Boyle's law

An animation showing the relationship between pressure and volume when mass and temperature are held constant.

Boyle's law (sometimes referred to as the Boyle–Mariotte law, or Mariotte's law^[1]) is an experimental gas law that describes how the pressure of a gas tends to increase as the volume of the container decreases. A modern statement of Boyle's law is

The absolute pressure exerted by a given mass of an ideal gas is inversely proportional to the volume it occupies if the temperature and amount of gas remain unchanged within a closed system.^[2][3]

Mathematically, Boyle's law can be stated as

${\displaystyle P\propto {\frac {1}{V}}}$

Pressure is inversely proportional to the volume.

or

${\displaystyle PV=k}$

Pressure multiplied by volume equals some constant ${\displaystyle k}$.

where P is the pressure of the gas, V is the volume of the gas, and k is a constant.

The equation states that the product of pressure and volume is a constant for a given mass of confined gas and this holds as long as the temperature is constant. For comparing the same substance under two different sets of conditions, the law can be usefully expressed as

${\displaystyle P_{1}V_{1}=P_{2}V_{2}.}$

The equation shows that, as volume increases, the pressure of the gas decreases in proportion. Similarly, as volume decreases, the pressure of the gas increases. The law was named after chemist and physicist Robert Boyle, who published the original law in 1662.^[4]

History[edit]

A graph of Boyle's original data

This relationship between pressure and volume was first noted by Richard Towneley and Henry Power in the seventeenth century.^[5][6] Robert Boyle confirmed their discovery through experiments and published the results.^[7] According to Robert Gunther and other authorities, it was Boyle's assistant, Robert Hooke, who built the experimental apparatus. Boyle's law is based on experiments with air, which he considered to be a fluid of particles at rest in between small invisible springs. At that time, air was still seen as one of the four elements, but Boyle disagreed. Boyle's interest was probably to understand air as an essential element of life;^[8] for example, he published works on the growth of plants without air.^[9] Boyle used a closed J-shaped tube and after pouring mercury from one side he forced the air on the other side to contract under the pressure of mercury. After repeating the experiment several times and using different amounts of mercury he found that under controlled conditions, the pressure of a gas is inversely proportional to the volume occupied by it.^[10] The French physicist Edme Mariotte (1620–1684) discovered the same law independent of Boyle in 1679,^[11] but Boyle had already published it in 1662.^[12] Mariotte did, however, discover that air volume changes with temperature.^[13] Thus this law is sometimes referred to as Mariotte's law or the Boyle–Mariotte law. Later, in 1687 in the Philosophiæ Naturalis Principia Mathematica, Newton showed mathematically that if an elastic fluid consisting of particles at rest, between which are repulsive forces inversely proportional to their distance, the density would be directly proportional to the pressure,^[14] but this mathematical treatise is not the physical explanation for the observed relationship. Instead of a static theory a kinetic theory is needed, which was provided two centuries later by Maxwell and Boltzmann.

This law was the first physical law to be expressed in the form of an equation describing the dependence of two variable quantities.^[12]

Definition[edit]

Play media

Boyle's law demonstrations

The law itself can be stated as follows:

For a fixed amount of an ideal gas kept at a fixed temperature, pressure and volume are inversely proportional.^[3]

Or Boyle's law is a gas law, stating that the pressure and volume of a gas have an inverse relationship, when temperature is held constant. If volume increases, then pressure decreases and vice versa, when temperature is held constant.

Therefore, when the volume is halved, the pressure is doubled; and if the volume is doubled, the pressure is halved.

Relation with kinetic theory and ideal gases[edit]

Boyle's law states that at constant temperature for a fixed mass, the absolute pressure and the volume of a gas are inversely proportional. The law can also be stated in an equivalent manner, that the product of absolute pressure and volume is always constant.

Most gases behave like ideal gases at moderate pressures and temperatures. The technology of the 17th century could not produce high pressures or low temperatures. Hence, the law was not likely to have deviations at the time of publication. As improvements in technology permitted higher pressures and lower temperatures, deviations from the ideal gas behavior became noticeable, and the relationship between pressure and volume can only be accurately described employing real gas theory.^[15] The deviation is expressed as the compressibility factor.

Boyle (and Mariotte) derived the law solely on experimental grounds. The law can also be derived theoretically based on the presumed existence of atoms and molecules and assumptions about motion and perfectly elastic collisions (see kinetic theory of gases). These assumptions were met with enormous resistance in the positivist scientific community at the time however, as they were seen as purely theoretical constructs for which there was not the slightest observational evidence.

Daniel Bernoulli in 1737-1738 derived Boyle's law using Newton's laws of motion with application on a molecular level. It remained ignored until around 1845, when John Waterston published a paper building the main precepts of kinetic theory; this was rejected by the Royal Society of England. Later works of James Prescott Joule, Rudolf Clausius and in particular Ludwig Boltzmann firmly established the kinetic theory of gases and brought attention to both the theories of Bernoulli and Waterston.^[16]

The debate between proponents of Energetics and Atomism led Boltzmann to write a book in 1898, which endured criticism up to his suicide in 1906.^[16] Albert Einstein in 1905 showed how kinetic theory applies to the Brownian motion of a fluid-suspended particle, which was confirmed in 1908 by Jean Perrin.^[16]

Equation[edit]

The mathematical equation for Boyle's law is:

${\displaystyle PV=k}$

where:

• P denotes the pressure of the system.
• V denotes the volume of the gas.
• k is a constant value representative of the temperature and volume of the system.

So long as temperature remains constant the same amount of energy given to the system persists throughout its operation and therefore, theoretically, the value of k will remain constant. However, due to the derivation of pressure as perpendicular applied force and the probabilistic likelihood of collisions with other particles through collision theory, the application of force to a surface may not be infinitely constant for such values of v, but will have a limit when differentiating such values over a given time. Forcing the volume V of the fixed quantity of gas to increase, keeping the gas at the initially measured temperature, the pressure p must decrease proportionally. Conversely, reducing the volume of the gas increases the pressure. Boyle's law is used to predict the result of introducing a change, in volume and pressure only, to the initial state of a fixed quantity of gas.

The initial and final volumes and pressures of the fixed amount of gas, where the initial and final temperatures are the same (heating or cooling will be required to meet this condition), are related by the equation:

${\displaystyle P_{1}V_{1}=P_{2}V_{2}.\,}$

Here P₁ and V₁ represent the original pressure and volume, respectively, and P₂ and V₂ represent the second pressure and volume.

Boyle's law, Charles's law, and Gay-Lussac's law form the combined gas law. The three gas laws in combination with Avogadro's law can be generalized by the ideal gas law.

Human breathing system[edit]

Boyle's law is often used as part of an explanation on how the breathing system works in the human body. This commonly involves explaining how the lung volume may be increased or decreased and thereby cause a relatively lower or higher air pressure within them (in keeping with Boyle's law). This forms a pressure difference between the air inside the lungs and the environmental air pressure, which in turn precipitates either inhalation or exhalation as air moves from high to low pressure.^[17]

See also[edit]

Related phenomena:

• Gardener's watering-pot

Other gas laws in chemistry:

• Dalton's law
• Charles's law

Citations[edit]

Sources[edit]

• Britannica Educational Publishing (1 December 2012). Scientists and Inventors of the Renaissance. Britannica Educational Publishing. ISBN 978-1-61530-884-2. 

External links[edit]

• Media related to Boyle's Law at Wikimedia Commons
• Boyle’s law calculator