The term gas refers to the state of any substance in its vaporous form. Many elements remain in the gaseous states whenever temperature and pressure conditions are considered "normal." These elements become solid or liquid at different temperatures or atmospheric pressures.
Some gases can be pure or mixed, such as the mixture that makes up the Earth's atmosphere. Other gases can be toxic even in their vaporous forms, existing as either a poison or as a force that pushes breathable oxygen up away from the breathing organism.
From a purely hypothetical standpoint, if you could look at a gas under a microscope, you would see a wide variety of molecules, atoms, electrons, and more. These particles wouldn't have a definite shape, and the particles would move in random arrangements, only changing direction when the particles bumped into each other. This theory of a gas is defined by the kinetic-molecular theory, and the random nature of the movements explains diffusion.
1. Toxic Gases
Carbon monoxide is one such toxic gas that harms people and animals by pushing the breathable oxygen up. Known by the formula CO, it has a lower density than oxygen and therefore is more accessible but doesn't supply enough oxygen for breathing.
2. Elemental Gases
Eleven elements-hydrogen, nitrogen, oxygen, fluorine, chlorine, helium, neon, argon, krypton, xenon, and radon-exist as a gas under standard pressure and temperature. Depending on the element, when the temperature or pressure is raised or lowered, then they will shift into another state.
3. Pure Gases
A pure gas has no other gas molecules mixed with it. One example is pure oxygen. While we don't breathe pure oxygen because our atmosphere is made up of a variety of gases, hospital patients with breathing difficulties will breathe tanked pure oxygen to help their lungs transfer that oxygen to the bloodstream more efficiently.
4. Gas in Industry
A number of mixed gases serve industrial purposes in manufacturing fields such as welding, steel production, refrigeration, propellants, and more. Acetylene, butane, propane, and many other gases are even used in household applications as high-temperature heat sources.