The chemical formula of sodium carbonate is Na₂CO₃. Its common names include soda ash, soda, alkali ash, and washing soda. Under normal circumstances, it is a white powder and is a strong electrolyte. It has a density of 2.532 g/cm³ and a melting point of 851 °C. It is readily soluble in water, has the general properties of salts, and is a weak acid salt. It is slightly soluble in absolute ethanol and insoluble in propanol. When dissolved in water, it undergoes a hydrolysis reaction, making the solution alkaline. It has a certain degree of corrosiveness and can undergo a neutralization reaction with acids to form corresponding salts and release carbon dioxide.
At high temperatures, it can decompose to form sodium oxide and carbon dioxide. When exposed to the air for a long time, it can absorb moisture and carbon dioxide from the air to form sodium bicarbonate and clump together. It has strong hygroscopicity and easily forms hard lumps, and does not decompose even at high temperatures. Sodium carbonate forms three hydrates with water: Na₂CO₃·10H₂O, Na₂CO₃·7H₂O, and Na₂CO₃·H₂O. Among them, Na₂CO₃·10H₂O is the most stable, and its heat of solution when dissolved in water is very small. It is widely used in the photography industry, and its commercial name is carbogen.
Na₂CO₃·10H₂O is also known as crystalline soda. It is prone to efflorescence in dry air. In the past, crystalline soda was commonly used for household laundry and washing wool, so it was also called "washing soda". In the past, the so - called "lump alkali" that was commonly used by Chinese people for both laundry and dough - rising was made by mixing a large amount of water with soda ash (with some sodium bicarbonate (NaHCO₃) added). Its water content is over 50%. When sodium carbonate dissolves in water, it is an endothermic reaction, and it is prone to efflorescence in the air.
Na₂CO₃·7H₂O is unstable and can only be precipitated from a saturated sodium carbonate solution in the temperature range of 32.5 - 36 °C. Sodium carbonate is a salt formed from a weak acid and a strong base (soda ash is a salt, not an alkali, although its solution shows strong alkalinity). The Na₂CO₃ produced by chemical methods is purer than natural alkali, which is why people call it "soda ash".
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