Acids and bases are two well-known categories of chemical compounds. Acids and bases can be distinguished from one another based on a variety of observable physical and chemical properties. 
 

Properties of Acids and Bases

Acids can be distinguished from bases on the basis of observable properties. You will have to make such distinctions in this question in order to determine which two representations are characteristic of acids and which are characteristic of bases. This Question Group will focus on the following three contrasting properties:

pH and pOH Scale
Acid and base solutions can have their pH value tested. This refers to a number that provides an indicator of how they shift the H-OH components of water. To keep it simple, acids have a low pH ... less than 7 (in aqueous solutions at 25°C) and bases have a high pH ... greater than 7 (in aqueous solutions at 25°C).

The pH scale is not the only scale used as an indicator of acids and bases. There is also the so-called pOH scale. While the pH scale provides an indicator of the amount of H⁺ ions in solution, the pOH scale provides a measure of the amount of OH^- ions in solution.  Since these two ion concentrations are inversely related to one another, the pH and the pOH values are also inversely related to one another. So a solution with a low pH value (acid) will have a high pOH value. And a solution with a high pH value (base) will have a low pOH value.


Phenoplphtalein Indicator Test
An acid-base indicator is a chemical compound that is added to an aqueous solution to determine if it is acidic (pH<7) or basic (pH>7). One of the most popular acid-base indicators is phenolphthalein. A phenolphthalein indicator will turn pink in a basic solution and colorless in an acidic solution. As such, adding a drop or two of an indicator to an aqueous solution is a quick means of determining if the solution is acidic or basic.


Hydronium and Hydroxide Ion Concentrations
Neutral water contains equal amounts of hydrogen ions (H⁺) and hydroxide ions (OH^-). Adding an acid to neutral water will shift this balance of hydrogen and hydroxide ions. Adding acid to water increases the amount of hydrogen ions and decreases the amount of hydroxide ions. And adding base to water decreases the amount of hydrogen ions and increases the amount of hydroxide ions.

But what's hydronium ion have to do with this? It ends up that hydrogen ions don't really exist as hydrogen ions in an aqueous solution. The hydrogen ion will latch onto a water molecule and form hydronium ions - H₃O⁺. For practical purposes you can simply think of hydrogen ions as being the same thing as hydronium ions (since that is how they exist in water-based solutions). So the last sentence in the previous paragraph can be worded like this:  Adding acid to water increases the amount of hydronium ions and decreases the amount of hydroxide ions. And adding base to water decreases the amount of hydronium ions and increases the amount of hydroxide ions.

As mentioned, a neutral aqueous solution will have equal amounts of hydronium ions (H₃O⁺) and hydroxide ions (OH^-). At a temperature of 25°C, this amount is 1.0 x 10^-7 moles H₃O⁺ per liter os solution (abberviated as 1.0 x 10^-7 M). An acidic solution will have a hydronium ion (H₃O⁺) concentration that is greater than 1.0 x 10^-7 M. And a basic solution will have a hydronium ion (H₃O⁺) concentration that is less than 1.0 x 10^-7 M. You should know that a number like  1.0 x 10^-3 M is greater than 1.0 x 10^-7 M; and a number like  1.0 x 10^-10 M is less than 1.0 x 10^-7 M.