Oftentimes when people hear that something has a low or high pH, they worry that it is dangerous. But this is not the case, pH alone cannot tell you if a substance is dangerous or not. Anyway, bases are typically considered more dangerous than acids! But what is pH? A substances pH is simply the concentration of hydrogen ions (H+) of the material when in solution (usually water). It is a numeric (logarithmic) scale used to specify how acidic or basic an aqueous solution is and an aqueous solution is just a solution where water is the solvent.
Now you might be wondering, what makes an aqueous (aq.) solution acidic or basic? Essentially, acids have more hydrogen ions (H+) than pure water and bases have less hydrogen ions than pure water. This is because an acid will release a hydrogen ion into an aq. solution, while a base will release a hydroxide ion (OH-) – by accepting the hydrogen ion. Did you know that this also makes them taste different? Not that you should try it, but if you taste an acid it will taste sour (like lemon juice) and a base will taste bitter (like milk of magnesia).
A substance’s pH is a measure of concentration of hydrogen ions, not of strength. When talking about acids and bases, strength has a very technical meaning. With weak acids, only a certain percentage can act as an acid and with a strong acid, all of it can act as an acid (and the same for bases). So yes, a strong acid or a strong base will be better at dissolving substances than a weak acid or weak base. But when referring to the strength of an acid/base as strong or weak, scientists are typically not referring to the pH of the aq. solution or how caustic/corrosive it is. They are referring to its ability to ionize (produce hydrogen or hydroxide ions) in water, its ability to act as an acid/base. This is important to remember because acids/bases with a pH closer to 7 (like pure water) are not necessarily safer. A good example is hydrofluoric (HF) acid. In terms of pH hydrofluoric acid is considered a weak acid – it does not fully ionize in water – but it is extremely dangerous because of how corrosive it is.
Now keeping all that in mind, let’s look at some average pH values for common substances: