The source said in a brief statement obtained by IraqiNews.com, "ISIS terrorist members executed 25 persons in Mosul on charges of spying and collaborating with Iraqi security forces", pointing out that, "ISIS put the citizens in a large tub containing nitric acid inside one of its headquarters".
Nitric acid is a highly corrosive mineral acid used in fertilizer and rocket fuel, and also used for etching steel.
The fuming liquid can be colourless, red or yellow.
The terrorist group has continued losing control over territory across Iraq and Syria, a U.S. Military spokesman said this week, including nearly half of what it had once held in Iraq.
It is believed the killings were part of an ISIS rebound, after the terror group lost control of territory they claimed in Iraq.
Since August 2014, the United States has led an worldwide coalition fighting back against the IS group, using a combination of air strikes and training and equipping local partners.
ISIS intolerance towards other faiths means Iraq is now the second most risky place in the world for Christians - who are considered apostates and often tortured to death by the terrorists - only beaten by North Korea.