5b. Bases

Question 1

Base

Definition

Answer 1

A base is a substance that reacts with an acid to form salt and water only.

Question 2

Alkalis

Definition

Answer 2

Alkalis are bases that dissociate in water to produce hydroxide ions.

Question 3

Strong Alkali

Definition

Answer 3

A strong alkali completely dissociates in water to give hydroxide ions.

e.g. potassium hydroxide KOH, sodium hydroxide NaOH, calcium hydroxide Ca(OH)₂
Note: Calcium hydroxide is not very soluble in water (slightly soluble). However, those that dissolve will completely dissociate in water to give calcium ions, Ca²⁺(aq), and hydroxide ions, OH^-(aq). Hence, aqueous calcium hydroxide is still considered a strong alkali.

Question 4

Weak Alkali

Definition

Answer 4

A weak alkali partially dissociates in water to give hydroxide ions.

e.g. aqueous ammonia NH3, sodium carbonate NaCO₃
Note: When ammonia gas dissolves in water, only a small fraction of ammonia molecules react with water to produce ammonion ions, NH₄⁺(aq), and hydroxide ions, OH^-(aq). Most ammonia still remains as simple molecules in the solution.

Question 5

Physical and Chemical Properties of Alkalis

Answer 5

Physical properties of alkalis:

• Bitter
• Soapy
• Turn moist red litmus paper blue
• pH > 7
• Can conduct electricity (are electrolytes)

Chemical properties of alkalis:

• Alkalis react with acids to form salt and water only (neutralisation).
• Alkalis react with ammonium salts to form salt, ammonia gas, and water ON WARMING. (Warming isnecessary as ammonia is very soluble in water. Solubility of gas decrases with increase in temperature.)
• Alkalis react with some metal cations to form precipitate.

Question 6

Uses of Bases

Answer 6

1. To neutralise acids - elaboration in notes
2. To dissolve dirt and grease - elaboration in notes