The boiling point of a liquid is the temperature at which its vapor pressure equals the external atmospheric pressure. For pure water at standard atmospheric pressure, this temperature is 100°C (212°F). When table salt is introduced, it dissolves and alters the water’s physical properties. This change in boiling behavior results from the salt interfering with water molecules’ ability to escape into the gas phase.
Defining Boiling Point Elevation
Adding salt to water immediately increases the water’s boiling temperature, a phenomenon known as boiling point elevation. This means the resulting salt solution requires a higher temperature than pure water to begin boiling. This alteration is classified as a colligative property, meaning the magnitude of the change depends only on the concentration of the solute particles, not on the solute’s chemical identity.
When table salt, which is sodium chloride (NaCl), dissolves in water, it dissociates into two separate ions: a positively charged sodium ion (Na+) and a negatively charged chloride ion (Cl-). This splitting effectively doubles the number of particles introduced compared to a non-ionic solute like sugar. Because boiling point elevation relies on the total number of dissolved particles, salt has a greater impact per gram than a compound that does not dissociate.
The Mechanism of Colligative Properties
Boiling point elevation is a direct consequence of vapor pressure lowering. A liquid boils when its vapor pressure reaches the surrounding atmospheric pressure. Adding a non-volatile solute, such as salt, lowers this vapor pressure, making water molecules less likely to escape the liquid surface.
The dissolved salt ions physically occupy space at the water’s surface, reducing the number of water molecules available to transition into the vapor phase. The charged ions also attract and hold onto nearby water molecules through strong ion-dipole interactions. This interference requires more energy, and thus a higher temperature, for the vapor pressure to equal the atmospheric pressure and allow the solution to boil.
The degree of this effect is quantified by the Van ‘t Hoff factor, denoted as \(i\), which represents the number of particles a solute dissociates into. For non-dissociating solutes like sugar, \(i\) is 1, but for sodium chloride, \(i\) is approximately 2, reflecting the two ions released. This factor confirms that ionic compounds like salt are effective at raising the boiling point compared to non-electrolytes.
Practical Impact in the Kitchen
Despite the scientific principle, the practical impact of adding salt to water in a home kitchen setting is minimal. The amount of salt typically added to a pot of water for cooking pasta, often one or two tablespoons per gallon, is far too small to create a significant temperature change.
A highly concentrated salt solution, such as 30 grams of salt per liter of water (roughly equivalent to seawater), only raises the boiling point by about 0.52°C. The small increase achieved with a typical pinch of salt is negligible for cooking purposes and will not noticeably speed up the cooking time. The necessary concentration of salt to make a difference would render the food inedible.