Bases in chemistry- Class X Notes

Base definition and its properties

Definition of base and its properties is most important topic of NCERT  Acids bases salt chapter 2  in Class 10.  Questions are frequently asked in the CBSE  board  and  ICSE Board exam from  bases  . 

“Chemistry  – Define base : class 10 notes “ will be very beneficial for the students who are engaged in the preparation of  upcoming board exam.

In this topic, the following terms will be illustrated.

Contents :

* 1. Definition of base

* 2. What are alkalis

* 3. Physical Properties of base

* 4. Chemical Reactions of base

* 5. Uses of common  Bases

Base definition

• Base are the substance which neutralize the acid to form salt and water only .

Bases exist in two form

• Metal oxide  
• Metal hydroxide

According to Arrhenius principle,

• Those substances which dissociate into hydroxide  ions in aqueous solution are called base.

NaOH    =  Na⁺     +  OH^-

NH₄OH  =  NH₄⁺    +  OH^-

^Alkalis:  

• Water soluble bases are also known as alkalis.
• Strong Alkalis : NaOH, KOH
• Moderately strong alkalis:  Mg(OH)₂, Ca(OH)₂, Ba(OH)₂
• Weak alkalis :  NH₄OH

• Soluble bases are much more useful than insoluble bases because most of chemical reaction take place only in aqueous solution.

Strong alkali: 

• A base whose all molecules dissociate into hydroxide  ( hydroxyl) ions is called strong alkali.

e.g.  NaOH, KOH

Moderately strong alkalis:

• A base whose few molecules dissociate into hydroxyl ions is called moderately strong alkali.

e.g.  Mg(OH)₂, Ca(OH)₂, Ba(OH)₂

Weak alkali:

• A base whose very  few molecules dissociate into hydroxyl ions is called weak alkali.

e.g.  NH₄OH

Common Name of Alkalis

* NaOH   -   Caustic soda

* KOH  -       Caustic potash

* Mg(OH)₂   -   Milk of magnesia

* NH₄OH   -  Ammonia solution, ammonia water, aqueous ammonia, or aqua ammonia

* Ca(OH)₂  -    slaked lime

Note : Limewater is the common name for a saturated solution of calcium hydroxide.

Properties of Bases:

 Physical Properties:

• Bases have bitter tase.

• Bases feel soapy to touch.

• Bases turn red litmus blue.

• Bases conduct electricity in aqueous solution. i.e bases are electrolyte.

Chemical Reactions:

Alkalis      NaOH, KOH	+     Metal                     →	Salt   +  H2
	+     Acid                       →	Salt   +  H2O
	+  Non-Metal Oxide   →	Salt   +  H2O

Reaction With Some Metals:

•  Bases react with some metals  like Zn, Al,  Pb, Sn   to form salt and hydrogen gas.

Base  +  Metal  →   Salt  +  H₂

e.g.

2NaOH  +  Zn   →   Na₂ZnO₂   +   H₂

2NaOH  +  Pb   →   Na₂PbO₂   +   H₂

2NaOH  +  Sn   →   Na₂SnO₂   +   H₂

2NaOH +  2Al + 2H₂O   →   2NaAlO₂   +   3H₂

Note :

Na₂ZnO₂ is sodium zincate.

Na₂PbO₂ is sodium Plumbate.

Na₂SnO₂  is sodium  Stanate.

NaAlO₂   is sodium meta aluminate.

  

Reaction With Acid:

• Base react with acid to form  salt and water.

Base  +  Acid  →  Salt  +   Water

NaOH  +  HCl  →  NaCl   +  H₂O

 

Reaction With Non-metal oxide (Acidic oxide ) :

• Base react with  Non-metal oxide to form  salt and water.

Base  +  Acidic oxide  →  Salt  +   Water

Ca(OH)₂  (aq) + CO₂ (g)  → CaCO₃ (s)  +  H₂O

Uses of Bases:

Mg(OH)₂ :

• It is used as antacid to neutralize excess acid in the stomach.
• It is called milk of magnesia.

Na₂CO₃ :

• It is used as washing soda.
• It is used for softening hard water.

NaHCO₃ :

• It is used as baking soda in cooking.
• It is used for making baking powder.
• It is used as antacid.
• It is used in fire extinguisher.
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