rate of reaction is always positive whereas rate of appearanace and disapppearance can be positive and negative respectively

Dear Student,

In chemical reaction the reactant forms product by conversion rate, where the reactant concentration decreases on increasing the concentration of the product. Hence the rate of disappearance been negative and appearance of of product been positive and the mathematical expression is as below,

$$\begin{gathered} A\to B \\ Rate = \frac{∆\left[B\right]}{∆t} \left\{appearance\right\} \\ overall is, \\ Rate =- \frac{∆\left[A\right]}{∆t}=\frac{∆\left[B\right]}{∆t} \end{gathered}$$
Regards.