In the present article, we will study the relationship between the boiling point and the pressure on its surface and the impact and effect on one another.

At a certain temperature point, the pressure inside the liquid treated as the vapour pressure equals the pressure of the surroundings around that liquid. The tendency of molecules of a liquid to escape from it is treated as the vapour pressure of that liquid.

In the next section, let us discuss boiling point and pressure definitions.

Boiling point and pressure are the two physical properties of a liquid.

The liquid state of a substance is transformed to a gaseous phase at a certain value of temperature, and this temperature is known to be the boiling point of that substance. In general, the magnitude of the force imposed vertically (perpendicularly) to the surface of a body (system) for a unit area and this applied force is said to be distributed over the region.

The following part deals with the changes in boiling point with the influence of pressure.

boiling point and pressure — **Image credits: Pixabay free images**

Does boiling point change with pressure?

Yes, the boiling point changes with pressure.

We learn that the boiling point is bound to vary concerning the surrounding pressure. The relation between boiling point and pressure is direct proportionality. Since the pressure is lower there, and in turn, it forces the liquid also to possess a lower vapour pressure, and this can be achieved by an expenditure of a very small amount of heat.

Next, we shall explore how the boiling point changes with the pressure.

How does boiling point change with pressure?

It is observed that less pressure is associated with the lower value of the boiling point and vice versa.

We can see that a lower boiling point is usually observed at comparatively higher elevations. Since the pressure is lower there, and in turn, it forces the liquid also to possess a lower vapour pressure, and this can be achieved by an expenditure of a very small amount of heat.

Now, let us study the effect of pressure on the boiling point.

What is the effect of pressure on boiling point?

The obvious effect of pressure on the boiling point of the liquid is given below,

An increase in the boiling point (upto a critical point) is always seen whenever the pressure of the surroundings changes to a higher value. For example, the liquid surrounded by a vacuum is found to have a lower boiling point than atmospheric pressure.

i.e., the boiling point keeps deviating from the influence of the environmental pressure in the liquid’s surroundings.

Here, we focus on the process involved in the increase of boiling point due to pressure.

Why does boiling point increase with pressure?

The internal process involved in the phenomenon of achieving the boiling point is,

If the atmospheric pressure around the liquid is large, there is a requirement for more heat to be supplied to increase the vapour pressure of the liquid so that it matches the environmental pressure on the surface.

So that it can overcome the atmospheric pressure and form vapours inside the liquid that would boil out of it into the surrounding, this forces the boiling point of the liquid to increase as pressure increases and vice versa.

How the boiling point of water is measured at different pressures?

Many formulas are available for calculating boiling points concerning pressure.

In general, the process of boiling occurs when the liquid’s vapour pressure matches the atmospheric pressure on the liquid. As mentioned above, we can use several formulas to calculate the boiling point of water. One of them is using an already known boiling point at sea level.

Next, we shall discuss vapour pressure and boiling point.

Vapour pressure and boiling point

Both the vapour pressure and boiling point are the physical properties of a liquid.

In a closed system with a specified temperature, the amount of pressure imposed by vapour at equilibrium (thermodynamic) conditions by its different phases is termed the equilibrium vapour pressure of that system. It is nothing but the rate of evaporation of a liquid.

The liquid state of a substance is transformed to a gaseous phase at a certain value of temperature, and this temperature is known to be the boiling point of that substance.

Vapour pressure and boiling point relationship

The vapour pressure of the liquid affects its boiling point directly.

Even though the vapour pressure and the atmospheric pressure are interlinked, they are related to the boiling point differently. When there occurs a decrease in the boiling point of the liquid, there is a noticeable increase in the vapour pressure of the liquid.

Can impart the above fact by plotting a graph of vapour pressure versus the temperature. For example, methyl chloride, which has the highest vapour pressure, has the lowest boiling point.

Vapour pressure and boiling point equation

The connection between the vapour pressure and the boiling point can be represented by an equation given below.

The Clausius-Clapeyron equation gives the rate of change in boiling point concerning the vapour pressure

(P1/P2)=−ΔH{ R(1/T1−1/T2)}. Where R=0.008314 Kj/mole⋅K

The following section is contributed to several frequently asked questions in this region.

Frequently asked questions| FAQS

What is the difference between evaporation and boiling?

Both the evaporation and boiling are the formation of vapours.

The difference is that evaporation is a phenomenon that is completely associated with the surface of the liquid, and here only those liquid molecules that are seen to be escaped from the liquid are at the surface and are not bounded by liquid pressure on one side.

