Composition of Air

• Air is present all around us, forming a blanket around the Earth called the atmosphere.
• The atmosphere extends up to about 1000 km above the Earth's surface.
• Air becomes thinner as altitude increases, requiring mountaineers to carry their own oxygen supply on high mountains.
• Ninety-nine percent of the air is found up to a height of 30 km above the Earth's surface.

Composition of Air

• Until the eighteenth century, air was thought to be a single substance.
• In 1774, Joseph Priestley showed that part of air was the gas oxygen, proving that air is a mixture.
• Air mainly contains nitrogen (about 78% by volume) and oxygen (about 21% by volume).
• The remaining 1% includes argon (about 0.9%), carbon dioxide (0.03%), and small amounts of other gases.
• Varying quantities of dust, smoke, and water vapor are also present in the air.
• The amount of dust, smoke and water vapour varies from place to place and time to time.
• Oxygen and nitrogen are the main components of air.

Activity: Presence of Oxygen in Air

• An experiment involves lighting a candle and covering it with a glass tumbler to show that air is required for combustion.
• The candle gets extinguished after some time because the air in the tumbler gets used up.
• Rusting of iron is used to determine if only one component of air is used up during burning.
• Burning is a chemical reaction that produces other gases, while rusting also uses the same gases as combustion but does not produce any gases.
• Rusting of iron occurs when iron is exposed to humid air and it is a chemical change.
• Steel wool is pressed into the bottom of a gas jar, which is then inverted into a bowl of water and rested on wooden blocks.
• The water level in the gas jar rises slowly over 2-3 days.
• The level of water rises up by about one-fifth in the gas jar.
• The component of air used up in rusting the steel wool forms about 20% of air, which means it is oxygen, forming 21% of air.