- Bases are compounds which react with acid to form a salt and water as only products.
- Bases that soluble in water are called alkalis.
- In aqueous solution, alkali it produces hydroxide ions (OH–). In short, alkalis are substances that form hydroxide ions (OH–(aq)) in water
Sodium hydroxide NaOH gives Na+(aq) and OH–(aq) ions,
NaOH → Na+ + OH–
calcium hydroxide Ca(OH)2 gives Ca2+(aq) and 2OH–(aq) ions.Ca(OH)2 → Ca2+ + 2OH–
Ammonia give NH4+ and OH–NH3 + H2O → NH4+ + OH–
[Note: an alkali is a base soluble in water.]
- In alkaline solution there are more OH– ions than H+ ions.
Physical Properties of Alkali
- Alkalis are bitter in taste.
- Alkalis turn litmus from red to blue.
Like acid, alkali can change the colour of litmus. In alkali solution, the colour of litmus turn blue.
- Alkalis are soapy to touch.
- Alkalis has pH value more than 7
pH value is a measurement of the concentration of hydrogen ions in a solution. Alkali has very low concentration of solution hydrogen ion, even lower than water. Hence the pH value of alkali is higher than 7. (Note: The pH value of water is 7. The lower the concentration of hydrogen ions, the higher the pH value.
- Alkalis can conduct electricity
When a base dissolve in water, it will dissociate and form hydroxide ions. The present of the freely move ions make alkali an electrolyte.
Chemical Properties of Bases/Alkalis
- Alkalis react with acids to form a salt and water — this is a neutralisation reaction:
- Alkalis, when warmed with ammonium salts, give off ammonia gas:
Reaction between Alkalis and Acids
KOH + HNO3 → KNO3 + H2O
Alkali heat with Ammonium Salts
Ammonium Chloride + Sodium Hydroxide → Sodium chloride + Water + Ammonia
NH4Cl + NaOH → NaCl + H2O + NH3
2NH4Cl + Ca(OH)2 → CaCl2 + 2H2O + 2NH3
(NH4)2SO4 + 2NaOH→ Na2SO4 + 2H2O+ 2NH3
Ammonia as an Alkali
- By nature, ammonia is a covalent compound.
- Ammonia exists as gas at room temperature.
- Ammonia gas is soluble in water.
- Physical properties of ammonia:
- Pungent smell
- Turn litmus from red to blue
Oxide: Basic, Acidic, Neutral, or Amphoteric
- Most oxides of metal, especially alkali and alkaline earth metals, are basic oxides.
- Basic oxides that dissolve in water are called alkalis.
- A basic oxide is an oxide that shows basic properties in opposition to acidic oxides and that either
- reacts with water to form an alkali; or
- reacts with an acid to form a salt.
- Sodium oxide, which reacts with water to produce sodium hydroxide
- Magnesium oxide, which reacts with hydrochloric acid to form magnesium chloride
- Copper(II) oxide, which reacts with nitric acid to form copper nitrate
- Most non-metal oxides dissolve in water to form acids, and are called acidic oxides.
- An acidic oxide is an oxide that either
- reacts with water to form an acid; or
- reacts with a base to form a salt.
- Carbon dioxide which reacts with water to produce carbonic acid.
- Sulfur dioxide, which does not form the non-existent sulfurous acid but does react with bases to form sulfites.
- Silicon dioxide, which does not react with water but will react with bases to form silicates
- The metal oxides that can behave as both acids and bases and are said to be amphoteric oxides.
- Examples of amphoteric oxides are lead oxide, aluminium oxide and zinc oxide.