So, why does the gas expand when the pressure decreases? Well, **Boyle's law** has the answer. Read on to find out more!

- This article discusses
**Boyle's law.** - First, we will review the components of Boyle's law: ideal gas, pressure, and volume.
- Next, we will define Boyle's law.
- Then, we will do an experiment to show how Boyle's law works.
- Subsequently, we will learn about the
**Boyle's law constant.** - Lastly, we will learn about an equation related to Boyle's law and use it in some examples.

## Boyle's Law Overview

Before we talk about Boyle's law, let's talk about the components involved: **ideal gases**, **pressure**, and **volume.**

First up, let's talk about **ideal gases**.

When looking at this law and other related gas laws, we are typically applying them to **ideal gases.**

An **ideal gas **is a theoretical gas that follows these rules:

- They are constantly moving
- The particles have a negligible mass
- The particles have negligible volume
- They do not attract or repel other particles
- They have full elastic collisions (no kinetic energy is lost)

Ideal gases are a way to approximate gas behavior since "real" gases can be a bit tricky. However, the ideal gas model is less accurate than the behavior of a Real Gas at low temperatures and high pressure.

Next up, let's talk **pressure**. Since (ideal) gases are constantly in motion, they often collide with each other and the walls of their container. Pressure is the force of the gas particles colliding with a wall, divided by the area of that wall.

Lastly, let's discuss **volume**. Volume is the space a substance takes up. Ideal gas particles are approximated to have negligible volume.

## Boyle's Law Definition

The definition of Boyle's law is shown below.

**Boyle's law **states that for an ideal gas, the pressure of a gas is inversely proportional to its volume. For this relationship to be true, the amount of gas and temperature must be kept constant.

In other words,** if volume decreases, pressure **

*and vice-versa (assuming gas amount and temperature have not changed).*

**increases**## Boyle's Law Experiment

To get a better understanding of this law, let's do an experiment.

We have a 5L container of 1.0 mol of hydrogen gas. We use a manometer (pressure reading instrument), and see that the pressure inside the container is 1.21 atm. In a 3 L container, we pump in the same amount of gas at the same temperature. Using the manometer, we find that the pressure in the container is 2.02 atm.

Below is a diagram to illustrate this:

As the volume decreases, the gas has less room to move. Because of this, the gas particles are more likely to collide with other particles or the container.

This relationship only applies when the **amount **and **temperature **of the gas are *stable*. For example, If the amount decreased, then the pressure might not change or even *decrease* since the ratio of moles of gas-particle to volume decreases (i.e there is more room for particles since there are fewer of them).

## Boyle's Law Constant

One way to visualize** Boyle's law** mathematically is this:

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

Where,

P is pressure

V is volume

∝ means "proportional to"

What this means is that for every change in pressure, the inverse volume (1/V) will change by the same amount.

Here's what that means in graph form:

The graph above is linear, so the equation is \(y=mx\). If we put this equation in Boyle's law terms, it would be \(P=k\frac{1}{V}\).

When we refer to a linear equation, we use the form y=mx+b, where b is the y-intercept. In our case, "x" (1/V) can never be 0 since we can't divide by 0. Therefore, there is no y-intercept.

So, what's the point of this? Well, let's rearrange our formula:

$$P=k\frac{1}{V}$$

$$k=PV$$

The constant (k) is a proportionality constant, which we call **Boyle's law constant**. This constant tells us how the pressure value will change when the volume does and vice-versa.

For example, let's say we know that k is 2 (atm*L). This means we can calculate the pressure or volume of an ideal gas when given the other variable:

Given a gas with a volume of, 1.5 L, then:

$$k=PV$$

$$2(atm*L)=P(1.5\,L)$$

$$P=1.33\,atm$$

On the other hand, if we are given a gas with a pressure of, 1.03 atm, then:

$$k=PV$$

$$2(atm*L)=1.03\,atm*V$$

$$V=1.94\,L$$

## Boyle's Law Relationship

There is another mathematical form of Boyle's law, which is more common. Let's derive it!

$$k=P_1V_1$$

$$k=P_2V_2$$

$$P_1V_1=P_2V_2$$

We can use this relationship to calculate the resulting pressure when the volume changes or vice-versa.

It's important to remember that this is an inverse relationship. When variables are on the same side of an equation, that means there is an inverse relationship (here P_{1} and V_{1} have an inverse relationship, and so do P_{2} and V_{2}).

**The Ideal Gas Law: **Boyle's law, when combined with other ideal gas laws (such as Charles's law and Gay-Lussac's law), forms the **Ideal Gas Law.**

The formula is:

$$PV=nRT$$

Where P is pressure, V is volume, n is the number of moles, R is a constant, and T is temperature.

This law is used to describe the behavior of ideal gases, and therefore approximates the behavior of real gases. However, the ideal gas law becomes less accurate at low temperatures and high pressure.

## Boyle's Law Examples

Now that we know this mathematical relationship, we can work on some examples

**A diver is deep underwater and is experiencing 12.3 atmospheres of pressure. In their blood, there is 86.2 mL of Nitrogen. As they ascend, they are now experiencing 8.2 atmospheres of pressure. What is the new volume of Nitrogen gas in their blood?**

As long as we use the same units on both sides, we don't need to convert from milliliters (mL) to liters (L).

$$P_1V_1=P_2V_2$$

$$V_2=\frac{P_1V_1}{P_2}$$

$$V_2=\frac{12.3\,atm*86.2\,mL}{8.2\,atm}$$

$$V_2=129.3\,mL$$

We can also solve this problem (and others like it) using the Boyle's law constant equation we used earlier. Let's try it out!

**A container of neon gas has a pressure of 2.17 atm and a volume of 3.2 L. If the piston inside the container is pressed down, decreasing the volume to 1.8 L, what is the new pressure?**

The first thing we need to do is solve for the constant using the initial pressure and volume

$$k=PV$$

$$k=(2.17\,atm)(3.2\,L)$$

$$k=6.944\,atm*L$$

Now that we have the constant, we can solve for the new pressure

$$k=PV$$

$$6.944\,atm*L=P*1.8\,L$$

$$P=3.86\,atm$$

## Boyle's Law - Key takeaways

- An
**ideal gas**is a theoretical gas that follows these rules:- They are constantly moving
- The gas particles have a negligible mass
- The gas particles have negligible volume
- They do not attract or repel other particles
- They have full elastic collisions (no kinetic energy is lost)

**Boyle's law**states that for an ideal gas, the pressure of a gas is inversely proportional to its volume. For this relationship to be true, the amount of gas and temperature must be kept constant.- We can use this equation \(P \propto \frac{1}{V}\) to visualize Boyle's law mathematically. Where P is pressure, V is volume, and ∝ means "proportional to"
- We can use the following equations to solve for the change in pressure/volume due to a change in volume/pressure
- $$k=PV$$ (Where k is the proportionality constant)
- $$P_1V_1=P_2V_2$$

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##### Frequently Asked Questions about Boyle's Law

What is Boyle's law simple definition?

**Boyle's law **states that for an ideal gas, the pressure of a gas is inversely proportional to its volume. For this relationship to be true, the amount of gas and temperature must be kept stable.

What is a good example of Boyle's law?

When the top of a spray can is pressed down, it greatly increases the pressure inside the can. This increased pressure forces the paint outward.

How do you verify Boyle's law experiment?

To verify that Boyle's law is true, all we need to do is measure the pressure using a pressure gauge or other pressure reader. If the pressure of a gas increase when volume is reduced, Boyle's law is verified.

What's constant in Boyle's law?

Both the amount of gas and the temperature of the gas are assumed to be constant.

Does Boyle's law have a direct relationship?

No, since pressure increases with a volume *decrease* (i.e the relationship is indirect/inverse).

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