Understanding the Boiling Point
To understand why water boils at a lower temperature at high altitudes, we must first define what it means for a liquid to boil. Boiling occurs when the vapor pressure of a liquid becomes equal to the external atmospheric pressure acting upon its surface.
The Physics of Boiling
- Vapor Pressure: Molecules at the surface of a liquid are constantly escaping into the gas phase. This creates a pressure known as vapor pressure. As temperature increases, the kinetic energy of molecules increases, leading to a higher vapor pressure.
- Atmospheric Pressure: This is the force exerted by the weight of the atmosphere above us. At sea level, this pressure is standardized at $1$ atmosphere (atm) or $101.3$ kPa.
- The Equilibrium: Boiling happens when the molecules have enough energy to push back against the weight of the air pressing down on them. Once the internal vapor pressure equals the external atmospheric pressure, bubbles can form throughout the bulk of the liquid, and boiling commences.
Why Altitude Matters
As you climb to higher altitudes (like a mountain top), there is less atmosphere above you. This means there is less air weight pushing down on the surface of your water. Consequently, the external atmospheric pressure is lower.
Because the external pressure is lower, the water's vapor pressure doesn't need to rise as high to match it. The water molecules reach this threshold at a lower temperature compared to sea level.
- At Sea Level: Atmospheric pressure is high, so water must reach $100^{\circ}C$ to boil.
- At High Altitude: Atmospheric pressure is low, so water might boil at $90^{\circ}C$ or even less.
Practical Implications
This is why cooking at high altitudes is more difficult! Because the boiling water is at a lower temperature, it doesn't provide as much thermal energy to food, making it take much longer for eggs, pasta, or vegetables to cook thoroughly. Chefs often use pressure cookers in these environments to artificially increase the pressure and force the boiling point back up.