Gas

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.

PROPERTIES OF GASES

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).

GAS LAWS

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:

• Boyle's Law of Gas
• Charles's Law of Gas
• Dalton's Law of Gas
• Avogadro's Law of Gas

Temperature

Temperature, in physics, property of systems that determines whether they are in thermal equilibrium (see Thermodynamics). The concept of temperature stems from the idea of measuring relative hotness and coldness and from the observation that the addition of heat to a body leads to an increase in temperature as long as no melting or boiling occurs. In the case of two bodies at different temperatures, heat will flow from the hotter to the colder until their temperatures are identical and thermal equilibrium is reached (see Heat Transfer). Thus, temperatures and heat, although interrelated, refer to different concepts, temperature being a property of a body and heat being an energy flow to or from a body by virtue of a temperature difference. See Energy.

Temperature changes have to be measured in terms of other property changes of a substance. Thus, the conventional mercury thermometer measures the expansion of a mercury column in a glass capillary, the change in length of the column being related to the temperature change. If heat is added to an ideal gas contained in a constant-volume vessel, the pressure increases, and the temperature change can be determined from the pressure change by Gay-Lussac's law (see Gases), provided the temperature is expressed on the absolute scale.

Molecule

Molecule, smallest unit of a substance that shows all the chemical properties of that substance. A molecule is a group of atoms that are bound tightly together by strong chemical bonds called covalent bonds. Every molecule has a definite size. If a molecule is broken up into its atoms or into smaller groups of atoms by chemical processes, these pieces will not behave like the original molecule. A molecule can contain atoms of the same element or atoms of different elements. A substance made up of molecules that include two or more different chemical elements is called a molecular compound. An example of a molecular compound is water. Water is made of molecules that contain two hydrogen atoms and one oxygen atom. See also Atom.

Many substances on Earth are made of molecules. Millions of molecules join together to make up the cells in humans or in any other plant or animal. The food we eat, the air we breathe, the clothes we wear, and the wood, paint, and carpeting that we use in homes are all made of molecules. Millions of different molecules exist in nature or can be made by chemists. The nature of each molecule depends on the atoms that it contains and how they link to each other. For example, the oxygen that animals require is made of molecules that have two oxygen atoms bound together. If one oxygen atom binds to a carbon atom, the molecule is instead the poisonous gas carbon monoxide.

Scientists study molecules and their structures so they can better understand why substances behave the way they do. For example, molecular structure helps explain why water boils at a high temperature. Scientists and manufacturers also use their knowledge of molecules and molecular structures to make substances with desirable properties. Plastics, for instance, are laboratory-made substances that consist of enormous molecules containing thousands of atoms. By manipulating the molecular structure of plastics, chemists have created materials that stretch better, resist fading, or can be used in microwave ovens without melting. Similarly, pharmaceutical chemists use their knowledge of molecular structure to develop new drugs that more effectively ease pain or fight disease. The discovery of the structure of deoxyribonucleic acid (DNA), the molecule that contains the genetic blueprint for living organisms, opened the door to tremendous advances in medicine and industry. Knowledge of the structure of DNA has enabled physicians to understand and treat certain genetic diseases. Moreover, by manipulating DNA structure, scientists have been able to modify—or genetically engineer—organisms, creating, for example, bacteria that produce valuable drugs (see Genetic Engineering).