Atmospheric Pressure — Definition & Overview

What is Atmospheric Pressure?

The air or atmosphere of our planet is made of gases. It's mostly composed of nitrogen, oxygen, and argon. The percent composition is 78%, 21%, and 1% respectively. Remember, gases fill their container. In this case, our system is the entire atmosphere. So, right now, there is an unfathomable number of gas molecules above and around you. All of these gases are pulled toward the earth by gravity. What's the result? Pressure! Every second, trillions of gas particles collide and bounce off you. It is the pressure of the earth's atmosphere!

The standard unit for atmospheric pressure measurement is the atm or standard atmosphere. At the sea level on Earth, the atmospheric pressure is equal to 1 atm. We can convert atm to other units of pressure.

1 atm is equal to:

• 101,325 Pa

• 760 mmHg

• 29.9212 inches Hg

• 14.696 psi

Atmospheric Pressure Changes

The atmospheric pressure changes based on elevation, so it is not the same everywhere on the planet. As the elevation increases, atmospheric pressure decreases.

Changes in atmospheric pressure are important because they can affect chemical reactions. A simple example is boiling water at different elevations. Water typically boils at 100$^{\circ}$C at 1 atm. Boiling occurs when the vapor pressure of water equals the atmospheric pressure. So, when the atmospheric pressure decreases (like at higher elevations) the boiling temperature of water decreases. This is great for boiling pasta at high elevations. Why? It will use less energy. For chemical reactions, it's important to understand that changes in atmospheric pressure will alter reaction conditions. That's why we often perform reactions at STP. We need a set of standard conditions to describe our reaction!

Measuring Pressure

So, how do we measure atmospheric pressure? We use a barometer! A barometer is an instrument with a thin tube and a basin. Both contain a liquid. But, the tube has a small section that's a vacuum. The atmosphere pushes on the liquid in the basin. This causes the liquid to flow up into the tube. If the pressure decreases, it flows out of the tube and back into the basin.

What liquid do we use? Historically, we used mercury. It was less dense than water. So, we could use a smaller barometer tube. That's also why sometimes you'll see pressure recorded in millimeters of mercury. We would measure the length that the mercury moved in the tube. Unfortunately, mercury is toxic. So, in the modern era we've switched to digital alternatives. Some modern barometers use capsules made of nontoxic metal alloys. In these, the alloy expanded or contracts with the pressure change.

(Video) Measuring Gas Pressure and Atmospheric Pressure

by Tyler DeWitt

In this video, Tyler DeWitt presents atmospheric pressure. He starts with an illustration of how the air presses down. So, you'll conceptually understand atmospheric pressure. Next, he explains how we can experimentally measure atmospheric pressure with a U-tube. Then, he explores u-tube measurements at different elevations. So, you'll see why pressure changes between sea level, mountains, and below sea level. Finally, he details how we measure the pressure in a tank. This video will tell you what you need to know to understand air pressure.

(Video) How Heavy is Air

by Ted-Ed

In this short video, TED-Ed illustrates air pressure. You'll learn...

• The number of molecules of air in a cubic centimeter.

• The weight of the air in a school gym.

• How you resist the air pressure so that you don't implode.

• Why air pressure causes weather.

• How your lungs create a pressure difference so you can breathe.

It's a fun short video that will tell you a lot about atmospheric pressure.