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 many of the following 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.


Litmus Paper Test
One of the quickest test to determine if a solution is an acid or a base is the litmus paper test. This involves dipping a special strip of paper (known as litmus paper) into the solution and observing the color of the paper. Litmus paper will turn red in acid solutions and blue in base solutions.



pH Paper
While a litmus paper test will only indicate if a solution is acidic or basic, a pH paper test will provide additional information - an estimate on the pH of the solution. The test uses a special strip of paper called pH paper or Hydrion paper. The paper strip is dipped in the aqueous solution and turns color. The resulting color depends on the pH. As such, a pH paper test provides an indicator of whether a solution is acidic or basic. The collection of colors for pH paper and their corresponding pH value is shown in the graphic below. As you can see, red-orange-yellow colors are indicators of acids and green-blue-violet colors are indicators of base.
 
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 a phenolphthalein to an aqueous solution is a quick means of determining if the solution is acidic or basic.


The Feel of a Base
In a lab environment, it is never a good idea to handle acids and bases with your bare hands. Yet many consumer products are alkaline or basic and they have a characteristic feel of being slippery. Soap is a good example of an alkaline or basic commercial product that has a slippery feel. Bases feel slippery.


Chemical Properties
Perhaps the most common chemical property of acids is their tendency to react with metals to produce hydrogen gas. While the reaction of an acid with a metal may be more vigorous with some metals rather than others, the metal reactivity test is always a great test to determine if an aqueous solution contains an acid.


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 re-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 a number like  1.0 x 10^-10 M is less than 1.0 x 10^-7 M.

The previous paragraph focused on how the hydronium ions (H₃O⁺) concentration value can be associated with the solution being an acid or a base. Similar statements can be made regarding the hydroxide ion (OH^-). At a temperature of 25°C, an acidic solution will have a hydroxide ion (OH^-) concentration that is less than 1.0 x 10^-7 M.  And a basic solution will have a hydroxide ion (OH^-) concentration that is greater than 1.0 x 10^-7 M.  



Check the Bottle
In a lab environment, acids and bases are dispensed in some sort of container. The container should be labeled and often it is labeled with the formula of its contents. The formula can be a tip-off as to whether the contents the contents of the bottle are acidic or basic. Acid formulas are typically written as HA where A is some anion accompanying the Hydrogen cation. And while not every base has the formula format of MOH (where M is the symbol of some metal ion), seeing that formula is a near certain indicator that the bottle contains a base.