "and when entrusted, he betrays."