What Is Air?

Air is invisible—you cannot see, touch, or hold it in your hands. But if it cannot be seen, can it really be considered a substance?
More than 2,000 years ago, people had already found the answer. Let's take a closer look at the discovery that proved the existence of something invisible, along with some of the fascinating properties of air.

Air Is a Substance — A Discovery Made Over 2,000 Years Ago

Although invisible, air occupies space and has mass. This idea was demonstrated by the ancient Greek scientist Heron, who lived around 200–150 BCE. His experiment was simple. When an empty bottle is turned upside down and pushed into water, water does not enter. However, if a small hole is made in the bottom of the bottle, water begins to flow inside. This happens because the accumulated air escapes through the hole, creating room for the water to enter. Even though it cannot be seen, the experiment shows that something was already inside the bottle. In other words, air is real matter that takes up space.

Does Air Have Weight?

Air has weight, even though we do not normally notice it. This can be demonstrated through a simple experiment. If a spray can filled with compressed air is weighed before and after the air is released, the can becomes lighter by the amount of air that has escaped. The weight of air can also change slightly depending on environmental conditions. As temperature or humidity increases, air generally becomes slightly lighter.

If Air Has No Color, Why Is the Sky Blue?

Although air itself is colorless, the sky appears blue because sunlight is scattered as it passes through the atmosphere. Sunlight contains many colors, just like the colors visible in a rainbow. When this light strikes tiny particles in the air, such as dust and water vapor, it scatters in different directions. Among these colors, blue light is scattered more easily. As a result, blue light spreads throughout the sky, giving it its familiar blue appearance.

Why Does the Sky Appear Red at Sunrise and Sunset?

The color of the sky changes throughout the day because the distance sunlight travels through the atmosphere also changes. When the sun is high overhead during the day, its light passes through a relatively short distance of atmosphere before reaching the Earth's surface. During this process, blue light is scattered throughout the sky, making it appear blue. In the morning and evening, however, the sun is lower in the sky. As a result, sunlight passes through the atmosphere at an angle and travels a longer distance before reaching us. During this journey, much of the blue light is scattered away before reaching our eyes. This makes red light, which has a longer wavelength, easier to see, causing the sky to appear red at sunrise and sunset.