Expert: Kristin - 5/26/2007

Question
what is the meaning of acids

Answer
In order to understand what an acid is, some basic chemistry is required.  Don't be scared away, it is not scary or hard!

All solutions can have a number of attributes - temperature, color, concentrations of compounds, and something called pH.  pH is a measure of a solution's acidity just like temperature is a measure of a solution's heat.

There are a few different definitions of acid because the idea of an acid has evolved as our understanding of chemistry has changed over the years.  I'm going to describe two definitions that are commonly used.  There is a third definition, but it requires a bit more understanding of chemistry and I think I can get my point across with the first two definitions.

The first definition I'm going to write about is an Arrhenius acid.  An Arrhenius acid is a compound that dissolves in water and dissociates to produce H+.  This means that one of the hydrogen atoms attached to the compound when it isn't in the water 'falls off' when the compound is dissolved in water.  When it falls off it doesn't take it's electron with it, so it is a charged ion, H+.  Adding this type of acid to water increases the number of H+ in the water.  pH measures the amount of H+ in the water, and thus measures the acidity.  (A slightly funny thing about the pH scale is that as the number of H  increases, the pH drops.  So an acidic solution will have a low pH, <7, and a basic solution, the opposite of acidic, will have a high pH, >7.)

The other definition that I will describe is the Bronsted-Lowry definition of an acid.  This definition says that an acid is a compound that can donate an H .  It sounds kind of similar to the Arrhenius definition, but it doesn't require that the compound dissociate and dissolve in water.  An example of a Bronsted-Lowry acid is water.  It can donate one of its protons to another molecule.  If you combine ammonia (NH3) with water (H2O), the water molecule will donate an H+ to the NH3 forming NH4+ (the water is left as OH-).  So in this case, water, H2O, is the acid.

I can get more specific, but I wanted to try and answer your question with out requiring a lot of previous knowledge on the subject.  I hope that I've succeeded in this!