PS-3.8   Classify various solutions as acids or bases according to their physical properties, chemical properties (including neutralization and reaction with metals), generalized formulas, and pH (using pH meters, pH paper, and litmus paper)
               
Key Concepts
Acid      pH    Base      Neutralization reactions       Neutral solution
    By definition, an acid is a substance that dissociates ( dissociate means it breaks apart)  to produce Hydrogen ions in an aqueous ( aqueous means water) solution.
    Look at the diagram shown below. It shows a formula unit of Hydrogen Chloride dissociating in water to form an aqueous solution of Hydrochloric acid. Its interesting to note that solid, crystalline, HCl is not Hydrochloric acid until it dissociates in water to form a solution. Until it dissolves it is solid, crystalline Hydrogen Chloride.
 
                                                                                                        An Acidic Solution of HCl(aq)
     The solution in the beaker shown above contains free hydrogen ions, H+. The free Hydrogen ions attach loosely to the water molecule forming  H3O+. H3O+ is referred to as the Hydronium ion.  This solution is an acid because it contains positively charged Hydrogen ions. The pH of this solution will fall somewhere below 7 on the pH scale. pH is a way to measure the Hydrogen ion concentration of the solution. Solutions that are acidic will have a pH below 7 on the pH scale. Strong acid solutions will have a pH closer to 0. Weaker acid solutions will  have a pH closer to 7.
    There are many acids. They all have one thing in common, they donate H+ ions to an aqueous solution. Click here to read about some of these acids and learn what they are used for.
A Base is a substance that dissociates to give a solution Hydroxide ions, OH-. One common Basic substance is Sodium Hydroxide, NaOH. Sodium Hydroxide is commonly used as drain cleaner. The diagram below shows Sodium Hydroxide dissociating in water to give the solution Hydroxide ions, OH-.
 
A Basic Solution of Sodium Hydroxide, NaOH(aq)
    The solution in this beaker is basic because it contains free Hydroxide (HO-)ions. The pH of this solution will fall somewhere above 7 on the pH scale.  Solutions that are basic will have a pH above 7 on the pH scale. Strong base solutions will have a pH closer to 14. Weaker base solutions will  have a pH closer to 7.
    There are many bases. They all have one thing in common, they contribute OH- ions to an aqueous solution. Click here to read about some of these bases and learn what they are used for.
    Shown below is the pH scale. Note that acids are below 7 and that the bases are above 7 on this scale. 
 
 
    On the pH scale, a pH of 7 is neutral. A neutral solution contains equal numbers of H+ and OH- ions. If  H+ and OH- ions are added to the same solution, they will combine chemically to produce water. If there are exactly the same number of H+ and OH- ions in the solution, there will be no leftover Hydrogen ions or Hydroxide ions. When there are no excess Hydrogen ions or Hydroxide ions, the solution is neutral. Of course, acids do not just contain Hydrogen and bases do not just contain Hydroxide ions. There is always something else that comes along with the ion. In a formula unit of HCl there is a Chlorine atom that comes along with every Hydrogen ion. In a formula unit of NaOH, there is a Sodium atom that comes along with every Hydroxide ion. In solution, the Hydrogen ions combine with the Hydroxide ions to form water, but at the same time the Sodium atom combines with the oppositely charged Chlorine atom making a formula unit if Sodium Chloride, NaCl. In general terms:
an acid + a base   a salt + water.
   This generalized chemical equation is called a neutralization reaction. Any acid plus any base produces a salt plus water. The term, "salt" simply means an ionically bonded, chemical combination, of a metal and a nonmetal. There are lots of salts, this one in this example just happens to be common table salt, NaCl.
Let's take a close look at what happens in the beaker of water when an acid is added to a base. The solution produced by adding equal quantities of acid and base is neutral. It consists of a salt plus water.
 
Acids can be distinguished from bases by their physical and chemical properties.
Acid solutions are electrolytes. Electrolytes are solutions that will conduct electricity.
Acid solutions have a tart or sour taste. This is why Citric acid and Phosphoric acid are added to candy and soda. It gives them a little sour kick. Citrus fruits, such as lemons, limes, and Oranges, contain Citric acid.
Indicators are used to determine the pH of a solution. The indicator blue litmus turns red in the presence of an acid. Other indicators will turn a specific color for each pH value. Indicators can change color to show if a solution is an acid or a base, because they change color in response to the concentration of H+ ions and OH ions in the solution.
Acid solutions have a pH less than 7
Acids react with active metals such as zinc and magnesium to form a salt and Hydrogen gas. We used an acid-metal reaction to fill the balloon with Hydrogen gas in the Hydrogen Demo.
For the purposes of Physical Science, the formula of an acid can be recognized because (except for water) the first element in the formula hydrogen.
Bases can be distinguished from acids by their physical properties.
Basic solutions have a slippery feel. The base Sodium Hydroxide, NaOH is used in soap making...soap is slippery!
Basic solutions conduct electricity (are electrolytes)
Basic solutions have a pH greater than 7.
Red litmus paper turns blue in the presence of a basic solution; other indicators will turn a specific color for each pH value
For the purposes of Physical Science, the formula of a base can be recognized because the formula ends in OH. Sodium Hydroxide, NaOH  is a base...note the OH at the end of the molecule. Calcium Hydroxide, Ca(OH)2 is a base, it also has hydroxide, OH, at the end of the molecule.