Composition of Air

Air is a mixture of distinct gases, primarily nitrogen, oxygen, carbon dioxide, and water vapor, rather than a single uniform element. Early scientists discovered these individual gases through experiments involving combustion, animal respiration, and plant life.

Joseph Priestley's experiments

In 1774, Joseph Priestley placed a burning candle under a beaker sealed with water, and the flame soon went out because the air lost the substance needed for burning. When he placed a mouse inside, the mouse died because the trapped air lacked what it needed to breathe, a gas later named oxygen by Antoine Lavoisier. Priestley then placed a living plant inside with a candle and a mouse. The mouse and candle survived because the plant took in the carbon dioxide they released and gave out oxygen, proving that air is a mixture of gases.

How nitrogen was identified

• Daniel Rutherford modified Priestley's experiment by passing the air left after a candle burned out into lime water.
• The carbon dioxide in that air reacted with the lime water to form a white precipitate.
• The remaining gas could not support a burning candle or allow plants to breathe.
• In 1790, Jean-Antoine Chaptal named this remaining gas nitrogen.

Testing for oxygen from plants

• Submerge small green leaves under an inverted funnel inside a beaker filled with water.
• Invert a test tube filled with water over the funnel and place the setup under sunlight.
• The leaves release gas bubbles through photosynthesis that collect inside the test tube.
• Bringing a small flame near this trapped gas makes it burn brightly, proving it is oxygen.

Testing for carbon dioxide

Carbon dioxide can be identified using clear lime water. Pouring lime water into a test tube and dipping a straw into it creates a simple test. When someone blows through the straw, the lime water turns white. This reaction confirms that carbon dioxide is present in the gas.