
The gas laws are a group of laws that model the behaviour of gases by describing the relationships between their pressure, volume, temperature, and amount. They were developed in the 17th and 18th centuries through experimental observations. The three fundamental gas laws are Boyle's Law, Charles' Law, and Avogadro's Law. These laws describe the relationship between pressure and volume, volume and temperature, and volume and the amount of gas, respectively. By combining these three laws, we arrive at the Ideal Gas Law, which relates all four variables. The Ideal Gas Law can be further combined with Gay-Lussac's Law to form the Combined Gas Law or General Gas Equation, which accounts for multiple sets of conditions. These gas laws are essential for understanding the behaviour of gases and have practical applications in various fields, including medicine and drug development.
| Characteristics | Values |
|---|---|
| Boyle's Law | The volume of a given amount of gas held at constant temperature varies inversely with the applied pressure when the temperature and mass are constant. |
| Charles' Law | The volume of a given fixed mass of a dry gas is directly proportional to its absolute temperature at a constant pressure. |
| Avogadro's Law | The volume of gas increases as the amount of gas increases. |
| Gay-Lussac's Law | The pressure exerted by a given mass and constant volume of an ideal gas on the sides of its container is directly proportional to its absolute temperature. |
| Ideal Gas Law | The ideal gas law is the combination of three simple gas laws: Boyle's Law, Charles' Law, and Avogadro's Law. It gives the relationship between pressure, volume, number of moles or molecules, and temperature. |
| Combined Gas Law | The combined gas law is obtained by combining Boyle's Law, Charles' Law, and Gay-Lussac's Law. It shows the relationship between pressure, volume, and temperature for a fixed mass of gas. |
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What You'll Learn
- Boyle's Law: The volume of a gas is inversely proportional to its pressure at a constant temperature
- Charles' Law: The volume of a gas is directly proportional to its absolute temperature at constant pressure
- Avogadro's Law: The volume of a gas is directly proportional to the number of moles of the gas when pressure and temperature are constant
- Gay-Lussac's Law: The pressure exerted by a gas on its container is directly proportional to its absolute temperature
- Ideal Gas Law: The relationship between pressure, volume, number of moles, and temperature

Boyle's Law: The volume of a gas is inversely proportional to its pressure at a constant temperature
Gas laws are a group of physical laws that model the behaviour of gases. They were developed from experimental observations made from the 17th century onwards. The three fundamental gas laws establish the relationship between pressure, temperature, volume, and the amount of gas.
Boyle's Law, put forward by Anglo-Irish chemist Robert Boyle in 1662, states that the volume of a gas is inversely proportional to its pressure at a constant temperature. In other words, as the volume of a gas increases, its pressure decreases, and vice versa, as long as the temperature and the quantity of gas are kept constant.
The mathematical expression for this relationship is P1V1 = P2V2, assuming temperature to be constant. For example, if the volume is halved, the pressure is doubled; and if the volume is doubled, the pressure is halved. This relationship can be visualised through graphical analysis, which shows a linear relationship between pressure and volume.
Boyle's Law can be used to calculate the change in volume at different altitudes. For instance, it can be used to calculate the change in volume of a patient with a pneumothorax at sea level, compared to at an altitude of 1 km.
Boyle's Law was discovered through experiments with air, which Boyle considered to be a fluid of particles at rest between small invisible springs. He used a closed J-shaped tube and poured mercury from one side, forcing the air on the other side to contract under the pressure of the mercury. Through these experiments, he found that under controlled conditions, the pressure of a gas is inversely proportional to the volume occupied by it.
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Charles' Law: The volume of a gas is directly proportional to its absolute temperature at constant pressure
Charles's Law, also known as the Law of Volumes, is an experimental gas law that describes the relationship between the volume of a gas and its absolute temperature at constant pressure. The law was formulated by French physicist Jacques Charles in the 1780s and states that the volume of a given mass of gas varies directly with its absolute temperature when pressure is kept constant. In other words, as the temperature of a gas increases, so does its volume, and conversely, a decrease in temperature leads to a decrease in volume.
Mathematically, the direct relationship of Charles's Law can be expressed as a proportion: V ∝ T, where V is volume and T is absolute temperature in Kelvin. This equation can be used to calculate the initial or final volume or temperature of a gas if the other value and the initial/final comparison value are known. For example, if a balloon with a volume of 2.20 L at 22°C is heated to 71°C, Charles's Law can be used to calculate the new volume of the balloon.
The Kelvin temperature scale is used in Charles's Law because zero on the Kelvin scale corresponds to a complete stoppage of molecular motion. As a gas is heated, its molecules increase in kinetic energy, causing them to move faster and exert more pressure, resulting in an increase in volume. Conversely, when a gas is cooled, its molecules slow down and move closer together, reducing the volume of the gas.
