Sodium carbonate
A versatile alkali used in glass, soap, and food.
Wikipedia / Wikimedia Commons
Sodium carbonate—also called washing soda, soda ash, sal soda, or soda crystals—is an inorganic compound with the formula Na₂CO₃ and several hydrated forms. All of these are white, odorless, water-soluble salts that create alkaline solutions when dissolved. In the past, it was obtained by burning plants that grew in sodium-rich soil. Because the ash from these plants differed from wood ash (which was used to make potash), the substance became known as "soda ash." Today, it is manufactured on a large scale from sodium chloride and limestone using the Solvay process, and also by carbonating sodium hydroxide produced via the chloralkali process.
Sodium carbonate exists as three hydrates and an anhydrous salt. The decahydrate (natron, Na₂CO₃·10H₂O) easily loses water to become the monohydrate. The heptahydrate (Na₂CO₃·7H₂O) is not found as a mineral. The monohydrate (thermonatrite, Na₂CO₃·H₂O) is also known as crystal carbonate. Anhydrous sodium carbonate (natrite), or calcined soda, is made by heating the hydrates, and also forms when sodium hydrogencarbonate is heated—for example, in the final step of the Solvay process. The decahydrate crystallizes from water solutions between −2.1 and 32.0 °C, the heptahydrate between 32.0 and 35.4 °C, and above that the monohydrate forms. In dry air, both the decahydrate and heptahydrate lose water to become the monohydrate. Other hydrates, such as one with 2.5 water molecules per sodium carbonate unit (called "penta hemihydrate"), have also been reported.
Washing soda is the common name for sodium carbonate decahydrate (Na₂CO₃·10H₂O), which contains ten molecules of water of crystallization. It is made by dissolving soda ash in water and then crystallizing it: Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O. It is one of the few metal carbonates that dissolve in water.
Sodium carbonate has many uses. As a cleaning agent, it is found in many dry soap powders and works by saponification—converting fats and grease into water-soluble salts (soaps). It also softens water by removing calcium and magnesium ions. It is used in making glass, soap, and paper, as well as in producing other sodium compounds like borax.
In glassmaking, sodium carbonate acts as a flux for silica (SiO₂, melting point 1,713 °C), lowering the melting point so that it can be reached without special equipment. The resulting "soda glass" is slightly water-soluble, so calci
- field
- Inorganic chemistry, industrial manufacturing
- known_for
- Key component in glass, soap, paper, water softening, and food additive E500
- formula
- Na2CO3
- common_names
- Washing soda, soda ash, sal soda, soda crystals
- hydrates
- Decahydrate, heptahydrate, monohydrate, anhydrous
Lore & Background
Sodium carbonate occurs naturally in arid regions, especially in mineral deposits (evaporites) formed when seasonal lakes evaporate. Deposits of the mineral natron have been mined from dry lake bottoms in Egypt since ancient times, when natron was used in the preparation of mummies and in the early manufacture of glass. The anhydrous mineral form is called natrite, and sodium carbonate also erupts from Ol Doinyo Lengai, Tanzania's unique volcano.
Sodium carbonate is obtained as three hydrates and as the anhydrous salt: the decahydrate (natron), the heptahydrate (not known in mineral form), and the monohydrate (thermonatrite). The decahydrate is formed from water solutions crystallizing in the temperature range −2.1 to 32.0 °C, the heptahydrate in the narrow range 32.0 to 35.4 °C, and above this temperature the monohydrate forms. In dry air the decahydrate and heptahydrate lose water to give the monohydrate.
Sodium carbonate decahydrate, also known as washing soda, is the most common hydrate. It is used as a cleansing agent for domestic purposes like washing clothes, as it has detergent properties through saponification. It is also used for lowering the hardness of water, in the manufacture of glass, soap, and paper, and in the manufacture of sodium compounds like borax.
Reader's Guide
Sodium carbonate is a foundational industrial chemical with broad applications spanning household cleaning, glassmaking, water treatment, and food production. Its role as a flux for silica in glass manufacture is historically significant, lowering the melting point of silica to produce soda-lime glass, which has been the most common form of glass for centuries. In water softening, it removes calcium and magnesium ions by forming insoluble precipitates, a process essential for both domestic and industrial water treatment. As a food additive (E500), it serves as an acidity regulator, anticaking agent, raising agent, and stabilizer, and is a key component in kansui for ramen noodles and in moon cakes. Its use in the production of sherbet powder exploits the endothermic reaction with citric acid. Sodium carbonate is preferred over sodium hydroxide in many chemical processes because it is cheaper and safer to handle. It is also used in photography, swimming pools, dyeing, froth flotation, and as a precursor to sodium bicarbonate and sodium bisulfite. The compound's natural occurrence as natron in ancient Egypt underscores its long history of human use, from mummification to modern industry. Its cost advantage is notable: as a precursor to an ingredient in sodium-ion batteries, sodium carbonate costs 1% of what lithium carbonate costs for lithium-ion batteries.
Did You Know?
- Sodium carbonate decahydrate (washing soda) contains 10 molecules of water of crystallization and readily effloresces to form the monohydrate.
- The integral enthalpy of solution of sodium carbonate is −26.7 kJ/mol, and the Mohs hardness of the monohydrate is 1.3.
- Sodium carbonate is used in the production of snus to stabilize the pH of the final product.
- Sodium carbonate erupts from Ol Doinyo Lengai, Tanzania's unique volcano.
More in Sodium Compounds 1-24
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