When a liquid turns into a gas, that temperature is known as the boiling point. Boiling is a physical change, and a boiling point is a physical property. If a liquid’s vapor pressure equals that of the gas surrounding it at atmospheric pressure, this facilitates the transition of the material between gaseous and liquid phases.
The boiling point will change along with variations in atmospheric pressure. Standard atmospheric pressure (atm) is the point at which a liquid boils. For instance, pure water boils at 100 °C at a pressure of exactly 1 atm. At this point, with the addition of heat, the liquid transforms into vapor without further increase in temperature. The standard boiling point of a liquid was defined by IUPAC in 1982 as the temperature at which the liquid boils at one bar of pressure.
What is Boiling Point (B.P.)?
The boiling point of a liquid is the temperature at which the liquid’s vapour pressure becomes equal to the surrounding atmosphere’s pressure.
At this temperature, the liquid transforms into a gas. The boiling point of a liquid depends on atmospheric pressure. The B.P becomes lower as the pressure is reduced.
For instance, at sea level, the boiling point of water is 100 °C (212 °F), but at 6,600 feet, the boiling point is 93.4 °C (200.1 °F).
The melting point of ice and the boiling point of water served as the basis for the invention of the Celsius scale. Each substance has a specific boiling point. All the liquid-phase particles have been transformed into gas-phase particles, and as long as heat is still being added to the environment, the temperature begins to rise again. The kinetic energy of the particle increases as the temperature starts to climb.
Properties of Boiling Point
- A liquid’s boiling point depends on its surroundings’ pressure of a liquid. Water boils at different temperatures depending on the altitude.
- The boiling point of a liquid can be influenced by the presence of other substances in the liquid.
- The boiling point of a liquid refers to its capacity to transition from a liquid state to a gaseous state. The ability of a liquid to resist changes in the state increases with its higher boiling point.
- The boiling point of a liquid is a measure of its ability to vaporize. The ability of a liquid to resist vaporization increases with its boiling point.
- The boiling point of a liquid is determined by its vapor pressure. Vapor pressure refers to the pressure that the vapor of a liquid exerts on the container walls.
- The boiling point of a liquid is determined by its volatility. The measure of a liquid’s ability to vaporize is known as its volatility.
- The boiling point of a liquid is influenced by the pressure in its surroundings. The boiling point increases as the pressure in the surrounding environment increases.
Factors That Affects Boiling Point
The boiling point of the liquid is affected by different types of factors that are discussed below:
Pressure
The pressure of the environment is one of the factors that determines the temperature at which a liquid will boil. In an open system, the Earth’s atmosphere is most likely the source of the external pressure.
Key points to Note
- When the atmospheric pressure is less than 1 bar, the B.P of a liquid is lower than its general boiling point.
- When the atmospheric pressure is 1 bar, the B.P is at its general boiling level.
- When the atmospheric pressure is higher than 1 bar, the boiling point of a liquid is higher than its general boiling point.
Example Of Pressure Affecting The Boiling Point
For instance, water achieves the normal atmospheric pressure at 100 degrees Celsius. This measure is taken at sea level, where the weight of the earth’s atmosphere presses down upon the water. As one travels to higher altitudes, one will notice that the temperature at which water boils drops. At the summit of Mount Everest, the boiling temperature of the water is approximately 72 degrees Celsius, which is due to the rise in atmospheric pressure.
Intermolecular Bonds
Other elements influence the boiling point while we evaluate various liquids. The strength of the molecules’ bonds is one of the most important of many.
Key Points to Note
- For elements with stronger molecular bonds, the boiling point of a liquid will be relatively high.
- For elements with relatively weaker molecular bonds, liquids will have a low boiling point.
Examples Of Intermolecular Bonds Affecting The Boiling Point
Ethyl alcohol has a boiling point of 78.5 degrees Celsius at sea level. It is a liquid at room temperature, and the bonds between its molecules are comparatively strong.
By contrast, methyl ether has a “boiling” point of -25 degrees Celsius. At room temperature and sea level, methyl ether is a gas.
Impurities Of The Substance
An effective way to raise the boiling point of a liquid is to add another ingredient. The boiling points of solute, solvent, and solution are all different. Adding different impurities will influence the boiling point.
Key Points To Note
- When the substance is in pure form it tends to have a normal boiling point.
- When impurities are added boiling point changes from normal.
Example Of Impurities Affecting The Boiling Point
While water at sea level has a boiling point of 100 degrees Celsius, its boiling point can be raised by adding a solute, such as salt, sugar, etc.
Trends That Affect The Boiling Point
We will look at the trends that affect the boiling point of the substances. To figure out boiling points, you need to learn how trends work. Remember that boiling temperatures measure the degree of molecular bonding. The more tightly they cling to one another, the more energy will be required to evaporate them into the atmosphere as gases. Three important trends are discussed below:
Intermolecular Forces
The molecules in the liquid are drawn to one another. The four different kinds of intermolecular forces are listed below in descending order of strength:
Ionic Bond
When two atoms form an ionic bond, one of the atoms (for example, NaCl or table salt) donates an electron to the other. Using sodium chloride as an example, an electrically neutral molecule results from the proximity of the positively charged sodium ion to the negatively charged chloride ion. The ionic bond’s strength and the reason it would require more energy to break than another sort of bond is due to this neutrality.
Hydrogen Bond
A hydrogen atom bonded to another atom by sharing its valent electron has low electronegativity (e.g., HF, hydrogen fluoride). The electron cloud around the fluorine atom is large and has a high electronegativity, whereas the electron cloud surrounding the hydrogen atom is smaller and has a significantly low electronegativity. This is an example of a polar covalent bond in which the electrons are distributed unevenly.
