Acids and Bases
Acids are substances that, when dissolved in water produce the aqueous ion, H+(aq), sometimes shown as H3O+(aq) which is known as the hydronium ion.   Acids are sour tasting and corrosive.  They react readily with active metals such as Zn, Ca, Mg and carbonate-based rock like limestone and marble.
Acids also cause natural and synthetic dyes to change colour and they "burn" organic materials such as skin or paper.
Bases are substances, which when dissolved in water, produce the hydroxide ion, OH-(aq).   Bases like acids are also electrolytes.   Bases tend to have a slippery feel, taste bitter, and also affect the colour of dyes.   Strong bases like NaOH (lye) and KOH are very corrosive and chemically breakdown organic substances such as fats, hair and vegetable matter.  For this reason they are used in drain cleaners.
A Neutralization Reaction occurs when an Acid and base react to produce water and a salt.
eg.  1                      HCl  +  NaOH  ------>   H-OH   +  NaCl
eg.  2            H2SO4   +   Ca(OH)2   ----->  2 H-OH  +   CaSO4
An indicator is a dye that shows a different colour in an acidic solution than it does in a basic solution.  Therefore indicators are used to show when a solution is acidic or basic.

Follow this link to see a list of acid-base indicators

The pH Scale
The pH scale was developed by S.P. Sorenson in 1909.  It is meant to show the "power of the hydrogen ion."
Let's say that a solution has a pH of 2.  This means that it is   1  M  =   1   = 1.0 X 10-2 mol/L
                                                                                                       102        100
A solution with a pH of 9 would be:      1 M    =        1   M          =  1.0 X 10-9 mol/L
                                                               109          1,000,000,000
for the H+ ion.  Therefore a solution with a pH of 9 is 10,000,000 X weaker than a solution with a pH of 2.
The strength of an acidic or basic solution is usually measured on the pH scale. On the pH scale a neutral solution has a pH of 7. (exactly the midpoint between 0 and 14).  Numbers lower than 7 mean acidic. The lower the number the stronger the acidic solution! Since the scale is based on the powers of 10 an acid solution with a pH of 2 is 10 X as strong as an acid solution with a pH of 3.
Any pH value larger than 7 indicates a basic solution. The higher the number the stronger the base. (A base with a pH of 12 is 100 X stronger than a base with a pH of 10.)
Follow this link to see an excellent pH demonstration
Go to the pH Calculations Worksheet
Go to the Acids & Bases Worksheet