There are several ways of defining the terms “acid” and “base”; we will use the Arrhenius model.
According to the Arrhenius model:
An acid is a substance that contains Hydrogen and ionizes in water to form Hydrogen ions.
Example
HCl (g)  H+ (aq)
+ Cl  (aq)
In reality, hydrogen ions (which would be protons without any electons) are not actually what
forms. Instead, the ionized hydrogen combines with water forming H3O+ (Hydronium) ions.
However, it is easier to think of them as H+ ions, so that is how we will describe them.
A base is a substance that contains Hydroxide and forms Hydroxide ions in aqueous solution.
Example
NaOH (c)  Na + (aq)
+ OH (aq)
The following substances form acids when dissolved in water
Strong Acids
Weak Acids
HCl
Hydrochloric acid
HF
Hydrofluoric acid
H2SO4 Sulfuric acid
H2CO3
Carbonic acid
HNO3 Nitric acid
H3PO4
Phosphoric acid
HBr
Hydrobromic acid
HC2H3O2
Acetic acid
HI
Hydroiodic acid
Properties of acids include:
1. Acids react with metals forming salts and Hydrogen gas
Example: HCl + Zn  ZnCl2 + H2
2. Acids react with carbonates forming salts, water, and Carbon dioxide gas
Example: HCl + MgCO3  MgCl2 + H2O + CO2
3. Acids react with bases forming salts and water
Example: HCl + NaOH  NaCl
+ H2O
A strong acid is one that completely dissociates in water, so that all of its Hydrogen atoms
become Hydrogen ions.
Examples: HCl and H2SO4 are strong acids
When 1 mole of HCl is dissolved in water, it forms 1 mole of H+ ions
When 1 mole of H2SO4 is dissolved in water, it forms 2 moles of H+ ions
A weak acid is one that does not completely dissociate in water, only some of its Hydrogen atoms
become Hydrogen ions.
Example: HF is a weak acid, when 1 mole of HF is dissolved in water, less than 1 mole of H+
ions are formed
The terms weak and strong can be applied to bases as well, substituting OH for H+
Just because an acid is a strong acid doesn’t mean that it is particularly powerful. This is because
the corrosiveness of an acid depends mostly on how concentrated it is.
A dilute strong acid can be much safer to handle than a concentrated weak acid.
Example: HCl is a strong acid and HC2H3O2 is a weak acid
A 0.01M HCl solution is far less dangerous to handle than a 6 M HC2H3O2 solution.
This is because the a 6 M HC2H3O2 solution has a much higher concentration of H+ ions.
Since it is the concentration of H+ ions that give a solution its acidic properties, acidity is
measured in terms of the concentration of H+ ions, or pH of the solution.
A solution that has equal amounts of H+ and OH is neither acidic nor basic, it is neutral
A neutral solution has a pH of 7
An acidic solution will have a pH between 0 and 6.9
A basic solution will have a pH between 7.1 and 14
pH stands for “negative log of the hydrogen ion concentration”
pOH stands for “negative log of the hydroxide ion concentration”
Even pure water has a certain amount of H+ ions in it. Water spontaneously dissociates to form
equal amounts of H+ and OH ions as shown in the following equation
H2O

H+ + OH
Because there are equal amounts of H+ and OH ions, water remains neutral.
The concentration of H+ ions in pure water is 1  107 M, and the concentration of OH ions is
also 1  107 M. If you take the log of either of these numbers and multiply by 1, you will get 7
So the pH of pure water is 7.
If you add an acid to water, the concentration of H+ ions goes up, from 1  107 M to perhaps
1  103 M. That means that the pH of the solution went from 7 when neutral, to 3 with the
addition of the acid.
Acidic solutions have a pH that is less than 7, ranging from 0 (highly acidic), to just under 7
(mildly acidic).
Basic solutions have a pH that is greater than 7, ranging from 14 (highly basic), to just over 7
(mildly basic).
If you add a base to water, the concentration of OH ions goes up (so the pOH) goes down, and
the concentration of H+ ions goes down so the pH goes up by the same amount.
Since H+ and OH ions combine to form water, which is neutral, adding a base to an acid will
bring the pH up, and adding an acid to a base will bring the pH down.
Increasing H+ ions
Increasing OH ions
Very Acidic
Neutral
Very Basic
pH
acidic
basic
0
7
substance
14
pH of some common substances
pH
battery acid
(sulfuric acid)
stomach juices (hydrochloric acid)
lemon juice
(citric acid)
soft drinks
(carbonic acid)
vinegar
(acetic acid)
milk
(lactic acid)
pure water
seawater
Tums
Laundry detergent
Oven cleaner
0.0
2.0
2.3
3.0
3.5
6.6
7.0
8.0
9.4
10.0
14.0
In pure water the concentration of H+ is 1  107 M and the concentration of OH is also
1  107 M if you multiply the two values together you get 1  1014 This is why the maximum
pH of a solution is 14.
In any water solution, the pH + pOH = 14
Ex: if the pH is 12, the pOH is 2
if the pH is 3 the pOH is 11
You can calculate the pH of a solution if you know the concentration H+ ions in a solution.
Remember that pH stands for the  log (H+ concentration in moles/liter)
A pH of 9 corresponds to an H+ concentration of 1.0  10  9
If you know the concentration of H+ ions and you want to know the pH take the negative log of
the concentration. Ex: if the concentration is 3.6  10 9 M then pH =  log(3.6  10 9) = 8.4
If you know the pH and you want to know the concentration in moles/liter, raise 10 to the
negative power of the pH. Ex: if the pH is 4.7, then the concentrations is 10 4.7 = 1.99  10 5
(to do this, use the second function of the LOG button)
Example: what is the pH of a solution contains 3.5  1011 M of H+ ions?
 log (3.5  1011) = 10.5 the solution has a pH of 10.5, this makes it basic