What is a reduction reaction?

Reduction involves a half-reaction in which a chemical species decreases its oxidation number, usually by gaining electrons. The other half of the reaction involves oxidation, in which electrons are lost. Together, reduction and oxidation form redox reactions (reduction-oxidation = redox).

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indicate valleys, hills, steepness and gentleness of slopes.

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contour lines on a map indicate elevation, or depth. numeric notations on contour lines refer to the level of elevation, or height about sea level.