A base is a substance which reacts with an acid to form salt and water only OR A base is a proton acceptor OR

A base is the oxide or the hydroxide of metals.

Examples are Copper II oxide (CuO), ammonia (NH₃), sodium hydroxide (NaOH).

An alkali is a base which is soluble in water. Eg: Sodium hydroxide (NaOH), ammonia (NH₃), calcium hydroxide (Ca (OH)₂), potassium hydroxide (KOH).

Strong and Weak Alkalis

A strong alkali is one which will ionize completely in solution. Eg: NaOH, KOH.

NaOH(aq)→Na(aq)++OH(aq)-

A weak alkali is one that will ionize partially in solution. Eg: Ammonia, calcium hydroxide.

NH3+H2O↔NH4 (aq)++OH(aq)-

Physical Properties of Alkalis

• They turn moist red litmus paper blue
• They are corrosive
• Alkalis have a soapy feel
• They have a bitter taste
• They act as electrolytes

Preparation of Bases

176. By the burning of metals in air.

Na(s)+O2 (g)→Na2O(s)

Ca(s)+O2 (g)→2CaO(s)

176. By reacting a metal with water.

Ca(s)+H2O(l)→Ca(OH)2 (aq)

Chemical Properties of Alkalis

• Bases react with acids to form salt and water only (neutralization).

CuO(s)+H2SO4 (aq)→CuSO4 (aq)+H2O(l)

• Alkalis precipitates are insoluble in metal hydroxide from their salt solution.

FeCl2 (aq)+NaOH(aq)→Fe(OH)2 (aq)+2NaCl(aq)

TYPES OF OXIDES

1. Amphoteric Substances (Oxides)

This is an oxide that will behave both as an acid and a base. Eg: Al₂O₃, ZnO, PbO

2. Acidic Oxide

This is an oxide that will react with water to form an acid. Eg: CO₂, SO₂

CO2 (g)+H2O(l)→H2CO3 (aq)

OR will react with a base to form salt and water only.

3. Basic Oxide

It is one that will react with an acid to form salt and water only.

CuO(s)+H2SO4 (aq)→CuSO4 (aq)+H2O(l)

INDICATOR

These are weak organic acids that change color in solution depending on the H⁺ concentration (PH) in that solution. Eg: Litmus, phenolphthalein, methyl orange.

PH SCALE

The PH scale is a means of comparing the acidity or Basicity of a substance in aqueous solution. The scale ranges from 0-14. Any solution with PH less than 7 is an acid. Any solution with PH greater than 7 is a base and any solution with PH equals to 7 is neutral.

• PH>7 is acid
• PH=7 is neutral
• PH<7 is base

Uses of Alkalis

• Making detergents. Eg: Magnesium hydroxide
• In making plasters, cement and in neutralizing acid soil. Eg: Calcium hydroxide [Ca (OH)₂].
• Used in making toothpaste.
• Used in making soap. Eg: NaOH
• In electroplating and in making liquid soap. Eg: KOH.