Gas is a state of matter that has no fixed shape and no fixed volume. Gases have lower density than other states of matter, such as solids and liquids. There is a great deal of empty space between particles, which have a lot of kinetic energy. The particles move very fast and collide into one another, causing them to diffuse, or spread out, until they are evenly distributed throughout the volume of the container.
When more gas particles enter a container, there is less space for the particles to spread out, and they become compressed. The particles exert more force on the interior volume of the container. This force is called pressure. There are several units used to express pressure. Some of the most common are atmospheres (atm), pounds per square inch (psi), millimeters of mercury (mmHg) and pascals (Pa). The units relate to one another this way: 1 atm = 14.7 psi = 760 mmHg = 101.3 kPa (1,000 pascals).
Besides pressure, denoted in equations as P, gases have other measurable properties: temperature (T), volume (V) and number of particles, which is expressed in a mole number (n or mol). In work involving gas temperature, the Kelvin scale is often used.
Because temperature and pressure vary from place to place, scientists use a standard reference point, called standard temperature and pressure (STP), in calculations and equations. Standard temperature is the freezing point of water — 32 degrees Fahrenheit (0 degrees Celsius, or 273.15 Kelvin). Standard pressure is one atmosphere (atm) — the pressure exerted by the atmosphere on Earth at sea level.
Gas laws
Temperature, pressure, amount and volume of a gas are interdependent, and many scientists have developed laws to describe the relationships among them.
Boyle's law
Boyle's law is named after Robert Boyle, who first stated it in 1662. Boyle's law states that if temperature is held constant, volume and pressure have an inverse relationship; that is, as volume increases, pressure decreases, according to the University of California, Davis' ChemWiki. Increasing the amount of space available will allow the gas particles to spread farther apart, but this reduces the number of particles available to collide with the container, so pressure decreases. Decreasing the volume of the container forces the particles to collide more often, so pressure is increased. A good example of this is when you fill a tire with air. As more air goes in, the gas molecules get packed together, reducing their volume. As long as the temperature stays the same, the pressure increases.