Salts:

            Many chemical compounds may be classified as salts.  The salt most familiar to all of us is table salt -- sodium chloride.  Baking soda is the salt sodium bicarbonate.  Magnesium sulfate, also called Epsom salts, is often found in the home.

             In general, salts are ionic compounds that are composed of metallic ions and nonmetallic ions.  For example, sodium chloride is composed of metallic sodium ions and nonmetallic chloride ions.  Some salts are composed of metallic polyatomic ions and nonmetallic polyatomic ions (ammonium nitrate is composed of ammonium ions and nitrate ions).

 Properties of Salts:

            The salty taste of ocean water is due to the presence of salts such as sodium chloride and magnesium bromide.  There are many different salts present in salt water:

                Salts dissociate in water.  Salts consist of tightly bonded ions.  In water, these bonds are weakened and the ions become mobile.  This accounts for the fact that salt solutions are generally electrolytes.  In water, for example, sodium chloride ionizes, or dissociates like this:

NaCl (s)   à   Na¹⁺ (aq)  +  Cl^1- (aq)

             Salts may react with water.  Some salt solutions, when tested with litmus show acid, others base, and others still nothing.  How does this happen? 

1)   When sodium carbonate dissolves in water, the salt liberates sodium ions and carbonate ions.  At the same time, the water itself ionizes slightly to form hydrogen and hydroxide ions (remember that water is a weak electrolyte):

Na₂CO₃ (s)  à   2 Na¹⁺  (aq)   +   CO₃^2- (aq)          HOH  à  H¹⁺ (aq)  +  OH^1- (aq)

2)  Thus, the following particles may be present in a solution of sodium carbonate:  water molecules, sodium ions, carbonate ions, hydrogen ions, and hydroxide ions.  The ions of opposite charge attract one another and combine to form sodium hydroxide and carbonic acid:

2 Na¹⁺ (aq)  +  CO₃^2- (aq)  +  2 H¹⁺ (aq)  +  2 OH^1- (aq)  à  2 NaOH (aq)  +  H₂CO₃ (aq)

The reaction of a salt and water to form an acid and a base is called a hydrolysis reaction.  Since acids and bases react to form water and salt (neutralization reactions)  hydrolysis reactions are the reverse of neutralization reactions.  Thus, when sodium carbonate is dissolved in water, carbonic acid and sodium hydroxide are formed.  Carbonic acid,  H₂CO₃ is the acid present in soda water.  Since carbonic acid decomposes on standing to form CO₂ gas and H₂O, it is called a weak acid.  From conductivity experiments we know that sodium hydroxide, NaOH, is a strong base. 

Na₂CO₃ (aq)  +  2 HOH (l)  à  H₂CO₃ (aq)  +  2 NaOH  (aq)

 When the acid formed in a hydrolysis reaction is stronger that the base, the effect of such a solution on litmus is that of an acid.  For example; a water solution of the weak base, ammonium hydroxide and water,  produces ammonium hydroxide (a weak base) and hydrochloric acid ( strong acid).  Ammonium hydroxide, on standing, decomposes to form gaseous ammonia, NH₃.

 When both the acid and the base in a hydrolysis reaction are equally strong (or equally weak), the effect of such a solution on litmus is neither that of an acid or a base.  For example, when sodium chloride is dissolved in water and the solution is tested with litmus, neither color changes, indicating that the solution is neither acidic nor basic.  Such a solution is said to be neutral and the sodium chloride has not undergone hydrolysis. 

 Uses of Salts:

Preparation of Salts:

            Salts may be prepared by three methods: 

1) neutralization of acid and base --  When an acid and base react, they counteract each other, that is, they neutralize each other.  Such a reaction, known as a neutralization reaction, results in the formation of water and a salt.  For example, when sodium hydroxide and hydrochloric acid react, water and the salt sodium chloride are formed.  This occurs because the hydrochloric acid and the sodium hydroxide first ionize, and then react.  The compounds ionize releasing hydrogen, chloride, sodium, and hydroxide ions.  Since these are mobile in solution, hydrogen ions meet hydroxide ions and unite to form water.  At the same time sodium ions and chloride ions remain as aqueous salt.

2) Direct combination  --  When a metal reacts with a nonmetal, a salt is generally formed.  For example, when the metal magnesium is burned in chlorine gas, the salt magnesium chloride is formed.

3) Metal oxide and acid  --  when a metal oxide reacts with an acid, a salt is formed.  For example, when calcium oxide reacts with nitric acid, the salt calcium nitrate is formed.