Fixed air is an early term for carbon dioxide, coined because it could be absorbed or "fixed" by alkaline substances like limewater.
What is fixed air?
Fixed air is an obsolete term for carbon dioxide or carbonic acid gas.
Back in the 1700s, chemists didn’t understand gases the way we do now. Dr. Joseph Black proved in the 1750s that heating limestone or treating it with acid released a gas he named “fixed air.” Why? Because it could bind with alkaline substances like quicklime. Turns out, that gas was carbon dioxide—a key player in respiration, combustion, and even the greenhouse effect that keeps Earth cozy.
Why CO2 is called fixed air?
Carbon dioxide was called fixed air because it could be chemically absorbed or “fixed” by strong bases like quicklime.
Joseph Black’s experiments showed something fascinating. When limestone (calcium carbonate) got heated or mixed with acid, it released a gas that snuffed out flames. Even better? That gas could be recaptured by reacting with lime (calcium oxide), turning back into chalk. This ability to “fix” itself to alkaline substances earned it the name fixed air—long before anyone knew its formula was CO₂.
Did carbon dioxide used to be called fixed air?
Yes, as of the mid-18th century, carbon dioxide was commonly referred to as fixed air.
Scottish chemist Joseph Black didn’t just stumble upon this discovery. In 1756, he isolated and named fixed air after heating limestone or treating chalk with acids. His work proved fixed air was a real substance in the air we breathe—and it could form new compounds. That breakthrough laid the foundation for modern chemistry.
Which gas is called fixed gas?
Carbon dioxide is the gas referred to as fixed air or fixed gas.
The phrase “fixed gas” doesn’t get much use these days. But back then, it described any gas that could bind to alkaline substances. Carbon dioxide was the main example early chemists like Black studied. (Honestly, it’s the only common gas that fits this description.)
Who introduced oxygen?
Oxygen was discovered by Joseph Priestley in 1774, though he called it “dephlogisticated air” at the time.
Priestley made an incredible find. By heating mercuric oxide, he isolated a gas that made candles burn brighter and let mice survive longer in sealed containers. His work shook up chemistry and medicine, paving the way for Lavoisier’s later naming of oxygen and its role in combustion and respiration.
What is Dephlogisticated air?
Dephlogisticated air is an old term for oxygen, based on the discredited phlogiston theory.
The phlogiston theory sounds wild today. It claimed burning substances released phlogiston, a fire-like element. Priestley thought oxygen was air that had lost its phlogiston—hence “dephlogisticated.” Though the theory was dead wrong, his experiments led directly to oxygen’s discovery and modern chemistry.
What does exhaled air contain?
Exhaled air contains about 16% oxygen, 4% carbon dioxide, and 79% nitrogen.
| Gas | Inhaled air (%) | Exhaled air (%) |
|---|
| Oxygen | 21 | 16 |
| Carbon dioxide | 0.04 | 4 |
| Nitrogen | 79 | 79 |
These numbers aren’t set in stone. Your breathing rate, activity level, and health can shift them slightly. The drop in oxygen and rise in CO₂? That’s your body using O₂ for metabolism and dumping CO₂ as waste.
What is the modern word for fixed air?
The modern word for fixed air is carbon dioxide.
“Fixed air” sounds like something from a dusty textbook now. Today, we know it as carbon dioxide (CO₂). This gas is everywhere—naturally part of Earth’s atmosphere—and it’s crucial for photosynthesis, climate regulation, and human respiration.
Who gave the name carbon dioxide?
Joseph Black first identified and studied carbon dioxide in the 1750s, though the formal name “carbon dioxide” was later popularized by Antoine Lavoisier.
Black showed fixed air (CO₂) could be made from limestone and absorbed by alkalis. Lavoisier later gave it the name “carbonic acid gas,” then “carbon dioxide.” He recognized its role in combustion and respiration. At room temperature, CO₂ is colorless, odorless, slightly acidic, and won’t catch fire.
Do we breathe out carbon dioxide?
Yes, humans continuously exhale carbon dioxide as a waste product of metabolism.
Every breath you exhale carries about 4% CO₂. This gas travels from your cells through your blood to your lungs, where it’s pushed out. That carbon in your CO₂ came from the food you ate—not the air you inhaled. So while you’re not recycling the exact same carbon atoms, you’re definitely releasing CO₂ with every exhale.
What was carbon dioxide known as?
Carbon dioxide was historically known as fixed air, and in solid form it’s called dry ice.
As a gas, CO₂ is invisible and scentless. Under pressure, it becomes a liquid used in fire extinguishers and soda fizz. Freeze it to −78.5°C, and it turns to dry ice, which skips melting and goes straight to gas. CO₂ also traps heat in the atmosphere, making it a major greenhouse gas.
Who gave term gas?
The term “gas” was coined by Flemish chemist Jan Baptist van Helmont in the early 17th century.
Van Helmont separated gases from liquids and solids through experiments. He named this new state of matter using the Flemish word “geest” (meaning spirit or ghost), which morphed into “gas.” He even identified carbon dioxide as the first known gas besides air—a huge leap for chemistry.
Which gas is chalky acid gas?
Carbon dioxide is the “chalky acid gas,” because it forms chalk (calcium carbonate) when it reacts with lime (calcium oxide) in water.
Chalk is mostly calcium carbonate (CaCO₃). It forms when CO₂ reacts with calcium hydroxide (limewater). This reaction works both ways—heat chalk, and it releases CO₂. That’s exactly how early chemists like Black produced and studied fixed air.
Why CO2 is not used as an inert gas?
CO₂ is not truly inert—it reacts with water to form carbonic acid and with strong bases to form carbonates.
Sure, CO₂ and nitrogen often get lumped in with noble gases under “inert” labels for convenience. But CO₂ isn’t chemically inert like argon or helium. It reacts with water to make carbonic acid and with strong bases to form carbonates. Unlike truly inert gases, CO₂ doesn’t just sit there—it forms compounds.
