“In” and “Out”: Understanding Inspired and Expired Air

 

What’s the Difference Between the Air We Breathe In and Out?

When you breathe in (inspired air) you take in air from around you. When you breathe out (expired air) you release air from your lungs. These two types of air don’t have the same amounts of gases.

Think of it like a sponge: when you “dip” it into water (breathing in), it picks up what’s there, and when you “squeeze” it (breathing out), what comes out isn’t exactly the same as what went in.

 

Key Differences in Composition

Gas	Inspired Air (Breathed In)	Expired Air (Breathed Out)
Oxygen $(O_2)$	Higher amount (more fresh oxygen)	Lower amount (we use some up)
Carbon Dioxide $(CO_2)$	Very low amount	Higher amount (we produce it)
Water Vapour	Variable (depends on surroundings)	Higher (lungs add moisture)

In short: Breathed-out air has less oxygen, more carbon dioxide, and is usually more moist than breathed-in air.

 

Testing for Carbon Dioxide with Limewater

Limewater is a special solution that turns milky (cloudy) when carbon dioxide is bubbled through it. This is how we can show expired air has more $CO_2$ than inspired air.

Step-by-Step Example:

1. Take two test tubes of limewater.
2. Gently blow into the first tube using a straw—this represents expired air.
3. Use a second straw and suck in air, then gently blow in fresh air (or just wave the open end) into the second tube—this represents inspired air.

What Happens?

• Tube with expired air (your breath): Limewater turns milky quickly because there’s more $CO_2$ in your breath. 
• Tube with inspired air: Stays clearer longer because there’s less $CO_2$ coming directly from the surroundings.