Sodium carbonate (\(\text{Na}_2\text{CO}_3\)), commonly known as soda ash or washing soda, is a white, odorless inorganic compound. It is classified as highly soluble, readily dissolving in water to form clear, alkaline solutions. This characteristic is fundamental to its many industrial and household uses.
The Chemical Reason for Solubility
Sodium carbonate is an ionic compound, composed of positively charged sodium ions (\(\text{Na}^+\)) and negatively charged carbonate ions (\(\text{CO}_3^{2-}\)). These ions are held together by electrostatic forces in a crystal lattice. For dissolution to occur, water must supply enough energy to overcome this lattice energy.
Water is a highly polar molecule. When sodium carbonate is introduced, the polar water molecules surround the individual ions, a process known as hydration. The negative end of the water molecule attracts the positive sodium ion, and the positive ends surround the negative carbonate ion.
The energy released during hydration is greater than the energy required to break the ionic bonds. This energy imbalance drives the dissolution process, causing the sodium carbonate to dissociate completely.
How Much Sodium Carbonate Dissolves
Sodium carbonate is highly soluble, but the exact quantity that dissolves is temperature-dependent. For the anhydrous form (\(\text{Na}_2\text{CO}_3\)), solubility at \(0^\circ\text{C}\) is about 7 grams per 100 milliliters of water. This increases sharply, peaking at 49.7 grams per 100 milliliters around \(35.4^\circ\text{C}\).
The solubility curve is complex because sodium carbonate forms several hydrates, which incorporate water molecules into their crystal structure. The most common is the decahydrate, known as washing soda. When dissolving the anhydrous form, solubility decreases slightly beyond the peak temperature. Even at \(100^\circ\text{C}\), solubility remains high at approximately 43.6 grams per 100 milliliters.
Why This Solubility Matters in Practice
The practical significance of sodium carbonate’s solubility stems from the chemical properties of the resulting solution. When the carbonate ion (\(\text{CO}_3^{2-}\)) dissolves, it undergoes hydrolysis, generating hydroxide ions (\(\text{OH}^-\)). This makes the solution alkaline; a one percent solution has a \(\text{pH}\) of approximately 11.3.
This alkalinity is leveraged in cleaning, where the high \(\text{pH}\) helps saponify fats and grease, making them water-soluble. Solubility is also exploited in water softening, which removes mineral ions that cause water hardness. Hard water contains dissolved metal ions, primarily calcium (\(\text{Ca}^{2+}\)) and magnesium (\(\text{Mg}^{2+}\)).
When sodium carbonate is added, the carbonate ions react with the calcium and magnesium ions. This forms calcium carbonate (\(\text{CaCO}_3\)) and magnesium carbonate (\(\text{MgCO}_3\)). These new compounds are highly insoluble and precipitate out as solids. Removing these ions softens the water, improving detergent performance and preventing scaling.