Robert Boyle in the seventeenth century first identified substances as either acids or bases (he called bases alkalis) according to the following characteristics:

Bases also damage protein .  This results in the "slippery" feeling on hands when exposed to base.   Strong bases that dissolve well in water , such as sodium or potassium lye are very dangerous because a great amount of the structural material of human beings is made of protein.

It has been observed that water can dissolve many compounds by separating them into their individual ions.  Acids are compounds which include hydrogen atoms. When they are dissolved in water they release hydrogen ions (H+) into solution.  A typical acid is hydrochloric acid (HCl)>

Bases dissolve in water to release hydroxide ions (OH-) into solution.   A typical base is sodium hydroxide (NaOH):

When a base is mixed with an acid the acidity is reduced and vice versa.  This is called Neutralisation.   As you can see above, acids release H+ into solution and bases release OH-.   If we were to mix an acid and base together, the H+ ion would combine with the OH- ion to make the molecule H₂O, or plain water:

The modern definition of Acids and Bases is based on Johannes Br�nsted's and Thomas Lowry's versions both definitions were published in the early 1920's and were similar.  Acids and bases are defined in accordance with Br�nsted-Lowry as follows.

An acid is any substance that can donate a hydrogen ion (which is a proton).   Acids are often referred to as proton donors.

A base is defined as any substance that can accept a hydrogen ion.    In essence, a base is the opposite of an acid.    NaOH and KOH, as we saw above, would still be considered bases because they can accept an H+ from an acid to form water.  The Br�nsted-Lowry definition also explains why substances that do not contain OH- can act like bases.    Baking soda (NaHCO₃), for example, acts like a base by accepting a hydrogen ion from an acid .

A salt is the combination of an anion (- ion) and a cation (a + ion) ( ref Ions and cations).   A salt results from the combination of the anion of a certain acid combined with the cation of a certain base.   The neutralization of potassium hydroxide with hydrochloric acid produces water and the salt, potassium chloride.   In a solid salt, the ions are held together by the difference in charge.    Solid salts generally form crystals, posssible including specific amounts of water, called water of hydration into the crystal.    If a salt dissolves in water solution, it normally separates into the anions and cations that make up the salt.

Table salt, NaCl, has a neutral pH in water.   Other salts may not be neutral e.g...
a) Salts made of the anion of a strong acid and the cation of a strong base will be neutral salts, that is, the water solution with this salt will have a pH of seven.
b) Salts made of the anion of a strong acid and the cation of a weak base will be acid salts, that is, the water solution with this salt will have a pH of less than seven. (example - ammonium chloride)
c) Salts made of the anion of a weak acid and a strong base will be an alkali salt.   The pH of the solution will be over seven. (example - sodium bicarbonate)

The common acids that are almost one hundred percent ionized are:

The acids on this short list are called strong acids, because the amount of acid quality of a solution depends upon the concentration of ionized hydrogens.     Other acids are incompletely ionized, existing mostly as the unionized form.   Incompletely ionized acids are called weak acids, because there is a smaller concentration of ionized hydrogens available in the solution.   Strong and weak acids are not the same as dilute and concentrated acids.   The differences in concentration of the entire acid will be termed dilute or concentrated. Muriatic acid is the name given to an industrial grade of hydrochloric acid that is often used in the finishing of concrete.

This is a short list of strong bases i.e ones that completely ionize into hydroxide ions and a conjugate acid.   All of the bases of Periodic Table Group I and Group II metals except for beryllium are strong bases.    The bases of Group II metals, magnesium, calcium, barium, and strontium are strong, but all of these bases have somewhat limited solubility.    Magnesium hydroxide has a particularly small solubility.  

Hydrogen ion concentrations are measured as the number of gram ions of hydrogen present per litre of solution.  Since these concentrations are usually small the concentration is generally expressed as the pH of the solution: the pH being the logarithm i.e pH = - log[H⁺]. The pH can be measured using a glass electrode or less accurately using coloured indicators...

An acid in the aqueous system is defined as a substance which is capable of forming hydrogen ions when dissolved in water.   Hydrogen ion concentration can cover a wide range from about 10^-14gram ions (or less) per litre in alkaline solutions to 1 gm ion(or more) per litre in acid solutions. Therefore the pH scale varies from 0 (Acid) to 14 (alkali).