pH is a scale that describes the acidity or basicity of a chemical. The scale goes from 1 to 14, with water being a 7. An acid has a low pH rating. A base (chemistry) has a high pH rating. So the lower the number, the more dangerous the acid.
The "H" stands for Hydrogen, because it's hydrogen ions that are released when an acid does it's work. The "p" in pH stands for, "potential", hence the "hydrogen potential" of a solution, expressed as the negative base ten logarithm of the (theoretical)1 hydrogen ion concentration. In laymans terms, the lower the number, the more hydrogen ions there are available to "be acidic".
Conversely if the concentration of theoretical hydrogen ions is very low (i.e. pH is high) then then (equilibria being what they are) water molecules will lose hydrogen atoms and create OH- ions2.
It's worth noting that pH alone doesn't determine how dangerous an acid is: other factors such as toxicity and corrosive action need to be taken into account - some acids corrode human flesh (or any other given substance) much more easily than others, and some have toxic properties that are far more dangerous than any corrosive hazard they may create.
Game and Story Use
- You can use pH rating to rate acids in your game. See also molarity, which is the measure of how concentrated or dilute the acid is.
- Our scale, with water in the middle, is earthcentric carbon chauvinism, based on us living on a planet where water is common and forms the solvent heart of our biochemistry. If alien life has created an equivalent scale, it might or might not put water in the exact center, depending on what hypothetical types of biochemistry their life revolves around.