Each of four test tubes contains a different concentration of hcl(aq) at 25°c. a 1-gram cube of zn is added to each test tube. in which test tube is the reaction occurring at the fastest rate?

From the experiment details; there are 4 test tubes containing different concentrations of aqueous HCl, and all test tubes at the same temperature (25°C), also to all test tubes 1 g of Zn cube is added at the same time.
- The reaction occurs with the fastest rate in  the test tube that contains highest concentration of HCl; due to the higher concentration of HCl will increases the velocity of the reaction   

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