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A negative sign placed before a reaction rate symbol signifies a decrease in concentration of the reactant with increase of time and a positive sign before the rate symbol signifies that the concentration of the product increases with increase in time. The concentration change may be positive or negative but the rate of reaction is always positive. The minus sign is always written when required but the plus sign is usually not mentioned.

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Expressions for reactants are given a negative sign. This is because the reactant is being used up or decreasing.

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