Everything you see, touch, and breathe is made of matter, which is anything that takes up space and has mass. Matter is built from tiny pieces called atoms and molecules that are constantly moving. Depending on how these pieces are arranged and how much energy they have, matter exists in three common forms: solid, liquid, and gas. Understanding the behavior of these small pieces helps us understand the differences between a hard ice cube, flowing water, and the air we breathe.
What Makes a Gas Different?
The tiny particles that make up a gas are spread far apart from each other, unlike the closely packed particles in solids and liquids. These gas particles are always moving incredibly fast, zooming in every direction. Because the particles are so separated and free, a gas has no fixed shape or fixed size.
If you put a gas into a container, its particles spread out to fill the entire space available, which is why the air inside a balloon takes on the balloon’s shape. Gases can also be easily compressed, allowing a large amount of gas to be squeezed into a smaller container (like air pumped into a tire). This is possible because of the empty space between the particles. Most gases, like the air around us, are invisible, even though they take up space and have mass.
Gases All Around Us
Gases are everywhere, even in places you cannot see. The air that surrounds our planet is a mixture of many different gases, mainly nitrogen and oxygen. Oxygen is the gas our bodies need to breathe and get energy, while nitrogen makes up the largest part of the air, about 78%.
Gases are also used in everyday ways. The bubbles that make soda fizzy are carbon dioxide. When you blow up a party balloon, you are filling it with a gas like air, or perhaps helium, a lighter gas that makes the balloon float. Even though air is invisible, you can feel it when the wind blows against your face, showing that the gas particles are moving and pushing on objects. Water vapor is another gas present in the air, visible as steam rising from a hot drink or a bath.
Making Gas Appear and Disappear
Matter can change from a liquid to a gas, and back again, by gaining or losing energy. When liquid water is heated, the particles gain energy and move fast enough to break away from each other, turning the water into an invisible gas called water vapor. This process of a liquid becoming a gas is called evaporation.
The opposite happens when the gas is cooled down. When water vapor touches a cold surface, like the outside of a glass of iced water, the gas particles lose energy and slow down. This causes the particles to move closer together and turn back into liquid water droplets, a process known as condensation. These phase changes show that gas can transform back into liquid just by changing its temperature.