The boiling point of a liquid is the temperature at which it transforms into a gas. This transformation occurs when the liquid’s vapor pressure equals the surrounding atmospheric pressure. At this specific temperature, vapor bubbles can form throughout the liquid, allowing it to rapidly convert into a gaseous state.
The Baseline: Pure Water’s Boiling Point
Pure water has a boiling point of 100°C (212°F) at sea level under standard atmospheric pressure. This temperature serves as a reference point. The specific temperature required for water to boil is linked to its molecular structure.
Water molecules are held together by strong intermolecular forces, primarily hydrogen bonds. A significant amount of thermal energy is necessary to overcome these attractive forces, allowing individual water molecules to escape from the liquid phase and transition into a gas.
Why Salt Water Boils at a Higher Temperature
Adding salt to water causes it to boil at a higher temperature than pure water, a phenomenon known as boiling point elevation. This occurs because dissolved salt, typically sodium chloride, introduces solute particles in the form of sodium and chloride ions into the water. These ions interact with the water molecules.
The presence of these solute particles interferes with water molecules attempting to escape into the gas phase. The dissolved ions occupy space at the surface of the liquid and create attractive forces, called ion-dipole interactions, with the water molecules. This interference effectively lowers the vapor pressure of the solution.
Because the vapor pressure of the salt solution is lower, more thermal energy is required to raise it to the level of the surrounding atmospheric pressure. Consequently, a higher temperature must be reached for boiling to commence. This increase in boiling point is an example of a colligative property, meaning it depends on the total number of solute particles in the solution, not their specific chemical identity. For instance, sodium chloride dissociates into two ions (Na+ and Cl-), contributing more particles per unit of mass than a non-dissociating solute like sugar.
The extent of boiling point elevation depends on the salt concentration. For example, typical seawater, which contains about 3.5% salt, boils at approximately 102°C, about 2°C higher than pure water. Heavily saturated salt solutions, containing around 360 grams of salt per liter of water, can raise the boiling point by about 3°C, reaching temperatures close to 108°C.
What Else Influences Salt Water’s Boiling Point?
Other factors also influence the boiling point of water, including salt water. However, the small quantities of salt typically used in home cooking, such as a pinch or a teaspoon, result in a minimal and often imperceptible increase in boiling temperature.
External atmospheric pressure also plays a significant role in determining the boiling point. Boiling occurs when the liquid’s vapor pressure matches the ambient atmospheric pressure. At higher altitudes, atmospheric pressure is lower, which means water boils at a reduced temperature. For instance, at an elevation of 5,000 feet, water boils around 95°C (203°F), and at 10,000 feet, it boils at approximately 89.6°C (194°F).
Conversely, increasing the external pressure raises the boiling point. This principle is utilized in pressure cookers, which trap steam to build up internal pressure. Inside a pressure cooker, the pressure can increase by about 15 pounds per square inch (psi) above normal atmospheric pressure. This elevated pressure allows water to boil at much higher temperatures, typically around 121°C (250°F), which significantly speeds up the cooking process.