In contrast, all present anywhere in the liquid tend to escape. In boiling, the vapours or bubbles are formed even interior of the liquid.

Describe the connection between the boiling point and pressure?

The relation between the two physical properties, namely the pressure and boiling point, is,

The pressure measured above the surface of the liquid directly affects the temperature at which it transforms into vapours. It must spend more energy for a liquid to start boiling when an atmospheric pressure of a higher value surrounds it.

What happens to the boiling point of the liquid as it goes up in elevation?

The atmospheric conditions of the region influence the process of boiling, in turn affecting the boiling point.

At higher elevations, it is found that the pressure of air in that region is lower in comparison with the lower elevations. This low pressure imparts a condition so that even with less heat, it becomes possible to achieve the boiling point as it is enough to obtain the vapour pressure equal to that of the atmospheric pressure.

How do you locate the boiling point in a temperature versus vapour pressure graph?

The temperature versus vapour pressure graph consists of a linear plot.

The horizontal line drawn from the standard pressure, when extrapolated, would meet at temperature if extended by a straight line. This particular temperature obtained would represent the boiling point of the liquid. Also, the pressure is equal to the vapour pressure inside the liquid at this point.

What happens when the vapour pressure of the liquid is decreased?

When the vapour pressure of any liquid is made to reduce, there is an obvious variation in the boiling point of that liquid.

By the observation, we conclude that the boiling point tends to increase during the decrease of the vapour pressure measured inside the liquid as a large amount of energy would be necessary for the liquid to synchronise its pressure with the environmental pressure. This synchronous behaviour is found to occur at the boiling point.

What is the reason for the decrease in boiling point due to pressure?

The atmospheric pressure has an obvious effect on the boiling point of the liquid.

The boiling point reaches a higher value of temperature when there is an increase in the atmospheric pressure that surrounds the liquid. Similarly, at higher elevations, when the air pressure tends to reduce, the liquid turns into vapours at a lower temperature value (boiling point);

this is due to the reason that the molecules inside the liquid would now require less speed to move out into the surrounding.

How differently do the melting point and boiling points affect by pressure?

Does the pressure have a similar yet different effect on the boiling and melting points?

When the phase of a substance changes from solid to liquid, i.e., when there is melting, changes in the volume also occur. Depending on this volume difference, the melting point tends to increase or decrease when there is an application of pressure.

In contrast, whenever the pressure is applied, it is always found that the boiling point is increased.

Why does an increase in pressure results in an increase in boiling point?

The increase of boiling point associated with the pressure increment is because of a change in the energy necessary for vapours to form.

Let us consider some amount of liquid and increase the atmospheric pressure at the surface. On observing, we can see that now the energy required to form bubbles or vapours and transform into a gaseous phase will be more. In turn, more molecules start colliding with high energy to escape from the liquid.

How can you explain the boiling point diagram?

A boiling point diagram can be defined as a plot of two quantities.

At constant pressure, the graph of boiling points of a mixture of liquids versus the vapour equilibrium of that binary mixture is a boiling point diagram.

Why is the boiling point of water at higher altitudes, not 100 degrees Celsius?

The boiling point is affected due to changes in altitudes also, and the density of air is less at higher altitudes.

At sea level, vapour pressure is achieved at a temperature of 100 degrees Celsius, and therefore water boils at this temperature. Accordingly, as we move higher and higher above the sea level, the temperature required to reach the sufficient vapour pressure becomes lesser and lesser; this implies that at a low temperature itself, the water may boil.

i.e., the water boils at a temperature where the atmospheric pressure is the same as the vapour pressure.

altitude — **Image credits: Pixabay free images**

What is the theory behind the working of a pressure cooker?

The principle on which the pressure cooker works is given by,

In a pressure cooker, the interior is at a higher temperature than the boiling point of water at high pressure; this is due to the fact that, At higher pressure, we need to reach a higher temperature to make water boil. This does not occur in a normal vessel.

What are the different methods for finding the boiling point?

There are many methods by which we can find the boiling point of a liquid,

By using equations
Using nomogram
By online calculators
With the use of graphs and tables.

Also Read:

Harshitha H N

Hi…..I am Harshitha H N. I have completed my master’s in Physics from the University of Mysore with a specialization in Nuclear physics. I enjoy exploring new things in my free time. My article always aims to develop and bring some value to the table with relevant topics.
Let’s connect through LinkedIn-