This article discusses about the relation between boiling point and atmospheric pressure. We all have this misconception that the boiling point of a liquid depends on the temperature. This is not true.
Temperature plays an important role in boiling but there is another factor which is responsible for boiling to take place. That is vapour pressure. This vapour pressure when reaches the value of atmospheric pressure outside the system, the liquid starts boiling.
What is vapour pressure?
Vapour pressure is the value of pressure exerted by the surface of the liquid when it is about to change its state to gaseous.
The temperature is increased to make the vapour pressure value reach the value of pressure of ambient. As temperature increases the pressure of the liquid also increases. This law is called as Gay-Lusaac’s law in thermodynamics.
Define boiling point using atmospheric pressure and vapour pressure
We all have seen boiling water atleast once in our life time. It is the point at which the liquid phase of a substance changes into gaseous phase.
Whenever this happens the ambient pressure and vapour pressure of the liquid will have same values. To achieve this equality, temperature of the liquid is increased continuously. As the temperature increases, pressure also increases. Boiling is observed by droplets of liquid rising up.
Does boiling point depend on atmospheric pressure?
Definitely yes. As we have discussed that vapour pressure when reaches the value of atmospheric pressure, boiling takes place.
If the value of atmospheric pressure is itself less, then it will take lesser amount of time for vapour pressure to touch the value of atmospheric pressure. This way the boiling point will be lower as compared to places where ambient pressure is more.
Effect of atmospheric pressure on boiling point
We have clearly discussed in above section regarding the effects of atmospheric pressure on the boiling point of a substance.
In a nutshell, we can say that atmospheric pressure and boiling both are directly dependent on each other, if one changes then other one also changes. They both are directly proportional to each other that is if one quantity increases other one will also increase. If the liquid starts boiling faster than surely the atmospheric pressure outside would also be low.
How to increase atmospheric pressure?
Atmospheric pressure mainly depends on the gravity. Due to gravity, the molecules are attracted to the surface.
So the atmospheric pressure will be more where the effect of gravity is more. In simple words, the places which are closer to the Earth’s surface (sea level), those places will have more atmospheric pressure than the places having more altitude.
Why does boiling point vary with atmospheric pressure?
Until now we have discussed a lot about effect of atmospheric pressure on boiling point in early sections of this article. They both are directly related to each other and boiling point is dependent on the value of atmospheric pressure outside the system.
This is because when the vapour pressure at the surface of the liquid starts getting higher and getting closer to the value of atmospheric pressure, then the boiling starts taking place. If the value of atmospheric pressure is low then the liquid will start boiling very easily ad faster.
How do atmospheric pressure and elevation affect boiling point?
Gravity. Gravity is the main reason behind the entire episode of boiling point’s dependency on elevation and atmospheric pressure.
Gravity is more at Earth’s surface and gets slightly lower as we go higher. Due to gravity, the molecules in the atmopshere get closely packed near the surface and are slightly scattered at higher elevations. We know the direct affect of atmospheric pressure and higher altitudes on boiling point of a substance.
Boiling point and atmospheric pressure equation
Clausius Clapeyron Equation gives us a direct mathematical relationship between atmospheric pressure and the boiling point.
Using this equation, a person can find the values of boiling point for different vapour pressure if only value of boiling point with its corresponding temperature is given. The equation is given as follows-
Where will the water boil faster, surface or at the hilltop?
After discussing so many points regarding affect of atmospheric pressure and elevation on the boiling point, now we can answer this question easily.
At the surface level, the atmospheric pressure is more and at the hill top the atmospheric pressure is less. So it is safe to say that the water will boil faster at the hill top due to low atmospheric pressure. It will take less time for the vapour pressure of water to reach a value equal to the atmospheric pressure.
What is cavitation?
Cavitation is related to boiling. It takes place inside a liquid when the local vapour pressure becomes lesser than the vapour pressurevery rapidly.
Cavitation is the localised boiling of the liquid particles that can be observed by bubbles and voids forming in the liquid. Cavitation is undesirable because it damages the mechanical components used in the machine.
What is Bernoulli’s equation?
The Bernoulli’s equation is given for incompressible fluids flowing in a streamline flow. This equation tells us that for a fluid moving in a stream line flow, the fluid particles will have constant energy throughout.
In simple words, Bernoulli’s equation tells us that: Static pressure+dynamic presusre= total pressure.
What happens to atmospheric pressure as we move upwards?
As we move upwards, the effect of gravity becomes lesser. Hence, the molecules in the air get scattered and are loosely packed.
Due to this, the atmospheric pressure value decreases. When we say this, the boiling point of a substance is getting directly affected. Due to low atmospheric pressure the boiling point of the substance will also go down.
What is the boiling point of water at standard pressure conditions?
At standard atmospheric conditions, the atmospheric pressure is considered as 1 atm or 1.01 x 10^5 Pa.
At this point, the water will boil at 100 degrees Celsius or 212 degrees Fahrenheit. The conversions of units can be done by using their respective conversion formulae. In Kelvin, the boiling point temperature is 373.15 Kelvin.