What are Bases? Definition, Properties and Uses of Bases

 Bases- Definition, Properties and Uses 

What are Bases? 

Base is a chemical substance that donates electrons ,accepts protons, or releases hydroxide (OH^-) ions in aqueous solution.They neutralize the acid by react with hydrogen ions.

According to Arrhenius theory, bases are substances that ionise in water to form hydroxyl ions (OH^-). There are some metal oxides which give salt and water on reaction with acids. These are also called bases. Bases that are soluble in water are called as alkalis. A base reacts with an acid to give salt and water only.

                Base + Acid → Salt + Water

For example, zinc oxide (ZnO) reacts with Hydrochloric acid (HCl) to give the salt zinc chloride and water. 

                ZnO_(s) + 2HCl_(aq) → ZnCl_2(aq) + H₂ O_(l)

Similarly, sodium hydroxide (NaOH) ionises in water to give hydroxyl ions (OH^-) and thus get dissolved in water. So it is an alkali.

                NaOH_(aq) → Na⁺ _(aq) + OH^- _(aq)

Bases contain one or more replaceable oxide or hydroxyl ions (OH^-) in solution.

" All alkalis are bases but not all bases are alkalis. For example: NaOH and KOH are alkalis whereas Al(OH)3 and Zn(OH)2 are bases. "                                                             

 Classification of Bases 

(a) Based on their Acidity 

Monoacidic Base: 

It is a base which ionises in water to give one hydroxide ion per molecule.Example: NaOH, KOH.

 Diacidic Base: 

It is a base which ionises in water to give two hydroxide ions per molecule.Example: Ca(OH)₂ , Mg(OH)_2.

Triacidic Base: 

It is a base which ionises in water to give three hydroxide ions per molecule.Example: Al(OH)₃ , Fe(OH)₃

(b) Based on concentration

Concentrated Alkali: 

It is an alkali having a relatively high percentage of alkali in its aqueous solution. 

Dilute Alkali: 

It is an alkali having a relatively low percentage of alkali in its aqueous solution.

 (c) Based on Ionisation 

Strong Bases: 

These are bases that ionise completely in aqueous solution. Example: NaOH, KOH.

Weak Bases: 

These are bases which ionise partially in aqueous solution. Example: NH₄ OH, Ca(OH)₂

" The term acidity is used for base, which means the number of replaceable hydroxyl groups (OH^-) present in one molecule of a base. "                              

Properties of Bases

a) They have bitter taste. 

b) Their aqueous solutions have soapy touch.

c) They turn red litmus blue. 

d) Their aqueous solutions conduct electricity. 

e) Bases react with metals to form salt with the liberation of hydrogen gas ( H₂).

                Zn + 2 NaOH → Na₂ ZnO2 + H₂ ↑

f) Bases react with non-metallic oxides to produce salt and water. Since this is similar to the reaction between a base and an acid, we can conclude that nonmetallic oxides are acidic in nature.

                Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

g) Bases react with acids to form salt and water.

                KOH + HCl → KCl + H₂O

The above reaction between a base and an acid is known as Neutralisation reaction.

 h) On heating with ammonium salts, bases give ammonia gas(NH₃ ↑).

             NaOH + NH₄Cl → NaCl + H₂O + NH₃↑ 

  "Few metals do not react with sodium hydroxide. Example: Cu, Ag, Cr"                      

Uses of Bases 

(i) Sodium hydroxide is used in the manufacture of soap. 

(ii) Calcium hydroxide is used in white washing of building. 

(iii) Magnesium hydroxide is used as a medicine for stomach disorder like used as a antacid. 

(iv) Ammonium hydroxide is used to remove grease stains from cloths.

(v) Sodium Bicarbonate used a baking soda for food and also used as a alkaliser for the treatment of acidosis,and used as a Systemic Antacid.

(vi) Potassium  hydroxide is used for the preparation of soft soap.