SS1 Chemistry Third Term: Bases and Alkalis

Introduction

Bases are substances that, in aqueous solution, are slippery to the touch, taste bitter, change the colour of indicators (e.g., turn red litmus paper blue), react with acids to form salts, and promote certain chemical reactions (base catalysis). Bases are usually metallic oxides or metallic hydroxides.

Bases that are soluble in water are called alkalis.

 

In aqueous solution, alkali produces hydroxide ions (OH^–). In short, alkalis are substances that form hydroxide ions (OH^–(aq)) in water

Examples

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^–

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In alkaline solution there are more OH^– ions than H⁺ ions

Strength of Alkalis

Similar to strength of acids, the strength of an alkali is defined by its ability to ionise and release hydroxide ions (OH^–) in the solution.

In a solution of strong alkali, all the alkali molecules are ionised in the water to produce hydroxide ions

In a solution of weak alkali, only small portion of the molecules are ionised to release hydroxide ions.

Table below shows some example of strong/weak alkalis.

Alkali
Strong	Weak
NaOH KOH LiOH	NH3

 

Physical Properties of Alkali

The following are the physical properties of alkali

1. Alkalis are bitter in taste.
2. 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.
3. Alkalis are soapy to touch.
4. 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.
5. 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…

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