If we leave a newspaper in the heat and another in the dark room. What chemical reactions will take placebetwen the two newspapers on exposure to light which results in change of color.
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photochemical reaction, a chemical reaction initiated by the absorption of energy in the form of light. The consequence of molecules’ absorbing light is the creation of transient excited states whose chemical and physical properties differ greatly from the original molecules. These new chemical species can fall apart, change to new structures, combine with each other or other molecules, or transfer electrons, hydrogen atoms, protons, or their electronic excitation energy to other molecules. Excited states are stronger acids and stronger reductants than the original ground states.
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Chain of fluorescent tunicates.
photochemical reaction
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Related Topics: photosynthesis photochemical equivalence law photosensitization photolysis quantum yield
It is this last property that is crucial in the most important of all photochemical processes, photosynthesis, upon which almost all life on Earth depends. Through photosynthesis, plants convert the energy of sunlight into stored chemical energy by forming carbohydrates from atmospheric carbon dioxide and water and releasing molecular oxygen as a byproduct. Both carbohydrates and oxygen are needed to sustain animal life. Many other processes in nature are photochemical. The ability to see the world starts with a photochemical reaction in the eye, in which retinal, a molecule in the photoreceptor cell rhodopsin, isomerizes (or changes shape) about a double bond after absorbing light. Vitamin D, essential for normal bone and teeth development and kidney function, is formed in the skin of animals after exposure of the chemical 7-dehydrocholesterol to sunlight. Ozone protects Earth’s surface from intense, deep ultraviolet (UV) irradiation, which is damaging to DNA and is formed in the stratosphere by a photochemical dissociation (separation) of molecular oxygen (O2) into individual oxygen atoms, followed by subsequent reaction of those oxygen atoms with molecular oxygen to produce ozone (O3). UV radiation that does get through the ozone layer photochemically damages DNA, which in turn introduces mutations on its replication that can lead to skin cancer.
ozone depletion
ozone depletion
Antarctic ozone hole, September 17, 2001.
NASA/Goddard Space Flight Center
Double exposure of science laboratory test tubes with bokeh and chemical reaction
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Types of Chemical Reactions
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Photochemical reactions and the properties of excited states are also critical in many commercial processes and devices. Photography and xerography are both based upon photochemical processes, while the manufacture of semiconductor chips or the preparation of masks for printing newspapers relies on UV light to destroy molecules in selected regions of polymer masks