2. Bases

Bases

Definition:  Bases are those substances, when dissolved in water, that give hydroxyl ions (OH–). 

Bases

Generally, bases are bitter in taste and feel soapy to touch. 

Examples:

  • Sodium hydroxide (NaOH) or caustic soda 
  • Potassium hydroxide (KOH) 
  • Calcium hydroxide: Ca(OH)2 

When a base like sodium hydroxide dissociates in water, it gives (Na+) and hydroxide(OH–) ions. 

Similarly, calcium hydroxide dissociates in water to give (Ca+) and hydroxide(OH–) ions. 

  

Alkalis

Alkalis are the bases that are soluble in water. 

Example of bases

 

Classification of Bases

Bases are classified into three categories based on various factors as follows. 

1. Based on Strength (Ionising Capability)

  • Strong Bases 
  • Weak Bases 

Strong Bases: 

The bases which completely dissociate in water and therefore produce a large number of hydroxyl ions (OH–) are known as strong bases. 

Examples:

  • Hydroxide of all reactive metals
  • Sodium Hydroxide (NaOH)
  • Potassium hydroxide(KOH)
  • Aluminium hydroxides (Al(OH)3) 

 

Weak Bases: 

The bases that do not completely dissociate in the water and give very few hydroxyl ions are known as weak bases. 

Examples:

  • Ammonium hydroxides (Al(OH)3), 
  • Copper hydroxide (Cu(OH)2),
  • Zinc hydroxide (Zn(OH)2),
  • Ferric hydroxide (Fe(OH)3) 

 

2. Based on Concentrations  

Based on the amount of water present in the given base sample, they are classified into two types. 

a. Concentrated Base: A given sample of a base that contains very little or no amount of water is called a concentrated base. 

b. Dilute base: If a given sample contains an amount of water very much than its own weight, then the given base is called the weak base. 

Based on Acidity of Base: 

 

3. Acidity

Acidity is defined as the number of hydroxyl ions [OH– ion] provided by one base molecule on complete dissociation in water. 

Types of Bases

i. Moacidic Bases

  • Acidity: Dissociates to give one OH– ion per molecule of base 
  • Examples: NaOH, KOH


ii. Diacidic Bases

  • Acidity: Dissociates to give two OH– ion per molecule of base 
  • Examples: Ca(OH)2, Mg(OH)

 

iii. Triacidic Bases

  • Acidity: Dissociates to give three OH– ion per molecule of base 
  • Examples: Fe(OH)3, Al(OH)3


Physical Properties of Bases 

  1. Taste: Bases are usually bitter. 
  2. Nature: Bases are slippery to feel/soapy in nature. 
  3. Action of an Indicator. 

When the red litmus paper dissolved in the solution of base, it turns into blue 

  1. Action of Electricity: Bases also conduct electricity when dissolved in water.  

This is because, when bases dissolved in the water, they dissociate into their respective ions (hydroxyl ions) which is helpful to conduct the electricity. 

 

Chemical Properties of Bases

  

Methods Of Preparation of Base

1. By the Action of Oxygen on Metals 

When the metals are heated with oxygen or air, they give their respective basic metal oxides. 

The general chemical reaction can be written as: 

Metal + Oxygen    →    Metallic oxide [Basic Oxide] 

For example:

  • when the metals like sodium, potassium, calcium burn in the air, they give their corresponding oxides. 

 

2. By the Action of Water / Steam on Reactive Metals 

Some reactive metals react with water or stream to give basic hydroxide with the evolution of hydrogen gas. 

Metal     +     Water/Steam     →   Basic hydroxide/oxide+ H2(g) 

Metals like sodium and potassium react with cold water to form hydroxides of sodium and potassium, respectively, with liberation hydrogen gas. This reaction is very violent. 

Magnesium reacts with steam to give magnesium oxide with the evolution of hydrogen gas. 

When superheated steam is passed over red-hot iron, ferric oxide is formed, and hydrogen gas is evolved. 

By Heating Carbonate s(Decomposition) of Some Metals: 

When the carbonates of some metal are heated, they decompose into their respective oxides with the evolution of carbon dioxide gas. 

When calcium carbonate is heated, calcium oxide and carbon dioxide are formed. 

 

Uses of Bases

1. Sodium Hydroxide (NaOH) 

  • Used in the manufacture of soaps, detergents. 
  • Used in the manufacture of textiles, papers. 

2. Magnesium Hydroxide (Mg(OH)2) 

  • Used as antacid to relief from acidity. 

3. Calcium Hydroxide (Ca(OH)2) 

  • Used to neutralize acidity of soil. 
  • Used as dry mixes in decorating, painting.