Sodium Carbonate (Washing Soda)

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Important Compounds of Sodium

Sodium Carbonate

The Solvay Process, Soda Ash, and Chemical Properties.

By chemca Team • Updated Aug 2026

Sodium carbonate is one of the most important industrial chemicals. It exists in various hydrated forms, the most famous being the decahydrate, commonly known as Washing Soda. Understanding its industrial preparation via the Solvay process is crucial for JEE Advanced.

1. Formulas and Common Names

• Washing Soda: $Na_2CO_3 \cdot 10H_2O$ (Sodium carbonate decahydrate)
• Soda Ash: $Na_2CO_3$ (Anhydrous sodium carbonate)
• Crystal Carbonate: $Na_2CO_3 \cdot H_2O$ (Sodium carbonate monohydrate)

2. Preparation: The Solvay Process (Ammonia-Soda Process)

Sodium carbonate is universally manufactured using the Solvay Process. This process is highly efficient because it recycles ammonia and uses cheap, readily available raw materials.

Raw Materials Required:

• Brine solution (Concentrated $NaCl_{(aq)}$)
• Limestone ($CaCO_3$) — source of $CO_2$ and $Ca(OH)_2$
• Ammonia ($NH_3$) — acting as an intermediate/catalyst that is recycled.

Step 1: Ammoniation and Carbonation of Brine

Ammonia gas and Carbon Dioxide gas (obtained by heating limestone) are passed through a saturated solution of Sodium Chloride (brine). They first react to form ammonium bicarbonate.

$NH_3 + H_2O + CO_2 \rightarrow NH_4HCO_3$
(Ammonium Bicarbonate)

Step 2: Precipitation of Sodium Bicarbonate (The Core Principle)

The ammonium bicarbonate reacts with the sodium chloride. Because Sodium Bicarbonate ($NaHCO_3$) has a relatively low solubility in the concentrated brine solution, it precipitates out as a solid, driving the reaction forward.

$NaCl_{(aq)} + NH_4HCO_3 \rightarrow NaHCO_{3(s)} \downarrow + NH_4Cl_{(aq)}$

Step 3: Calcination (Formation of $Na_2CO_3$)

The precipitated Sodium Bicarbonate is filtered off and heated strongly (calcined). It decomposes to form Sodium Carbonate, releasing $CO_2$ (which is fed back into Step 1).

$2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + CO_2 \uparrow + H_2O \uparrow$

Step 4: Ammonia Recovery (Crucial for Economy)

The filtrate from Step 2 contains Ammonium Chloride ($NH_4Cl$). To make the process economically viable, ammonia is recovered by treating this filtrate with Calcium Hydroxide (slaked lime, obtained from limestone).

$2NH_4Cl_{(aq)} + Ca(OH)_{2(aq)} \rightarrow 2NH_{3(g)} \uparrow + CaCl_{2(aq)} + 2H_2O_{(l)}$

*The only true by-product (waste) of the Solvay process is Calcium Chloride ($CaCl_2$).

3. The Ultimate JEE Trap: Why not Potassium Carbonate?

Question: Can Potassium Carbonate ($K_2CO_3$) be manufactured by the Solvay Process?

Answer: NO.

The Chemical Reason:

The entire Solvay process relies on the precipitation of the intermediate bicarbonate in Step 2. Sodium bicarbonate ($NaHCO_3$) is sparingly soluble and precipitates out easily.

However, Potassium Bicarbonate ($KHCO_3$) is highly soluble in water. If we used $KCl$ instead of $NaCl$, the $KHCO_3$ formed would remain dissolved in the solution along with $NH_4Cl$. We would not be able to filter it out to calcine it. Therefore, $K_2CO_3$ cannot be made this way (it is made using the Leblanc process or electrolysis).

4. Properties: Efflorescence and Heating

Efflorescence in Air

Washing soda ($Na_2CO_3 \cdot 10H_2O$) is a transparent crystalline solid. When exposed to dry air, it spontaneously loses 9 molecules of its water of crystallization to form a white powder of the monohydrate. This phenomenon is called efflorescence.

$Na_2CO_3 \cdot 10H_2O \xrightarrow{\text{dry air}} Na_2CO_3 \cdot H_2O + 9H_2O$

Action of Heat (Formation of Soda Ash)

When the monohydrate is heated above $373\text{ K}$ ($100^\circ\text{C}$), it loses its remaining water molecule to become completely anhydrous. This anhydrous white powder is called Soda Ash.

$Na_2CO_3 \cdot H_2O \xrightarrow{\gt 373\text{ K}} Na_2CO_3 \text{ (Soda Ash)} + H_2O$

Note: Anhydrous Sodium Carbonate ($Na_2CO_3$) does not decompose upon further heating (even up to its melting point of $852^\circ\text{C}$) because alkali metal carbonates are thermally very stable.

5. Chemical Properties

A. Hydrolysis (Alkaline Nature)

Sodium carbonate is a salt of a strong base ($NaOH$) and a weak acid ($H_2CO_3$). In aqueous solution, it undergoes anionic hydrolysis to yield an alkaline solution (turns phenolphthalein pink).

$CO_3^{2-} + H_2O \rightleftharpoons HCO_3^- + OH^-$

B. Reaction with Acids

It neutralizes acids with brisk effervescence due to the rapid evolution of Carbon Dioxide gas.

$Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2 \uparrow$

C. Reaction with excess $CO_2$

When carbon dioxide gas is passed through an aqueous solution of sodium carbonate, it gets converted into the less soluble Sodium Bicarbonate.

$Na_2CO_3 + H_2O + CO_2 \rightarrow 2NaHCO_3$

6. Industrial Uses

• Extensively used in the manufacture of glass, soap, borax, and caustic soda.
• Used for the softening of hard water (it precipitates calcium and magnesium ions as insoluble carbonates).
• Used in the paper, paint, and textile industries.
• Used as an important laboratory reagent for qualitative and quantitative analysis (e.g., standardizing acids).

Knowledge Check

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