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.

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.Almost everything is made up of atoms (your dog, your science book, this computer...). The atom has a nucleus...more

Density Definition Page

Density is a measure of how much mass is contained in a given unit volume (density = mass/volume). Put simply, if mass is a measure of how much ‘stuff’ there is in an object, density is a measure of how...more


Most things around us are made of groups of atoms connected together into packages called molecules. Molecules are made from atoms of one or more elements. Some molecules are made of only one type of...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 has three different forms, called states: solid, liquid and gas. The state...more


Have you ever left a glass of water out for a long time? Did you notice that the water disappears after a few days? That's because it evaporated! Evaporation is when water passes from a liquid to a gas....more


There is more nitrogen gas in the air than any other kind of gas. About four out of five of the molecules in Earth's atmosphere is nitrogen gas! A molecule of nitrogen gas is made up of two nitrogen atoms....more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA