Boiling Point of Water: Definition, Values and Factors Affecting Boiling Point.

Boiling Point of Water: We are familiar with the word boiling point. What does it mean? Well, here is the answer: The temperature at which the liquid and the vapour states of a substance coexist is called its boiling point. It is also defined as the temperature at which liquid boils. The boiling point of water at sea level is \(100\,^\circ {\rm{C}}.\) It is equal to \(373.15\) on the Kelvin scale. Is this value the same at all temperature and pressure conditions?

The boiling point of water is determined by air pressure, which varies with altitude. As you gain altitude (e.g., ascending a mountain), water boils at a lower temperature, and as atmospheric pressure rises, it boils at a higher temperature (coming back down to sea level or going below it). Let us discuss more about it in this article.

Boiling Point (B.P)

Let us discuss what the boiling point is:

Boiling

When a liquid is heated, the kinetic energy of the particles in it increases on average. As a result, the rate of evaporation increases, allowing an increasing number of particles to escape from the liquid’s surface and enter the vapour state.

Learn Everything About Evaporation Here

After a period of time, all of the particles in the liquid will have enough kinetic energy to vaporise. This is the point at which the liquid begins to bubble and eventually boils.

Boiling Point

Boiling point is defined as the temperature at which the vapour pressure of a liquid becomes equal to the prevailing atmospheric pressure. It is also defined as the temperature at which liquid boils. Every liquid has a standard or normal boiling point.

Standard Boiling Point of Water

The boiling point of a substance at standard atmospheric pressure is called its standard boiling point. Therefore, we can now say that the standard boiling point of water is \(100\,^\circ {\rm{C}}.\)

The standard boiling point of water in the Kelvin scale is \(373.15\;{\rm{K}}.\)

Generally, it is said that the boiling point of water is high. This is because of the presence of hydrogen bonding in water molecules. That is, the presence of hydrogen bonding increases the boiling point of a compound.

The boiling point of water or any liquid depends on the temperature, pressure, and the vapour pressure of the liquid. That is, for example, a liquid in a partial vacuum has a lower boiling point than when that liquid is at atmospheric pressure. Similarly, a liquid at high pressure will have a higher boiling point than when that liquid is at atmospheric pressure.

The Factors Affecting Boiling Point or Boiling Point of Water

The various factors affecting the boiling point of water are discussed below.

1. Effect of Temperature on Boiling Point

The temperature of a liquid remains constant during boiling. This is because the external heat supplied during boiling is used for the vaporisation of water (the latent heat of vaporisation).

Therefore, water will remain liquid at \(100\,^\circ {\rm{C}}\) provided the conditions such as a pressure of \(1\,{\rm{ atm }}\) or \(101.3\,{\rm{kPa}}\) during boiling.  The temperature will only increase once the water is entirely converted into its vapour state.

2. Effect of Atmospheric Pressure on Boiling Point

The atmospheric pressure has a major effect on the boiling point of a liquid. That is, the boiling point of a substance increases with an increase in pressure. When the pressure increases, greater energy is required for a liquid to boil. As a result, the boiling point of the liquid increases. Similarly, when the pressure decreases, a liquid needs less energy to get converted into its vapour state. Therefore, we can say that the boiling point decreases with the decrease in pressure.

Now, we can say that the boiling point of water increases when the atmospheric pressure increases. Similarly, when the atmospheric pressure decreases, the boiling point of water decreases. Hence, at high altitudes, the boiling point of water will be very low because of the lower atmospheric pressure.

Do you know why cooking is easier in the pressure cooker?  This is because the steam trapped inside the pressure cooker increases the atmospheric pressure inside the cooker. As a result, the boiling point increases. Similarly, the boiling point of a substance decreases with the decrease in pressure. Therefore, cooking is difficult on hill stations.

3. Effect of Vapour Pressure on Boiling

The vapours formed from a liquid create an upward pressure as they collide with air molecules. This pressure is called vapour pressure. Different substances will have different vapour pressures, and hence, they will have different boiling points. The liquids with high vapour pressure will have very low boiling points. This is because the vapour pressure of a liquid lowers the amount of pressure exerted on the liquid by the atmosphere.

The vapour pressure of a liquid can be increased by heating the liquid, and as a result, more molecules enter the atmosphere. When the vapour pressure is equal to the atmospheric pressure, boiling will start.

Now, we can say that when the vapour pressure of water increases, the boiling point of water decreases. Similarly, when the vapour pressure decreases, the boiling point increases.

Calculation of Boiling Point

During the conversion of a liquid into its vapours (during boiling), the heat of vapourisation (It is the energy required to transform a given quantity of a substance from a liquid into a gas at a given pressure) is involved.

