Would a newspaper bleaching in the sun be a physical or chemical change?
Bleaching is usually an oxidation reaction, whether it is initiated by chemical bleach or photonically by ultraviolet rays in sunlight. The result is that the formerly colored ink or pigment loses its intensity and may eventually become almost white. The chemicals, which were originally colored, have been transformed into new chemicals. This is both a chemical and a physical change.
To be entirely general, all chemical changes or reactions are considered physical since all chemicals and their reaction products are physical. The converse of this is generally not true: most physical changes like bending a wire or lifting a block do not directly involve chemical changes. The wire and the block are chemically the same after the physical change.
An example of a chemical change that is obviously physical is the explosion of a mixture of gasoline and oxygen in air that propels most cars. The chemical reaction releases heat that causes the gasses to expand inside the cylinder and push the piston, ultimately rotating the crankshaft and finally the tires. The initial mixture of gasoline (a hydrocarbon) and oxygen are converted by the chemical reaction into water and carbon dioxide plus a number of undesirable pollutants including carbon monoxide, oxides of nitrogen and unburned hydrocarbons.
Scott Wilber, President, ComScire - Quantum World Corporation
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