Acids and bases .....

Acids and Bases   

Introduction :

      By the 15th century , chemists recognized a group of substances which had sour taste called Acids (In Latin acidus meaning "sour"). They also recognized another group of substances which had bitter taste and were used as good cleaning agent ,called Bases.

       In 16th century, it was recognized that bases react with acids to 'destroy' or "neutralize" them,forming an ionic compound called salt .

       Nearly all fruits and foods contain acids.All citrous fruits contains large amount of ascorbic acid(C6 H8 O6) or vitamin C.Ascorbic acid also acts as an anti-oxidant .Apples contain maleic acid.The souring of milk produces lactic acid,butter on rancidity gives butyric acid.The extract of vinegar is acetic acid .Chemists prepare large quantities of important industrial acids.They are manufactured from minerals and are known as inorganic acids or more commonly mineral acids.

    

Properties of Acids and Bases :

Acids:

1:Acids have sour taste .

2:They change the color of blue litmus into red .

3:Aqueous acid solutions,conduct electricity .

4:They react with bases to form salt and water .

Bases:

1:Bases have bitter taste .

2:Bases have slippery touch .

3:They change the color of red litmus into blue .

Aqueous solution of bases,conduct electricity .

Acids and Bases (Arrhenius Theory):

        Svante-Arrhenius , a Swedish chemist in (1887) first gave the clue to chemical nature in his theory of ionization .It states that an acid can be difined as a substance that yields (H+) ions when dissolved in water and base can be defined as a substance that yields (OH-) when dissolved in water .

        Arrhenius acid  HCL(aq) -----------------H+(aq)+cl-(aq)

        Arrhenius base NaOH(aq -----------------Na+(aq)+OH-(aq)