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activation energy (in chemistry) The minimum energy needed for a particular chemical reaction to take place.

atomThe basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and neutrally charged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

bond(in chemistry) A semi-permanent attachment between atoms or groups of atoms in a molecule. Its formed by an attractive force between the participating atoms. Once bonded, the atoms will work as a unit. To separate the component atoms, energy must be supplied to the molecule as heat or some other type of radiation.

carbonThe chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

catalyst A substance that helps a chemical reaction to proceed faster. Examples include enzymes and elements such as platinum and iridium.

catalytic converter A device made of honeycomb-shaped ceramic structures that is put onto the tailpipe of a vehicle. As exhaust gases flow through it, they encounter two different types of catalysts, each able to foster a different type of chemical reaction. One or more metals, usually platinum, rhodium, palladium and sometimes even gold coat the inside of the system. All of the walls of the devices honeycomb structure increase greatly the area of catalyst-covered surfaces now available to react with the exhaust. As the gases from the engine hit these metal coated surfaces, they break apart the pollutants, turning them into less harmful materials. A sensor in the converter also measures how much oxygen is in the exhaust. If it finds too much, it tells a computer to adjust the air-to-fuel ratio in the engine so that it will burn more cleanly.

chemicalA substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O. Chemical can also be an adjective that describes properties of materials that are the result of various reactions between different compounds.

chemical bondsAttractive forces between atoms that are strong enough to make the linked elements function as a single unit. Some of the attractive forces are weak, some are very strong. All bonds appear to link atoms through a sharing of or an attempt to share electrons.

chemical reactionA process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).

electricityA flow of charge, usually from the movement of negatively charged particles, called electrons.

engineA machine designed to convert energy into useful mechanical motion. Sometimes an engine is called a motor.

enzymesMolecules made by living things to speed up chemical reactions.

exhaust(in engineering) The gases and fine particles emitted often at high speed and/or pressure by combustion (burning) or by the heating of air. Exhaust gases are usually a form of waste.

fuel cellA device that converts chemical energy into electrical energy. The most common fuel is hydrogen, which emits only water vapor as a byproduct.

geneticHaving to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.

hydrogenThe lightest element in the universe. As a gas, it is colorless, odorless and highly flammable. Its an integral part of many fuels, fats and chemicals that make up living tissues.

iridiumDiscovered in 1803, its name comes from the Latin for rainbow. Its a hard, brittle and corrosion-resistant metal in the platinum family. Slightly yellowish, the principle use for this element is as a hardener for platinum. Indeed, its melting point is more than 2,400 Celsius (4,350 Fahrenheit). The elements atomic number is 77.

manufacturingThe making of things, usually on a large scale.

metalSomething that conducts electricity well, tends to be shiny (reflective) and malleable (meaning it can be reshaped with heat and not too much force or pressure).

moleculeAn electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

nutrientA vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.

oxygenA gas that makes up about 21 percent of the atmosphere. All animals and many microorganisms need oxygen to fuel their metabolism.

palladiumA soft, ductile, steel-white, tarnish-resistant, metallic element occurring naturally with platinum, especially in gold, nickel, and copper ores.

petroleumA thick flammable liquid mixture of hydrocarbons. Petroleum is a fossil fuel mainly found beneath the Earths surface. It is the source of the chemicals used to make gasoline, lubricating oils, plastics and many other products.

plasticAny of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.

platinumA naturally occurring silver-white metallic element that remains stable (does not corrode) in air. It is used in jewelry, electronics, chemical processing and some dental crowns.

pollutantA substance that taints something such as the air, water, our bodies or products. Some pollutants are chemicals, such as pesticides. Others may be radiation, including excess heat or light. Even weeds and other invasive species can be considered a type of biological pollution.

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Explainer: What is a catalyst? - Science News for Students (blog)