Sodium carbonate

Na 2
C 1
O 3

6 atoms in one formula unit of an ionic lattice.

The drawing shows one formula unit of an ionic lattice, not a molecule; sodium carbonate has no molecular shape.

IUPAC name: disodium;carbonate
Molar mass: 105.9880(70) g/mol
Atoms: 6
SMILES: C(=O)([O-])[O-].[Na+].[Na+]
InChIKey: CDBYLPFSWZWCQE-UHFFFAOYSA-L
PubChem CID: 10340

Made of

Sodium carbonate gives off heat as it takes up water, and the solution it makes is strongly alkaline. A 1 per cent solution by weight reads pH 11.37 and a 10 per cent one 11.70, both at 25 °C, in HSDB, the US Hazardous Substances Data Bank. Carbonates are much weaker bases than hydroxides, yet that alkalinity lets sodium carbonate stand in for sodium hydroxide in some jobs.

The salt pairs two sodium ions with every carbonate ion, for a molar mass of 105.99 g/mol. The US National Cancer Institute’s thesaurus calls it the disodium salt of carbonic acid, and like other carbonates it acts as a base. It dissolves readily, 30.7 g in 100 g of water at 25 °C. Industry buys it as soda ash, the anhydrous (water-free) salt, while the decahydrate goes by sal soda or washing soda.

Structure and bonding

Six atoms make up the drawing, in three pieces with no bond between them: two sodium ions marked +1 each, and the carbonate ion. Its carbon atom holds one oxygen by a double bond and two by single bonds, each of those two marked −1. Na₂CO₃ records the fewest ions whose charges cancel: one carbonate, 2−, for two sodium ions. The drawing fixes the double bond on one oxygen; the real ion’s bonding is set out on the calcium carbonate page.

Those pieces are one formula unit of the anhydrous salt’s ionic lattice, not a free-standing particle. Washing soda is a different solid, the decahydrate. It is a hydrate, a salt that builds water molecules into its crystal lattice, the class set out on the copper(II) sulfate page. Its transparent crystals melt at 34 °C, a figure for the decahydrate alone.

The two forms answer air in opposite ways. The anhydrous powder is hygroscopic, slowly absorbing water from the air. The decahydrate is efflorescent, readily losing water to the air. A hydrate tends to do so when the air’s water vapour pressure falls below its own vapour pressure, the pressure of the water vapour it gives off.

Where it turns up

The world’s largest deposit of trona, a mineral of sodium carbonate, sodium bicarbonate and water together, lies in Wyoming’s Green River Basin, according to the US Geological Survey (USGS). Trona was first found at depth there in January 1938, by an oil test well in the Westvaco area. Fossil Lake, preserved in the Green River Formation, shrank and grew saltier in its final stage, leaving sediments rich in trona (National Park Service).

In California the USGS estimates soda ash reserves of 810 million tons at Searles Lake and Owens Lake. Soda Lake in the Mojave, near Baker, wears a white crust of alkaline salts, mostly sodium carbonate and bicarbonate, left by evaporation. Near Tanzania’s Lake Natron, the volcano Ol Doinyo Lengai produces molten sodium and calcium carbonates, NASA’s Earth Observatory reports. Beyond Earth, the bright spots on the dwarf planet Ceres are crusts mostly of sodium carbonate, left where salty water rose and evaporated, NASA/JPL reports.

Why it matters

Glass takes the largest part of US soda ash, 45 per cent in 2024 on USGS figures. In the furnace sodium carbonate acts as a flux, an additive that lowers the temperature at which silica, the principal component of glass, melts. Recycled glass, called cullet, cuts the soda ash that container makers need. Another 3 per cent went to neutralising sulfur dioxide in the flue gases of power plants.

Synthetic soda ash comes from limestone and salt by the Solvay process. Ammonia is added to brine, carbon dioxide bubbled through precipitates the less soluble sodium bicarbonate, and heating gives the carbonate. About 70 per cent of world capacity uses it, on HSDB’s figure; China, the leading producer in 2024 by USGS estimates, made 36 million tons, mostly synthetic. The United States has relied almost entirely on trona since 2000, according to the US Environmental Protection Agency. High energy costs and strict pollution controls drove that change, on HSDB’s account, and Solvay plants discharge large volumes of chloride-bearing waste.

Properties

Properties of sodium carbonate Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 10340.
Property	Value	Unit	Source
Formula (Hill order)	CNa₂O₃
Molar mass	105.9880(70) g/mol	g/mol	IUPAC/CIAAW 2024
Sodium atoms	2
Carbon atoms	1
Oxygen atoms	3
PubChem molecular weight	105.988 g/mol	g/mol	PubChem

Safety

Sodium carbonate’s main hazard is to the eyes. Under the EU’s harmonised classification it carries the signal word Warning and the statement that it causes serious eye irritation. The International Chemical Safety Cards call it severely irritating to the eyes and mildly irritating to the skin and airways, and repeated or prolonged skin contact may cause dermatitis. The PubChem Laboratory Chemical Safety Summary for sodium carbonate gathers these classifications.

Questions

What is the difference between soda ash and washing soda?

Both are sodium carbonate. Soda ash is the anhydrous salt that the formula Na₂CO₃ and the molar mass of 105.99 g/mol describe, and the Haz-Map database puts its technical grade at about 99 per cent pure. Washing soda, or sal soda, is the decahydrate, a hydrate whose crystals carry water in their lattice. It is sold in very small quantities next to the anhydrous form, and occurs in nature as the mineral natron.

Why is soda ash used to make glass?

Sodium carbonate is a flux: mixed with silica, the principal component of glass, it brings down the temperature at which the silica melts, according to the US Environmental Protection Agency and Chem1. The National Park Service lists the ingredients of glass as sand, soda ash, potash and lime. At Jamestown in 1608 the colonists made glass from sand off nearby beaches, with soda ash from seaweed they burned.

How does washing soda soften hard water?

Hard water carries calcium and magnesium ions, which form unwanted precipitates with soap. Washing soda's main laundry job was to remove them first as insoluble carbonates, while its alkaline solution also emulsified grease (Chem1). The US Geological Survey lists soda ash's roles as a detergent builder: emulsifying oil stains, keeping dirt from settling back, supplying alkalinity and softening laundry water. Modern detergents have largely ended that market.

Is sodium carbonate the same as baking soda?

No. Baking soda is sodium bicarbonate, NaHCO₃, a separate salt. Trona, the mineral mined in Wyoming, holds both of them together with water. Damp can turn one into the other: in high humidity, the US Environmental Protection Agency notes, stored sodium carbonate may degrade into sodium bicarbonate.