In what order should strong acids and water be mixed?
This is a very important practical tip. Especially as at A level students may sometimes deal with concentrated acids. A small quantity of water in concentrated acid will get very hot as it hydrates the acid, and it may spit. And if you get such a drop in your eye, that's your eye gone. On the other hand, when conc. acid is added to water, it won't spit; the solution will still heat up, but it will do so gradually.
When you work with acids, you MUST ALWAYS wear safety spectacles; or glasses if you are a speccy. Chemists routinely wear lab coats to protect their clothing from acid spills.