The electronegativity of an atom to which a hydrogen bond is formed determines its strength, which varies among hydrogen bonds.
For example:
- When hydrogen is bonded to fluorine, the connection is much stronger.
- When it is bonded to chlorine, the bond is moderately strong.
- When bonded to another hydrogen, the bond is non-polar, and the molecule is relatively weak.
Dipole – Dipole
A dipole force is generated when the positive end of one polar molecule attracts the negative end of another polar molecule.
Dipole-dipole interactions that result from the polarized C-O bonds hold diethyl ether molecules, C4H10O, together.
Van der Waals Force
The least intense intermolecular forces are known as dispersion forces or London forces. In a molecule, these represent the attraction between instantaneous dipoles.
Consider the electrons present in the valence shell. On average, they are distributed equitably. There may be a disparity between the number of electrons on one side and the other at any moment, resulting in an instant distinction in charge.
Examples
Argon is an inert gas; however, it can be condensed into a liquid state by cooling it to -186 °C. The fact that it is liquid suggests that something is holding it together. This “something” consists of dispersal forces.
Butane, C4H10, contains no polar functional groups. The Van der Waals dispersion forces are the only forces of attraction between individual butane molecules. Butane boils at the temperature at which water freezes (i.e., 0° C), which is significantly lower than the boiling point of diethyl ether.
Molecular Weight
A larger molecule is more polarizable, which is the force that keeps molecules together. The larger molecule has a higher boiling point because it requires more energy to transition into the gaseous state. Based on molecular weight and boiling point, sodium nitrate and rubidium nitrate can be compared.
| Chemical Formula | Molecular Weight | Boiling Point (°C) |
|---|---|---|
| Sodium Nitrate (NaNO3) | 85.00 | 380 |
| Rubidium Nitrate (RbNO3) | 147.5 | 578 |
Shape
Molecules that form lengthy, straight chains are more attracted to surrounding molecules because they can approach them. A straight-chain molecule like butane (C4H10) has a minor change in how electronegativity affects carbon and hydrogen.
A molecule with double-bonded oxygen, such as butanone (C4H8O), has a central apex where the oxygen is attached to the carbon chain. The geometry of the molecule, which creates an attractive force between the oxygen on one molecule and the hydrogen on a neighboring molecule, explains why the boiling point of butane is close to 0 degrees Celsius while the boiling point of butanone is higher (79.6 degrees Celsius).
Determination Of Boiling Point Of Organic Compounds
- A simple method for determining the boiling point of an organic compound is to utilize the capillary method.
- In this arrangement, an empty glass capillary tube is inverted into a liquid-phase container containing the pure compound.
- As the liquid is heated, the sample’s vapor pressure rises, and vapor begins to penetrate the glass capillary tube.
- This causes confined air to escape and bubbles to emerge from the bottom of the capillary tube. At this juncture, the liquid must be cooled.
- Once the sample’s vapor pressure equals the atmospheric pressure inside the glass capillary tube, the liquid will begin to penetrate the tube.
- When this phenomenon occurs, the temperature of the solution corresponds to the boiling point of the liquid compound.
Determination Of Boiling Point Using Boiling Point Apparatus
- By using a boiling point apparatus, the boiling point can be determined. This technique takes advantage of the fact that liquids boil when their vapor pressure equals atmospheric pressure.
- A liquid container, a heater, and a mercury manometer are included. First, the liquid is deposited in the container and heated until the container is filled with vapors.
- The mercury manometer is then utilized to determine the internal atmospheric pressure of the container. When the mercury column in the manometer reaches the same level as the liquid in the container, atmospheric pressure is attained, and the boiling point is obtained.
Boiling Point Elevation
The elevation of boiling points is a colligative property of matter that depends on the ratio of solvent to solute rather than on the solute’s individuality. This demonstrates that the increase in a solution’s boiling point is proportional to the quantity of solution added. The longer the simmering period, the greater the solute concentration in the solution.
The boiling point will change along with variations in atmospheric pressure. Standard atmospheric pressure (atm) is the point at which a liquid boils. For instance, pure water boils at 100 °C at a pressure of exactly 1 atm. At this point, with the addition of heat, the liquid transforms into vapor without further increase in temperature.
Formula For Calculating Boiling Point Elevation
The formula for calculating the boiling point is:
ΔTb = i×Kb×m
Where,
- i is the Van’t Hoff factor
- Kb is the ebullioscopic constant
- m is the molality of the solute
If the heat of vaporization and, consequently, the vapor pressure of a liquid at a specific temperature are known, the boiling point is frequently calculated with the assistance of the Clausius-Clapeyron equation as:
ln(P1/P2)=ΔHvap /R(1/T2 – 1/T1)
Here,
- P1 and P2 are the vapor pressures.
- T1 and T2 are the temperatures.
- ΔHvap is the enthalpy (heat) of vaporization and
- R is the universal gas constant (8.3145 J mol-1 K-1).
References
- Atkins, P.W. and Julio de Paulo, Atkins’ Physical Chemistry, Oxford University Press, UK, Indian Edition 9, 2011.
- R. Chang, “Physical Chemistry for the Chemical and Biological Sciences”, University Science Books, Sausalito, California (2000).
- https://www.britannica.com/science/boiling-point
- https://byjus.com/chemistry/elevation-boiling-point/
- https://collegedunia.com/exams/boiling-point-elevation-chemistry-articleid-1298
- DeVoe, Howard (2000). Thermodynamics and Chemistry (1st ed.). Prentice-Hall. ISBN 0-02-328741-1.
- https://www.geeksforgeeks.org/boiling-point-formula/
