What are Bases?

What are Bases?

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 alkalis. A base reacts with an acid to give salt and water only.

Base + Acid → Salt + Water

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

ZnO_(s) + 2HCl_(aq)  → ZnCl_2(aq) + H₂O_(l)

Similarly,sodium hydroxide ionises in water to give hydroxyl ions 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 in solution. Table 6.3 enlists various bases and ions formed by them in water.

1. Classification of Bases

Based on their Acidity

a) Monoacidic Base:

It is a base that ionises in water to give one hydroxide ion per molecule.

Example: NaOH, KOH

b) Diacidic Base:

It is a base that ionises in water to give two hydroxide ions per molecule.

Example: Ca(OH)₂. Mg(OH)₂

c) Triacidic Base:

It is a base that ionises in water to give three hydroxide ions per molecule.

Example: Al(OH)₃, Fe(OH)₃

Based on concentration

a) Concentrated Alkali

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

b) Dilute Alkali

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

Based on Ionisation

a) Strong Bases:

These are bases which ionise completely in aqueous solution.

Example: NaOH, KOH

b) Weak Bases

These are bases that ionise partially in aqueous solution.

Example: NH₄OH, Ca(OH)₂

2. 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.

Zn + 2 NaOH → Na₂ZnO₂ + 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 non-metallic 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.

NaOH + NH₄Cl → NaCl + H₂O + NH₃ ↑

In the above activity you can observe that the bulb will start glowing only in the case of acids. But you will observe that glucose and alcohol solution do not conduct electricity. Glowing of the bulb indicates that there is a flow of electric current through the solution. The electric current is carried through the solution by ions.

Repeat the same activity using alkalis such as sodium hydroxide and calcium hydroxide.

3. Uses of Bases

·           Sodium hydroxide is used in the manufacture of soap.

·           Calcium hydroxide is used in white washing of building.

·           Magnesium hydroxide is used as a medicine for stomach disorder.

·           Ammonium hydroxide is used to remove grease stains from cloths.