The chemical formula of washing soda is:

Washing soda is sodium carbonate decahydrate. Each unit of sodium carbonate is combined with ten molecules of water of crystallisation. It is a white, transparent crystalline solid.

Washing soda is prepared from baking soda or directly by the Solvay process (ammonia-soda process). In this process, brine (sodium chloride solution), ammonia and carbon dioxide react to form sodium hydrogen carbonate. When this is heated, it gives sodium carbonate (soda ash). Recrystallising soda ash from water gives washing soda.

When washing soda crystals are exposed to air, they lose nine molecules of water and form a white powder, the monohydrate (Na2CO3·H2O). This loss of water of crystallisation on exposure to air is called efflorescence.

Uses of washing soda:

• It is used as a cleaning agent in homes, especially for washing clothes.
• It removes the permanent hardness of water by reacting with calcium and magnesium salts to form insoluble carbonates.
• It is used in the manufacture of glass, soap and paper.
• It is used to make other sodium compounds, such as borax.
• Washing soda is a basic salt; its solution in water turns red litmus blue, because it is formed from a strong base (sodium hydroxide) and a weak acid (carbonic acid).

Common sodium compounds compared:

• Baking soda: sodium hydrogen carbonate (sodium bicarbonate), NaHCO3, used in cooking and as an antacid.
• Soda ash: anhydrous sodium carbonate, Na2CO3.
• Caustic soda: sodium hydroxide, NaOH, made by the chlor-alkali process.
• Common salt: sodium chloride, NaCl.

Why not the other options?

• ✗NaHCO3 is baking soda.
• ✗Na2CO3 is soda ash, the anhydrous form.
• ✗NaOH is caustic soda.