Charles's Law has practical applications, such as in the action of a gas thermometer, where the change in volume of a gas (e.g. hydrogen) is used to indicate the change in temperature. Another example is placing a balloon filled with gas into a freezer and observing the reduction in volume as the temperature decreases.
It is important to note that Charles's Law implies that the volume of a gas will decrease as the temperature decreases, approaching zero volume at absolute zero temperature (-273.15°C). However, Gay-Lussac clarified that the law does not apply at extremely low temperatures, as the compressed vapours may transition to a liquid state.
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Avogadro's Law: The volume of a gas is directly proportional to the number of moles of the gas when pressure and temperature are constant
Avogadro's Law, also known as Avogadro's hypothesis or principle, is an experimental gas law that relates the volume of a gas to the amount of substance of gas present. The law was first proposed in 1811 by Amedeo Avogadro, a professor of higher physics at the University of Turin, although it was not generally accepted until after 1858.
Avogadro's Law states that the volume of a gas is directly proportional to the number of moles of gas when the temperature and pressure are held constant. This can be expressed mathematically as:
> V = k x n and V1/n1 = V2/n2
Where n is the number of moles of gas and k is a constant. In simpler terms, this means that if you have two equal volumes of different gases at the same temperature and pressure, they will contain the same number of molecules.
Avogadro's Law is evident in everyday situations, such as blowing up a balloon. The volume of the balloon increases as you add more moles of gas by blowing into it. Avogadro's Law is a specific case of the ideal gas law, which combines Avogadro's Law with Boyle's Law, Charles' Law, and Gay-Lussac's Law.
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Gay-Lussac's Law: The pressure exerted by a gas on its container is directly proportional to its absolute temperature
Gay-Lussac's Law is a gas law that describes the relationship between the pressure exerted by a gas and its absolute temperature. Formulated by French chemist Joseph Gay-Lussac in the early 19th century, the law states that the pressure exerted by a gas on the walls of its container is directly proportional to its absolute temperature, provided the mass and volume of the gas remain constant.
Mathematically, Gay-Lussac's Law can be expressed as:
\[ \frac{P}{T} = \text{constant} \]
Or:
\[ \frac{P_1}{T_1} = \frac{P_2}{T_2} \]
Where P is pressure and T is absolute temperature. This equation illustrates that the ratio of pressure to temperature remains constant for a fixed mass of gas at a constant volume.
Gay-Lussac's Law is particularly applicable in situations where the volume of the gas is held constant. For example, when a pressurised aerosol can, such as a deodorant or spray paint can, is heated, the increase in temperature leads to a corresponding increase in the pressure exerted by the gas on the container, which can potentially result in an explosion. Similarly, in pressure cookers, heating the cooker increases the pressure exerted by the steam inside, causing the food to cook faster.
Gay-Lussac's Law also helps explain everyday phenomena, such as the decrease in tyre pressure during winter due to lower temperatures, or the increase in pressure inside a propane tank on a hot day. Additionally, it has clinical applications, such as understanding the change in volume of a pneumothorax at different altitudes, where the pressure-volume relationship described by Boyle's Law can be used to calculate the change in volume at different atmospheric pressures.
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Ideal Gas Law: The relationship between pressure, volume, number of moles, and temperature
The ideal gas law combines several empirical gas laws to establish the relationship between four variables: pressure (P), volume (V), the amount of gas (n) and temperature (T). It assumes that the gas particles are extremely small, exhibiting constant, random, and straight-line motion, with no forces between them. They only collide elastically with each other and with the container's walls.
The ideal gas law is derived from Charles' Law, which states that the volume of a given mass of gas at constant pressure is directly proportional to its temperature. For example, if the temperature of a gas is increased, its volume will increase, and vice versa. This can be observed by placing a balloon filled with gas into a freezer, where it will decrease in volume.
The ideal gas law also incorporates Boyle's Law, which states that the volume of a given mass of gas at a constant temperature is inversely proportional to its pressure. This means that if the volume of a gas increases, its pressure decreases, and conversely, if the volume decreases, its pressure increases.
Additionally, Avogadro's Law, which states that the volume of a gas is directly proportional to the amount of gas when pressure and temperature remain constant, is included in the ideal gas law. This law is based on the hypothesis that equal volumes of gas, at the same pressure and temperature, contain the same number of molecules.
The ideal gas law can be expressed as PV = nRT, where R is the gas constant. This equation can be used to calculate changes in volume, temperature, pressure, and the number of moles of an ideal gas. It is a good approximation for most gases under moderate pressure and temperature.
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Frequently asked questions
Boyle's Law states that the volume of a given amount of gas held at a constant temperature varies inversely with the applied pressure when the temperature and mass are constant.
Charles' Law, or the law of volumes, was founded in 1787 by Jacques Charles. It states that, for a given mass of an ideal gas at constant pressure, the volume is directly proportional to its absolute temperature, assuming a closed system.
Avogadro's Law states that the volume of gas increases as the amount of gas increases, when pressure and temperature are held constant.











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