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The phenomena you have written can be explained with the help of an example:
The reaction of sucrose and water (Catalyst used: Hydrogen Ion)

The rate of reaction between the reactant is measured by the angle of rotation of polarized light passing through the solution. 

Sucrose is dextrorotatory, but the resulting mixture of glucose and fructose is slightly levorotatory. As the sucrose is used up and the glucose-fructose mixture is formed, the angle of rotation to the righ
becomes less and less, and finally the light is rotated to the left. 
This is because of the touchin of the substance with the liquid.