Boiling water is a familiar process, but the temperature it reaches is not unlimited. Many people assume that if you keep turning up the heat, the water will simply get hotter and hotter. Under normal conditions, the temperature of water is fixed once it begins to boil. This maximum temperature is determined by the pressure exerted on the water’s surface.
The Standard Boiling Temperature
The standard temperature at which water boils is a precise figure established under specific conditions. For pure water at standard atmospheric pressure (the average pressure at sea level), the boiling point is 100 degrees Celsius (212 degrees Fahrenheit). These values represent the temperature at which water’s vapor pressure overcomes the surrounding atmospheric pressure, allowing steam bubbles to form throughout the liquid. This temperature remains constant as long as the conditions of purity and surrounding pressure are maintained.
Why the Temperature Stops Rising
Once water reaches its boiling point, additional energy supplied does not increase the liquid’s temperature. This occurs because the added heat is converted into the latent heat of vaporization, which is used exclusively for the phase change. This energy breaks the intermolecular bonds holding the water molecules together in their liquid state. Instead of raising the average kinetic energy of the molecules, the energy is consumed to transform the liquid into steam. Increasing the heat simply increases the rate at which the liquid converts to steam, maintaining the temperature equilibrium until the water has fully transitioned into the vapor phase.
How Altitude Affects the Boiling Point
The boiling point of water is directly linked to the atmospheric pressure above it. At higher altitudes, the air pressure is lower, making it easier for water molecules to escape into the vapor state. With less external pressure to oppose, the water requires less energy and a lower temperature for its vapor pressure to equal the surrounding pressure. This means water boils below the standard 100°C (212°F) found at sea level. In a city like Denver, Colorado, which sits at a high altitude, water boils at about 93°C (200°F). For every 500 feet of elevation gain, the boiling point of water is lowered by nearly 1°F. While the water reaches the boil faster at altitude, the lower temperature means that foods prepared by boiling or simmering require longer cooking times to achieve the same result as at sea level.
Other Ways to Change the Temperature Limit
The boiling point can be manipulated through two main methods: adding solutes and applying external pressure.
Adding Solutes
When a non-volatile substance like salt or sugar is dissolved in water, the boiling point is raised, a concept known as boiling point elevation. The solute particles interfere with the water molecules’ ability to escape into the gas phase. This interference requires more energy and a higher temperature to initiate boiling.
Applying External Pressure
Applying pressure to a system is the most effective way to significantly increase the boiling temperature. A pressure cooker is a common household example, where the sealed lid traps steam, causing pressure to build inside the vessel. This increased internal pressure forces the water to remain liquid at temperatures well above 100°C, often reaching 121°C (250°F) or higher.
Superheating
A final, unstable way the temperature can exceed the normal limit is through superheating. This occurs when very pure water is heated in an exceptionally smooth container without nucleation sites for bubbles to form. The water temperature can temporarily rise above its boiling point without boiling, creating a dangerous, unstable state that can erupt violently into steam when disturbed.