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# Boiling Point of Water: Examples and Calculations

Laura Foist, Scott van Tonningen
• Author
Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

• Instructor
Scott van Tonningen

Scott has a Ph.D. in electrical engineering and has taught a variety of college-level engineering, math and science courses.

Study how to calculate the boiling point of water at different pressures and altitudes. See water boiling point pressure and altitude charts to see how they impact boiling point. Understand how various added constituents to water can affect boiling point. Updated: 01/04/2022

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## The Boiling Point of Water

The boiling point refers to the temperature at which a liquid begins to bubble and convert to a gas. Calculations are commonly performed for the boiling point of water. The boiling point of water is very important in many scenarios, such as for food processing and making canned foods, or using steam as a power source.

The boiling point of a liquid can change based on several factors, including altitude, air pressure, or the presence of other constituents in the liquid (such as salt or sugar). When heating up water the temperature of the water continues to rise, but remains a liquid until it reaches a critical point. At this boiling point, a few things begin to happen:

• The liquid converts to gas
• Temperature of the liquid no longer increases
• Vapor pressure of the liquid is equal to surrounding air pressure

At a liquid's boiling point bubbles will form on the surface of the water, because the water now has enough energy from the heat to break the surface tension of the water, thus forming bubbles. When water reaches this critical point it does not increase in temperature any further. Instead any additional energy (heat) added to the water changes the state of the water from a liquid to a gas, creating steam. This critical point is achieved when the vapor pressure of the water is equal to the air pressure surrounding the water, which is why the boiling point changes at different air pressures. This also explains why the boiling point changes with altitude, because different altitudes have different air pressures.

### What is the Boiling Point of Water at Sea Level?

The boiling point of water is often calculated at sea level, which correlates to zero feet or meters in elevation. The boiling point of water at sea level is 100 degrees Celsius, or 212 degrees Fahrenheit. However, as mentioned earlier, the boiling point of water changes with altitude. This means that water will boil at different temperatures in locations that are at different altitudes.

City Elevation Boiling Point of Water in Degrees Celsius
Washington D.C. 0 m 100
Bogota, Columbia 2619 m 91.3
Lhasa, China 3490 m 88.2
La Rinconada, Peru 5100 m 82.5

As seen in this chart, as the elevation of the city increases, the boiling point of water decreases. As altitude increases, the air pressure decreases. As the air pressure decreases, less energy (heat) is required in order to increase the vapor pressure of the water so that it equals the surrounding air pressure. As a result, the boiling point decreases as elevation increases.

### Water Boiling Point Pressure Chart

The reason that boiling point decreases as elevation increases is because the air pressure decreases as elevation increases. A lower air pressure means a lower boiling point, and a higher air pressure means a higher boiling point. This graph shows how the boiling point changes as the pressure changes:

As shown in this graph, the boiling point temperature of a liquid increases as the air pressure increases. Also notice that the relationship isn't strictly linear (a straight line). Instead it curves, such that the increase in boiling point initially changes rapidly with any change in air pressure. However, as the air pressure gets higher and higher, the change in boiling point temperature doesn't change as quickly. This means the relationship between boiling point and air pressure is a logarithmic function, which is demonstrated by the equation used to find the boiling point of water from the pressure.

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## Boiling Point Calculations for Water

As mentioned, the boiling point of water at sea level standard pressure is 100 degrees Celsius, or 212 degrees Fahrenheit. Standard pressure refers to the air pressure that is standard at sea level. The value for standard pressure is:

• 14.7 atm
• 760 mm Hg
• 101325 Pa

However, it is important to know how to calculate the boiling point of a liquid at different pressures and altitudes as well.

### How to Calculate Boiling Point of Water at Different Pressures

The following formula can be used in order to calculate the boiling point of water at different pressures:

{eq}ln \frac{pressure}{101325} = 4890(\frac{1}{373} - \frac{1}{boiling point}) {/eq}.

In this formula, pressure is in pascals (Pa) and temperature is in Kelvin (K). Let's start with finding the boiling point of water if the air pressure is 245,675 Pa. We substitute the known air pressure value into the formula:

{eq}ln \frac{245675}{101325} = 4890(\frac{1}{373} - \frac{1}{boiling point}) {/eq}

And solve for boiling point:

• Simplify fraction: {eq}ln (2.42) = 4890(\frac{1}{373} - \frac{1}{boiling point}) {/eq}
• Take the natural log: {eq}0.886 = 4890(\frac{1}{373} - \frac{1}{boiling point}) {/eq}
• Divide both sides by 4890: {eq}1.811 \times 10^-4 = \frac{1}{373} - \frac{1}{boiling point} {/eq}
• Isolate boiling point fraction: {eq}\frac{1}{boiling point} = \frac{1}{373} - 1.811 \times 10^-4 {/eq}
• Solve subtraction: {eq}\frac{1}{boiling point} = 0.0025 {/eq}
• Isolate boiling point: {eq}\frac{1}{0.0025} = boiling point {/eq}
• Solve for boiling point: 400.02 K

This can then be converted into degrees Celsius by subtracting 273.15:

• {eq}400.02 - 273.15 = 126.87 {/eq}

Now let's look at another example, where a business wants to use steam from water for energy. They know that they need the temperature to be 254 degrees Fahrenheit to achieve the desired results. What pressure would the system need to be under to reach this temperature?

In this case the same equation is used, but we need to solve for pressure instead of boiling point. First, 254 degrees Fahrenheit needs to be converted into Kelvin. To convert from Fahrenheit to Kelvin we use to following formula:

{eq}Degrees Kelvin = ((Degrees Fahrenheit - 32) \times \frac{5}{9})+273.15 {/eq}

Substitute the known temperature value of 254 degrees Fahrenheit into this formula:

{eq}((254 - 32) \times \frac{5}{9})+273.15 {/eq}

Solving the equation results in a temperature value of 396.5 Kelvin. This value can now be used for boiling point in the logarithmic equation to solve for air pressure:

• {eq}ln \frac{pressure}{101325} = 4890(\frac{1}{373} - \frac{1}{396.5}) {/eq}
• Subtract fractions: {eq}ln \frac{pressure}{101325} = 4890(1.59 \times 10^-4) {/eq}
• Simplify: {eq}ln \frac{pressure}{101325} = 0.777 {/eq}
• Take inverse of natural log: {eq}\frac{pressure}{101325} = e^{0.777} {/eq}
• Multiply both sides by 101325: {eq}pressure = 101325 \times e^{0.777} {/eq}
• Solve for pressure: {eq}pressure = 220375 Pa {/eq}

### How to Calculate Boiling Point of Water at Different Altitudes

To find the boiling point of water at different altitude, first the air pressure at that altitude must be calculated. That new pressure value can then be plugged into the previous equation to find the boiling point. The equation to find air pressure at a specific altitude is:

{eq}P = P_0e^{\frac{-gMh}{RT}} {/eq}

Where:

• P is the new pressure being calculated
• {eq}P_0 {/eq} is the pressure at sea level. This can be in Pascals (101325), atm (1 atm), or psi (14.7 psi)).
• g is the gravitational acceleration. On Earth this is {eq}9.8 m/s^{2} {/eq}

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#### How do you find the boiling point of water at different elevations?

To find the boiling point of water at different elevations, first find the air pressure at that elevation, and then use that air pressure to calculate the boiling point.

#### Does pressure affect the boiling point of water?

Yes. Since the boiling point of water is the temperature at which the vapor pressure of water equals the surrounding air pressure, as air pressure increases the necessary temperature to raise the water's vapor pressure also increases. And as air pressure decreases, the necessary temperature also decreases.

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