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:

 

Acids react with metals forming salts and Hydrogen gas 

Example:           HCl  +  Zn  ®  ZnCl2   +   H2

 

Acids react with carbonates forming salts, water, and Carbon dioxide gas

Example:           HCl   +   MgCO3   ®    MgCl2   +   H2O   +   CO2

 

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.