Therefore, if we know the heat of vaporisation and the vapor pressure of a liquid at a certain temperature, we can calculate the boiling point of a liquid. That is,

\({{\rm{T}}_{\rm{B}}}{\rm{ = }}{\left( {\frac{{\rm{1}}}{{{{\rm{T}}_{\rm{0}}}}}{\rm{ – }}\frac{{{\rm{Rln}}\frac{{\rm{P}}}{{{{\rm{P}}_{\rm{0}}}}}}}{{{\rm{\Delta }}{{\rm{H}}_{{\rm{vap}}}}}}} \right)^{{\rm{ – 1}}}}\)

The above equation is known as the Clausius–Clapeyron equation.

Here, \({{\bf{T}}_{\rm{B}}} \Rightarrow \) The boiling point at the pressure of interest.

\({{\bf{T}}_{\rm{0}}} \Rightarrow \) Boiling temperature.

\({\bf{R}} \Rightarrow \) Ideal gas constant.

\({\bf{P}} \Rightarrow \) Vapour pressure of the liquid at the pressure of interest.

\({{\bf{P}}_0} \Rightarrow \) Some pressure where the corresponding \({{\rm{T}}_0}\) is known (usually data available at \(1\;{\rm{atm}}\) or \(100\,{\rm{kPa}}\)).

\(\Delta {{\rm{H}}_{{\rm{Vap}}}} \Rightarrow \) Heat of vapourisation of  the liquid.

\(\ln \Rightarrow \) Natural logarithm.

Presence of a Salt and Boiling Point of Water

Liquid water molecules escape and enter into their gaseous phase while boiling. But if salt is present in water, it makes it harder for the water molecules to escape and enter the gas phase. This increases the boiling point or boiling time of water. Therefore, we can say that saltwater will have a higher boiling point than pure water.

Note: The presence of sugar, salt or other non-volatile solutes in water will make the boiling point higher. In contrast, alcohol, which is a volatile chemical, lowers the boiling point of water.

Summary

We have now understood that the boiling point is the temperature at which liquid boils. It is also defined as the temperature at which the vapour pressure of a liquid becomes equal to the prevailing atmospheric pressure. We have also learnt about the boiling point of water. That is, at standard atmospheric pressure conditions, water will boil at \(100\;^\circ {\rm{C}}.\) The major factors affecting the boiling point, such as the temperature, atmospheric pressure and the vapour pressure of the liquid, are also explained in this article.

Frequently Asked Questions

We have provided some frequently asked questions on Boiling Point of Water here:

Q1. What are thezing and boiling points of water?
Ans: Thezing point of water is \(0\,^\circ {\rm{C}}\left( {32\,^\circ {\rm{F}}} \right)\), and the boiling point of water is \(100\,^\circ {\rm{C}}\left( {212\,^\circ {\rm{F}}} \right).\)

Q2. How does salt affect the boiling point of water?
Ans: The presence of salt in the water makes it harder for the water molecules to escape and enter the gas phase. This increases the boiling point or boiling time of water.

Q3. Why does salt increase the boiling point of water?
Ans: The addition of salt in water increases the boiling point of water. This is because when salt is added to water, it dissociates in water and form bonds with the water molecules. Such bonds formed are more strong than the bonds between the molecules of water. As a result, more energy is required by the water to break apart and to get converted into its vapours.

Q4. What is the boiling point of water in Fahrenheit scale?
Ans: The boiling point of water in Fahrenheit scale is \(212\,^\circ {\rm{F}}.\) On Celsius scale, it is \(100\,^\circ {\rm{C}}.\)

Q5. How to calculate the boiling point of water at different pressures?
Ans: Using Clausius–Clapeyron equation, if we know the heat of vaporization and the vapour pressure of water at a certain temperature, we can calculate its boiling point. That is,
\({T_{\rm{B}}} = {\left( {\frac{1}{{{T_0}}} – \frac{{R\ln \frac{P}{{{P_0}}}}}{{\Delta {H_{{\rm{vap}}}}}}} \right)^{ – 1}}\)
The above equation is known as the Clausius–Clapeyron equation.
Here, \({{\bf{T}}_{\rm{B}}} \Rightarrow \) The boiling point at the pressure of interest.
\({{\bf{T}}_{\rm{0}}} \Rightarrow \) Boiling temperature.
\({\bf{R}} \Rightarrow \) Ideal gas constant.
\({\bf{P}} \Rightarrow \) Vapour pressure of the liquid at the pressure of interest.
\({{\bf{P}}_0} \Rightarrow \) Some pressure where the corresponding \({{\rm{T}}_0}\) is known (usually data available at \(1\;{\rm{atm}}\) or \(100\,{\rm{kPa}}\)).
\(\Delta {{\rm{H}}_{{\rm{Vap}}}} \Rightarrow \) Heat of vapourisation of  the liquid.
\(\ln \Rightarrow \) Natural logarithm.

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