The Earth's Air

An Invisible Mix

Look all around you right now. You cannot see the air. But it is always there.

You feel it on a windy day. You fill party balloons with it. You breathe it in every second.

Air is not just one empty thing. It is a busy mix of different gases.

Think of a big bowl of mixed nuts. You see peanuts. You see almonds. You see cashews.

Air is exactly like that snack bowl. It holds a mix of invisible gases 1.

Each gas has its own special job. Together, they make our planet a great home.

We learned about light waves before. Sunlight travels through empty space.

Then the sunlight reaches our planet. It must pass through this thick mix of gases.

What exactly is in this air? We can measure it to find out.

The Biggest Piece is Nitrogen

Imagine 100 glass marbles in a jar. Count out 78 of those marbles.

Make all 78 marbles bright blue. These blue marbles represent nitrogen.

Nitrogen makes up most of our air 2. It is the biggest part of the mix.

Nitrogen is a very lazy gas. Scientists call it inert.

In plain terms: nitrogen gas does not like to change. It stays exactly the same.

It does not catch on fire. It does not react with other things.

It just hangs around in the sky. This lazy gas is very important.

It keeps our whole planet safe. It gives the light waves a place to travel.

The Oxygen We Breathe

Let us go back to our marble jar. Count out 21 more marbles.

Make these 21 marbles bright red. These red marbles represent oxygen.

Oxygen is the second biggest part of air 3.

You need oxygen to live. You take a deep breath. Your body uses the oxygen for energy.

Long ago, Earth had no oxygen at all. The air was very different back then.

In plain terms: tiny sea creatures made the very first oxygen. They pumped it up into the sky.

These tiny creatures changed our air forever 4. Now we have plenty of oxygen.

Measuring the Air

How do we know the parts of the air? We cannot see the gases with our eyes.

Scientists do special tests to find out. They trap air inside glass tubes.

They remove the oxygen from the tube 3. They measure the leftover gas.

This simple test shows the exact mix. The test shows a lot of nitrogen.

The test shows less oxygen. Science tools help us see the invisible.

Plants and Trace Gases

We have one single marble left in our jar. This last marble represents trace gases.

Trace gases are tiny bits of other stuff. We can list the most common ones.

• Argon is the biggest trace gas.
• Carbon dioxide is a very famous trace gas.
• Water vapor floats in the air too.
• Neon gas makes up a tiny fraction.

Plants love carbon dioxide. They pull it right out of the air.

Trees swap these gases every single day 5. They take in carbon dioxide.

They give us back fresh oxygen. This trade keeps the air perfectly balanced.

The trace gases also trap heat. They act like a warm winter coat for Earth.

Without them, our planet would be freezing cold.

Let us look at the whole mix of gases.

Gas Name	How Much in Air	What It Does
Nitrogen	78 parts out of 100	Stays stable and lazy
Oxygen	21 parts out of 100	Helps animals breathe
Trace Gases	1 part out of 100	Keeps Earth warm

Getting Ready for the Sky

We learned about sunlight color waves. We know the rainbow color order.

ROYGBIV shows us the visible spectrum. These light waves fly toward Earth.

They zoom into our mix of gases. The air is full of tiny molecules.

We will learn about these tiny bits in the air next.

The light waves hit the nitrogen. The light waves hit the oxygen.

The light bounces off the gases. Something amazing happens next.

The sky lights up with bright color!

Key Terms

• Nitrogen 2 — The most common gas in Earth's air. It is very stable and makes up 78 parts out of 100.
• Oxygen 3 — The second most common gas in Earth's air. Animals and humans need it to breathe and live.
• Trace gases 5 — Tiny amounts of other gases in the air, like carbon dioxide and argon. They help keep the Earth warm.
• Inert 2 — A science word for something that is very stable, lazy, and does not change or react easily.