In physics and chemistry, the properties of gases have been a subject of study for many years. The behavior of gases is described by several laws, including Boyle’s Law and Charles’s Law. The two laws relate to the physical properties of gases, and when working with them, certain quantities are held constant. In this article, we will explore which quantity is held constant when working with Boyle’s Law and Charles’s Law.

## Boyle’s Law

Boyle’s Law is a fundamental principle of the physical properties of gases. It 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. This means that when pressure is increased, the volume of a given amount of gas decreases proportionally. Conversely, when pressure is decreased, the volume of a given amount of gas increases proportionally.

When working with Boyle’s Law, temperature is held constant. This is because temperature affects the kinetic energy of gas molecules and can change the number of collisions between gas molecules. If the temperature were not constant, it would be challenging to determine the effect of pressure on the volume of a gas.

For example, suppose we have a glass syringe with a piston placed at the top of it. The syringe contains air, and we know that the initial volume of the air inside the syringe is 20 ml. If we apply pressure to the piston, the volume of the air inside the syringe will decrease. If the pressure applied is doubled, the volume of the air will reduce to 10ml, as per Boyle’s Law. The temperature of the air in the syringe is kept constant throughout the process.

## Charles’s Law

Charles’s law is a concept in chemistry and physics that relates the temperature and volume of a gas at a constant pressure. It states that the volume of a gas held at a constant pressure increases or decreases proportionally with the temperature of the gas, provided that the mass and pressure of the gas are kept constant.

Temperature in Charles’s Law is held constant. This is because temperature affects the kinetic energy of gas molecules and can change the number of collisions between gas molecules. If the temperature varies, it is challenging to determine the effect of temperature on the volume of a gas.

For example, suppose we have a plastic container with a hole in the middle. The container contains air, and we know that the initial volume of the air inside the container is 500 mL at a temperature of 20°C. We then heat the container to 50°C while keeping the pressure and mass of the air constant. As per Charles’s Law, the volume of air inside the container will now be 800 mL.

## Conclusion

In conclusion, when working with Boyle’s Law and Charles’s Law, certain quantities are held constant. When working with Boyle’s Law, temperature is held constant, while in Charles’s Law, pressure is held constant. The physical properties of gases are complex, and understanding these laws provides a foundation for understanding other principles like the ideal gas law. Knowledge of these laws is useful in fields such as engineering, meteorology, and the study of atmospheric gases.

## FAQ

### What is the constant quantity in Charles’s law?

Charles’s law is a fundamental concept in the field of thermodynamics that describes the relationship between the volume of a gas and its temperature. According to the law, the volume of a gas at a constant pressure is directly proportional to its temperature, provided that the pressure and the amount of gas remain constant. In other words, if the temperature of a gas increases, its volume will also increase proportionally.

One important aspect of Charles’s law is the constant quantity that governs the relationship between the volume and temperature of a gas. This constant quantity is the pressure of the gas. In other words, if the pressure of a gas is kept constant while its temperature is increased, the volume of the gas will increase proportionally. This relationship can be expressed mathematically as V/T = constant, where V is the volume of the gas, T is the absolute temperature, and the constant is equal to the pressure of the gas.

It is important to note that Charles’s law applies only to ideal gases, which are defined as gases that lack intermolecular forces and occupy no volume. Real gases, on the other hand, exhibit some deviation from ideal behavior due to the intermolecular interactions between their particles, which can affect their volume-temperature relationship.

The pressure of the gas is the constant quantity in Charles’s law, which describes the relationship between the volume and temperature of an ideal gas at a constant pressure. Understanding this relationship is essential for studying the behavior of gases and for applications such as the design of thermodynamic systems, engines, and refrigeration systems.

### Which is a constant quantity?

In physics and other sciences, a constant quantity is a value that remains the same and does not change over time or under varying circumstances. These constant quantities are fundamental to our understanding of the physical world and are used in a wide range of applications, from engineering and technology to medicine and environmental science.

A constant quantity can be expressed in many different ways, depending on the type of measurement being considered. For example, in the case of distance, a constant quantity might be expressed as a fixed value, such as 3 meters or 10 feet. This value would remain the same regardless of external factors, such as temperature, pressure, or illumination.

Similarly, in the case of speed, a constant quantity might be expressed in terms of miles per hour or kilometers per second. Again, this value would remain constant regardless of the surrounding conditions that may affect the physical system under analysis.

It is important to note that constant quantities are distinct from function-quantities. While constant quantities remain fixed and constant over time, function-quantities refer to relationships between different variables that change over time. For example, the height of a ball thrown into the air is a function-quantity, because it changes over time according to the laws of physics.

Constant quantities are fundamental to our understanding of the physical world and play a critical role in a wide range of scientific and engineering applications. Whether expressed in terms of distance, speed, temperature, or any other physical quantity, these values remain constant and provide a reliable baseline for our understanding of the natural world.

### Which two variables must be held constant for Boyle’s law to apply?

Boyle’s law is one of the fundamental laws of gases that describes the relationship between the pressure and volume of a gas when the temperature and the number of gas molecules are kept constant. This law states that the pressure and volume of a gas are inversely proportional to each other, i.e., when the pressure of a gas is increased, its volume decreases, and vice versa.

However, to apply Boyle’s law accurately in any given situation, the temperature and the number of gas molecules must remain constant. Let us first understand the significance of these two variables and how they affect the pressure and volume of a gas.

Temperature: The temperature of a gas is directly proportional to its volume, i.e., as the temperature of a gas increases, its volume also increases, and vice versa. Therefore, to apply Boyle’s law, the temperature of a gas must be constant because any change in temperature will affect the volume of the gas, and this will, in turn, affect the pressure, making it difficult to measure the true relationship between pressure and volume.

Number of gas molecules: The number of gas molecules, or more specifically, the number of moles of gas, also affects the pressure and volume of a gas. In a closed container, the more the number of gas molecules, the higher the pressure created by the gas. Similarly, more gas molecules occupy a larger volume, consistent with Avogadro’s law. Hence, to apply Boyle’s law accurately, the number of gas molecules in the container must remain constant, or else it will disturb the true relationship between pressure and volume.

Therefore, to summarize, Boyle’s law is applicable only when the temperature and the number of moles of the gas are constant. Any variation in these two variables will invalidate the law, and the relationship between pressure and volume of the gas will no longer remain proportional.