Question: What makes acid acid and what makes some dangerous?

Martin Zaltz Austwick answered on 16 Mar 2012:

Acid is usually said to be a solution (liquid) with hydrogen ions floating around. These hydrogen ions can react with other chemicals (like metals, or your skin!) and when that happens they tend to destroy that chemical, or at least dissolve it. The more hydrogen ions are around, the more aggressive the acid is in dissolving things.

There is a number called pH which tells you how many hydrogen ions are in an acid – the lower the pH, the more ions and the more acidic it is. Water has a pH of 7 – some hydrogen ions, but not enough to damage human beings…