Preparation of Bases

1. Direct Combination of Metal and Oxygen:

Metals like sodium, magnesium, potassium, calcium burn in air or oxygen to give the corresponding metallic oxide e.g

(i) Na_(s) + O_2(g)\( \rightarrow \) 2Na₂O_(s)

(ii) 2Ca_(s) + O_2(g) \( \rightarrow \)2CaO_(s)

(iii) 2K_(s) + O_2(g) \( \rightarrow \)2K₂O_(s)

(iv) 4Fe_(s) + 2O_2(g) \(\overset{heat}{ \rightarrow} \) 2Fe₂O_3(s)

2. Action of a Soluble Base on Water:

Soluble oxides of metals dissolve in water to give alkalis

(i) Na₂O_(s) + H₂O_(l)\( \rightarrow \) 2NaOH_(aq)

(ii) CaO_(s) + H₂O_(l) \( \rightarrow \)   Ca(OH)_2(aq)

3. Reaction of Metals with Steam or Water:

Some metals react with water or steam to give alkalis or basic oxide

(i) 2Na_(s) + 2H₂O_(l) \( \rightarrow \) 2NaOH_(aq) + H_2(g)

(ii) Ca_(g) + 2H₂O_(l) \( \rightarrow \)Ca(OH)_2(aq) + H_2(g)

(iii) Mg_(s) + H₂O_(g) \(\overset{heat}{ \rightarrow} \)Mg(OH)₂

4. Decomposition Reaction:

Insoluble basic oxides are obtained by the action of heat on the metallic hydroxide, trioxonitrate (V) and trioxocarbonate(IV) salts. Example:

(i) 2Al(OH)_2(s) \( \rightarrow \) Al₂O_3(s) + H₂O_(g)

(ii) CaCO_3(g) \( \rightarrow \)CaO_(g) + CO_2(g)

(iii) 2Cu(NO₃)₂ \( \rightarrow \) 2CuO_(s) + 4NO_2(g) + O_2(g)

5. Precipitation Method:

Insoluble hydroxides are precipitated by the action of sodium hydroxide solution on the aqueous solution of their salts e.g when excess sodium hydroxide solution is added to a solution of a copper (II) salt, a blue precipitate of copper (II) hydroxide is obtained.

\( \scriptsize NaOH_{(aq)} \: + \: CuSO_{4(aq)} \: \rightarrow \: Na_2SO_{4(aq)} \: + \: \underset {blue}{Cu(OH)_{2(aq)}} \)