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ACIDS AND BASES

ACIDS AND BASES   

For thousands of years, people have known that vinegar, lemon juice, amla, tamarind and many other food items taste sour. However, only a few hundred years ago it was proposed that these things taste sour because they  contain 'acids'. The term acid comes from Latin term 'accre' which means sour . It was first used in the seventeenth century by robert boyle to label substance as acids and bases according to the following characteristics.  

While Robert Boyle was sussessful in characterising acids and bases he could not explain their behaviour on the basis of their chemical structure.This was accomplished by Swedish scientist  Svante Arrhenius in the late nineteenth century. He proposed thaton dissolving in water , manycompounds dissociate and form ions and their properties are mainly the properties of the ions they form. Governed by this, he identified the ions furnished by acids and bases responsible for thier characteristics behaviour and gave their definitions.

Arrhenius concept of acids and bases: The Arrhenius definition of acids says that they are compounds that give off H+ ions in water and that bases are compounds that give off OH- ions in water. Thus, according to this theory only protic acids are allowed and only hydroxide bases are allowed to be classified as an acid or a base.

Brοnsted-Lowry definition: The Bronsted-Lowry definition of acids is that acids are compounds that give off H+ ions when they react with another compound. Likewise, this definition says that bases are compounds that accept H+ ions from other compounds. Furthermore, it also brings a new concept of conjugate acids and conjugate bases. Since an acid after donating its proton is technically a base according to this definition and is referred to as a conjugate acid. So every acid has its conjugate base and vice versa. Also, the stronger is an acid, the weaker is its conjugate base and vice versa. 

Lewis concept of acids and bases: Acids are electron pair acceptors while bases are electron pair donors. Thus, electron deficient species like BF3 are Lewis acids while electron rich species such as tertiary amines are Lewis bases. Lewis acids may combine with Lewis bases to generate a salt. 

pH and pOH 

The basic criterion for distinguishing acids and bases in aqueous solutions  is the concetration of hydrogen and hydroxide ions. 

              solution , pH , pOH , and ion concetrations 

Solution

pH

pOH

[H+]

[OH-]

Neutral

7

7

1.0×10-7              

1.0×10-7

 Acid 

<7 

 >7

>1.0×10-7

<1.0×10-7

 Base 

 >7

<7

<1.0×10-7 

>1.0×10-7 






     ACIDS

  SOME EXAMPLES OF ACIDS ARE: 

1. Hydrochloric acid (HCL)  in gastric acid.

2. Ascorbic acid (vitamin c) in lemon and many fruits. 

3.Citric acid in orange and lemons.

4. Acetic acid in vinegar.

5. tannic acid in tea.

Properties of acids

BASES 

The term alkali is often used for water soluble bases.  

SOME EXAMPLES OF BASES ARE : 

1. sodium hydroxide (NaOH) or caustic soda used in washing soaps.

2. potassium hydroxide (KOH) or potash used in bathing soaps.

3. ammonium hydroxide use in hair dyes.




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