Boiling Point of Water – What Temperature Does Water Boil?

The normal boiling point of water is 100 °C, 212 °F, or 373.1 K. The “normal” refers to sea level or an elevation of 0 meters or feet. But, the boiling point of water changes with elevation. The boiling point is a higher temperature below sea level and a lower temperature above sea level.

Key Takeaways: Boiling Point of Water

• The normal boiling point of water is 100°C (212°F, 373.1 K) at sea level.
• Boiling occurs when vapor pressure equals atmospheric pressure.
• Higher elevations = lower boiling point (e.g., 87.8°C in La Paz, Bolivia).
• Lower elevations = higher boiling point (e.g., 101.4°C at the Dead Sea).
• Adding impurities (like salt) increases the boiling point slightly.
• Pressure cookers work by increasing pressure, raising the boiling temperature.
• Water boils at room temperature if you reduce the surrounding pressure.
• Boiling does not always mean sterilization—some microorganisms survive boiling temperatures.

Why Water Boils

Water boils when its vapor pressure equals atmospheric pressure. In a liquid state, water molecules are constantly moving, but intermolecular forces keep them together. As temperature increases, molecules gain energy and move faster. When they reach a point where their energy is enough to overcome atmospheric pressure, they transition into the gas phase—boiling occurs.

Factors That Affect the Boiling Point of Water

Key factors that determine when water boils include:

• Atmospheric Pressure: At sea level, water boils at 100°C (212°F). At higher altitudes, where atmospheric pressure is lower, water boils at a lower temperature.
• Impurities in Water: Adding dissolved substances (e.g., salt or sugar) increases the boiling point due to boiling point elevation.
• External Pressure: In a pressure cooker, increased pressure raises the boiling point, allowing food to cook at higher temperatures.
• Heat Source: Boiling requires heat. Adding more heat increases the rate of boiling. It does not affect the boiling temperature.

The reason boiling point changes with elevation is because atmospheric pressure changes. The effect is noticeable when you compare boiling point in a valley compared to a mountain top. For every 150 m (500 ft) increase in elevation, water’s boiling point decreases about half a degree Celsius or single degree Fahrenheit. But, even daily barometric pressure changes affect boiling point, although the temperature difference is too small to notice.

Impurities increase boiling point through a process called boiling point elevation. For example, adding salt to water increases its boiling point. While some people add salt to boiling water because they think it will cook food faster at the higher temperature, the effect is really too small to make a difference.

Boiling Point in Denver, La Paz and Other Places

Water boils at a lower temperature in cities like Denver and La Paz, but at a higher temperature in places like Death Valley and the Dead Sea. If you live in a high altitude location, food cooks at a lower temperature, so it often takes longer to cook. You can’t make water hotter by boiling it longer or applying more heat. This is why many recipes include high altitude cooking directions or recommend using a pressure cooker.

Calculating the Boiling Point of Water

The boiling point of water varies depending on atmospheric pressure, which changes with altitude and weather conditions. The Clausius-Clapeyron equation describes the relationship between pressure and phase changes.

Boiling Point vs. Altitude

The graph shows how the boiling point of water decreases with altitude in a linear manner (over a relatively narrow pressure range). At higher elevations, lower atmospheric pressure means water boils at a lower temperature. Conversely, at lower elevations (such as below sea level), water boils at a slightly higher temperature.

Boiling Point Calculator

Want to find the boiling point of water for a specific atmospheric pressure? Use the interactive calculator below! Simply enter the pressure in kPa, and it will calculate the boiling temperature of water:

This tool is particularly useful for high-altitude cooking, pressure cooker adjustments, scientific experiments, and understanding how weather affects boiling points.

Boiling Water at Room Temperature

Most people assume water only boils at 100°C (212°F), but it actually boils at room temperature if you lower the surrounding pressure enough. This happens because boiling occurs when vapor pressure equals atmospheric pressure, and atmospheric pressure can be reduced artificially.

How to Boil Water at Room Temperature

Here are a few ways to demonstrate this effect:

1. The Syringe Experiment
   • Take a plastic syringe and pull a small amount of water into it.
   • Seal the open end with your finger and pull back on the plunger.
   • As the pressure drops inside, you may see bubbles forming—this is boiling at room temperature!
2. Using a Vacuum Chamber
   • Place a beaker of water inside a vacuum chamber.
   • Turn on the vacuum pump to lower the surrounding pressure.
   • Eventually, the water starts boiling at room temperature.
3. The Flask Experiment
   • Boil water in a flask until it fills with steam.
   • Seal the flask and turn it upside down into ice water.
   • As the steam condenses, the pressure inside the flask drops. The water boils again even though it’s cooling down.

Real-World Applications of Low-Pressure Boiling

Boiling water at a lower temperature is more than just a fun science demonstration. It has practical applications:

• Freeze-Drying Food and Pharmaceuticals: Water boils off at low temperatures in a vacuum, preserving food and medicine.
• Astronauts and Space Exposure: In the near-vacuum of space, a pressurized space suit prevent the boiling of bodily fluids.
• High-Altitude Effects: At extreme altitudes, such as on Mount Everest, water boils at just 69.9°C (157.8°F), making cooking a challenge.

Which Comes First? Does Water Freeze or Boil in Space?

Water immediately boils in the vacuum of space. Then, the vapor immediately crystallizes into ice. If you watch a rocket launch into space you sometimes see ice forming on surfaces. Also, when astronauts release urine into space, it vaporizes before it forms frozen crystals.

References

• DeVoe, Howard (2000). Thermodynamics and Chemistry (1st ed.). Prentice-Hall. ISBN 0-02-328741-1.
• Goldberg, David E. (1988). 3,000 Solved Problems in Chemistry (1st ed.). McGraw-Hill. section 17.43. ISBN 0-07-023684-4.
• West, J. B. (1999). “Barometric pressures on Mt. Everest: New data and physiological significance.” Journal of Applied Physiology. 86 (3): 1062–6. doi:10.1152/jappl.1999.86.3.1062