Question 5
When diluting a concentrated acid, what must always be done?
The right answer is the one marked above.
Add the acid to the water. A great deal of heat is given out. Adding acid slowly to a large volume of water spreads that heat through all of it, and the water warms gently.
Doing it the other way puts all that heat into a few drops, which can flash into steam and throw boiling acid out of the vessel. This is the one rule in the skill that is about safety rather than accuracy.