Air and Gases

Air is a mixture of gases that surrounds Earth and makes life possible. Although it appears empty, it is made of tiny particles that are constantly moving. Dry air contains about 78% nitrogen, 21% oxygen and small amounts of argon, carbon dioxide and other gases. Water vapor is also present, but its quantity changes with the weather and location.

Oxygen is essential for most living organisms because it allows cells to produce energy, while nitrogen helps maintain the balance of the atmosphere. Carbon dioxide, even in small amounts, is an important part of the carbon cycle and is used by plants during photosynthesis. Air also carries heat, moisture, pollen and tiny particles that influence weather, climate and air quality.

Like all gases, air has mass, occupies space and exerts pressure on everything around us. These properties explain why balloons inflate, why airplanes can fly and why changes in air pressure affect the weather. Gases expand when heated and usually contract when cooled, although the exact change depends on pressure and other conditions.

Scientists describe this behavior with gas laws that relate temperature, pressure and volume. Many gases have practical uses beyond the atmosphere. Helium fills balloons because it is lighter than air and does not burn, while oxygen is used in medicine and industry. Some gases can be harmful if inhaled in high concentrations or if they replace oxygen, making good ventilation important in homes, workplaces and laboratories.