Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Gas, one of the three ordinary states of matter. The other two ordinary states of matter are solid and liquid. Both solids and liquids are made up of particles that touch one another. The attraction between the particles of a solid is so strong that the particles hold rigidly together. This rigidity gives solids a definite shape and volume. The attraction between particles in a liquid is great enough to hold the particles near each other but too weak to prevent the particles from sliding around. Liquids have a definite volume but take the shape of their container. The particles that make up a gas, however, are completely separated from one another. Empty space accounts for more than 99 percent of the total volume of air, for example. Because gas particles are separated, the attractive forces between them are extremely small and are insufficient to hold gases in a definite shape or volume. Gases expand freely to fill their containers.


The characteristics or properties of gases vary widely. Some gases are transparent, some have a strong smell, some dissolve in water, and some react violently with almost any substance. Other gases exhibit exactly the opposite properties. The chemical structure of gases also varies greatly.

A. Color

A number of gases have a characteristic color. For example, fluorine gas appears green, chlorine appears yellow-green, and nitrogen dioxide (a component of smog) appears red-brown. The majority of gases, however, are colorless.

B. Odor

Many gases, including nitrogen, oxygen, and hydrogen, are odorless. Ammonia, however, has a sharp, pungent odor. Because fuel gases such as methane, propane, and butane are odorless, an intensely odorous sulfur compound is added to them to ensure early detection should these gases leak from their containers.

C. Solubility

Some gases, such as carbon dioxide, dissolve well in water. Many others, including nitrogen, hydrogen, and oxygen, are only slightly soluble in water. The solubility of any gas decreases as the temperature of the gas increases.

D. Chemical Reactivity

Some gases can react with other substances to form new chemical compounds. Oxygen, for example, reacts with iron to form rust. The chemical reactivity of gases varies widely. Oxygen, chlorine, and fluorine are extremely reactive gases. In fact, fluorine will react with almost any other substance; even water and glass will burn in a fluorine atmosphere. At the other extreme are the noble gases, which are generally considered inert (unreactive). Neon, a noble gas, is not known to react with any other substance.

E. Structure

Gas particles are the smallest units into which a gas can be divided without changing the chemical properties of the gas. These particles can either be single atoms or molecules (combinations of atoms). The noble gases, such as neon and helium, are composed of individual atoms. Other gases, including carbon dioxide (CO2), methane (CH4), and ammonia (NH3), contain atoms of more than one element chemically bound together in molecules. Some gases that contain only a single element, such as hydrogen, oxygen, and nitrogen, are also composed of molecules. The oxygen in Earth’s atmosphere, for example, consists mostly of oxygen molecules (O2) rather than individual oxygen atoms (O).


From the 17th to the 19th century, scientists noticed that gases respond to changes in temperature, pressure, and volume in predictable ways. Scientists established four laws that govern the behavior of gases: Boyle’s law, Charles’s law, Dalton’s law, and Avogadro’s law. These four gas laws can be combined and expressed as a single equation known as the combined ideal gas equation.

Ideal Gas Equation:

The gas laws can be combined as a more general expression called the ideal gas equation or ideal gas law:

PV = nRT

In this equation, n represents the number of moles of a gas. The constant R on the right-hand side of the equation is a universal constant and has a value of 0.0821. This single equation can predict the behavior of a gas even if multiple conditions are changed simultaneously. If both the pressure and volume of a gas double, for example, its temperature will increase by a factor of four.
related topics: