Metal oxides can be prepared by several methods depending on the metal's reactivity.

1.1 Direct Combination

Very reactive metals like magnesium and sodium burn in oxygen to form oxides:

$2Mg(s) + O_2(g) \rightarrow 2MgO(s)$

$4Na(s) + O_2(g) \rightarrow 2Na_2O(s)$

1.2 Thermal Decomposition of Carbonates

Less reactive metal carbonates decompose on heating to form oxides and carbon dioxide:

$CaCO_3(s) \xrightarrow{\text{heat}} CaO(s) + CO_2(g)$

$ZnCO_3(s) \xrightarrow{\text{heat}} ZnO(s) + CO_2(g)$

1.3 From Hydroxides

Some hydroxides decompose when heated to give oxides and water:

$Cu(OH)_2(s) \xrightarrow{\text{heat}} CuO(s) + H_2O(g)$

Worked Example: How would you prepare copper(II) oxide in the laboratory?

Answer: Heat copper(II) hydroxide in a crucible until it stops giving off steam. The greenish-blue hydroxide turns into the black oxide, copper(II) oxide.

2.1 Soluble Hydroxides (Alkalis)

Soluble hydroxides of sodium, potassium, and calcium are prepared by reacting the metal oxide with water:

$Na_2O(s) + H_2O(l) \rightarrow 2NaOH(aq)$

$CaO(s) + H_2O(l) \rightarrow Ca(OH)_2(aq)$

For potassium:

$K_2O(s) + H_2O(l) \rightarrow 2KOH(aq)$

2.2 Insoluble Hydroxides

Insoluble hydroxides are prepared by precipitation — mixing two soluble salts to form an insoluble product.

General method:

1. Dissolve a soluble salt of the required metal (e.g., copper(II) nitrate) in water
2. Dissolve a soluble hydroxide (e.g., sodium hydroxide) in water
3. Mix the two solutions — the insoluble hydroxide precipitates out
4. Filter, wash, and dry the precipitate

Example:

$Cu(NO_3)_2(aq) + 2NaOH(aq) \rightarrow Cu(OH)_2(s) + 2NaNO_3(aq)$

The blue precipitate of copper(II) hydroxide is filtered and washed with distilled water.

3.1 Soluble Carbonates (Sodium and Potassium)

Sodium carbonate is prepared industrially by the Solvay process. In the laboratory:

• React sodium hydroxide with carbon dioxide:

$2NaOH(aq) + CO_2(g) \rightarrow Na_2CO_3(aq) + H_2O(l)$

• Evaporate the solution to get crystals of sodium carbonate ($Na_2CO_3 \cdot 10H_2O$, washing soda)

3.2 Hydrogen Carbonates

When carbon dioxide is passed through a solution of sodium hydroxide or potassium hydroxide:

$NaOH(aq) + CO_2(g) \rightarrow NaHCO_3(aq)$

The solution on evaporation gives crystals of sodium hydrogen carbonate (baking soda).

3.3 Insoluble Carbonates

Insoluble carbonates are prepared by precipitation:

$Pb(NO_3)_2(aq) + Na_2CO_3(aq) \rightarrow PbCO_3(s) + 2NaNO_3(aq)$

$CuSO_4(aq) + Na_2CO_3(aq) \rightarrow CuCO_3(s) + Na_2SO_4(aq)$

Key property: All carbonates (except those of sodium, potassium, and ammonium) react with dilute acids to produce carbon dioxide gas:

$CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$

4.1 By Neutralization

React nitric acid with a base (metal oxide, hydroxide, or carbonate):

$HNO_3(aq) + NaOH(aq) \rightarrow NaNO_3(aq) + H_2O(l)$

$2HNO_3(aq) + CaCO_3(s) \rightarrow Ca(NO_3)_2(aq) + H_2O(l) + CO_2(g)$

4.2 By Action of Nitric Acid on Metals

Only less reactive metals like magnesium, zinc, lead, and copper react with dilute nitric acid:

$Mg(s) + 2HNO_3(aq) \rightarrow Mg(NO_3)_2(aq) + H_2(g)$

$Zn(s) + 2HNO_3(aq) \rightarrow Zn(NO_3)_2(aq) + H_2(g)$

The solution is then evaporated to obtain crystals.

5.1 Soluble Chlorides

Soluble chlorides are prepared by reacting dilute hydrochloric acid with a metal, metal oxide, hydroxide, or carbonate:

Procedure in the laboratory:

1. Add the metal, oxide, or carbonate gradually to dilute hydrochloric acid in a beaker
2. The mixture may effervesce (release gas) — continue adding until no more reaction occurs
3. Filter to remove any excess solid
4. Evaporate the filtrate to get crystals of the chloride

5.2 Insoluble Chlorides (Precipitation)

Some chlorides (e.g., silver chloride, lead(II) chloride) are insoluble and must be prepared by precipitation:

$AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)$

$Pb(NO_3)_2(aq) + 2NaCl(aq) \rightarrow PbCl_2(s) + 2NaNO_3(aq)$

Procedure: Mix solutions of two soluble salts. The insoluble chloride forms as a white precipitate, which is filtered and dried.

6.1 Soluble Sulphates

Prepare soluble sulphates by reacting dilute sulphuric acid with a metal, metal oxide, hydroxide, or carbonate:

Note: Concentrated sulphuric acid should NOT be used for this preparation — it can cause excessive charring.

6.2 Insoluble Sulphates

Insoluble sulphates like barium sulphate are prepared by precipitation:

$BaCl_2(aq) + Na_2SO_4(aq) \rightarrow BaSO_4(s) + 2NaCl(aq)$

• Always add acid to water, not water to acid
• Use fume cupboard when gases are released
• Handle hot apparatus with care using tongs
• Wash hands after handling chemicals

In Tanzania, sodium carbonate (washing soda) is used in many households for cleaning clothes and removing grease. Local small-scale industries and soap makers in places like Arusha and Mwanza prepare cleaning products using sodium hydroxide (caustic soda), which they obtain by reacting sodium carbonate with calcium hydroxide. Understanding these preparation methods helps Tanzanian students appreciate how common household items are made from metal compounds that can be prepared in the laboratory.