Shop Windows to the Universe

Learn about planets outside our solar system through Exoplanets and Alien Solar Systems by Tahir Yaqoob, Ph.D., a book in our online store book collection.
This weather balloon is full of helium gas. It is surrounded by Earth's atmosphere, which is mostly nitrogen and oxygen gasses. Helium is "lighter" (less dense) than nitrogen or oxygen, so the balloon will rise when the scientist lets go of it.
Click on image for full size
Image courtesy of the University Corporation for Atmospheric Research.


Gas is one of the four common states of matter. The three others are liquid, solid, and plasma. There are also some other exotic states of matter that have been discovered in recent years.

The air in Earth's atmosphere is mostly a mixture of different types of gases. A gas usually has much lower density than a solid or liquid. A quantity of gas doesn't have a specific shape; in this way it is like a liquid and different from a solid. If a gas is enclosed in a container, it will take on the shape of the container (a liquid will too).

The volume of a gas changes if the temperature or pressure changes. There are several scientific laws, called the "gas laws", that describe how the volume, temperature, and pressure of a gas are related.

The molecules or atoms in a gas are much further apart than in a solid or a liquid. Gas molecules or atoms are usually flying around at very high speeds, occasionally bouncing off each other or the walls of the container the gas is in.

When a gas is cooled or placed under high pressure, it can condense and turn into a liquid. If a liquid boils or evaporates, it will become a gas. Under some circumstances, usually very low pressure, a solid can turn directly into a gas (without first melting and becoming a liquid). When a solid turns directly into a gas, it is called "sublimation".

Most of the air in Earth's atmosphere is either nitrogen or oxygen gas. Balloons are often filled with helium gas; since helium is lighter (less dense) than air, helium balloons "float" or rise up in air. When liquid water boils or evaporates, it turns into a gas called "water vapor". Most of the gas in the atmospheres of the giant planets Jupiter and Saturn is hydrogen gas. In recent years, carbon dioxide gas has become quite famous because of its role in the Greenhouse Effect and global warming.

Last modified June 25, 2008 by Randy Russell.

Shop Windows to the Universe Science Store!

Our online store includes books on science education, classroom activities in The Earth Scientist, mineral and fossil specimens, and educational games!

Windows to the Universe Community



You might also be interested in:

Traveling Nitrogen Classroom Activity Kit

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more


Solid is one of the four common states of matter. The three others are gas, liquid, and plasma. There are also some other exotic states of matter that have been discovered in recent years. Unlike liquids...more

The Plasma State

Plasma is known as the fourth state of matter. The other three states are solid, liquid and gas.In most cases, matter on Earth has electrons that orbit around the atom's nucleus. The negatively charged...more

Density Definition Page

Density is a measure of how much mass is contained in a given unit volume (density = mass/volume). It is usually expressed in kg/m^3. Put simply, if mass is a measure of how much ‘stuff’ there is in an...more


Most things around us are made of groups of atoms bonded together into packages called molecules. The atoms in a molecule are held together because they share or exchange electrons. Molecules are made...more

Changes of State: Solids, Liquids, and Gases

A snowman, glass of water and steam might look very different but they are made of the same stuff! Just like any substance, water can exist in three different forms, called states: solid, liquid and gas....more


One process which transfers water from the ground back to the atmosphere is evaporation. Evaporation is when water passes from a liquid phase to a gas phase. Rates of evaporation of water depend on things...more


There is more nitrogen gas in the air than any other kind of gas. About 4/5ths of Earth's atmosphere is nitrogen gas! A molecule of nitrogen gas is made up of two nitrogen atoms. There are other molecules...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part by the National Science Foundation and NASA, our Founding Partners (the American Geophysical Union and American Geosciences Institute) as well as through Institutional, Contributing, and Affiliate Partners, individual memberships and generous donors. Thank you for your support! NASA AGU AGI NSF