Based on the data given and a rate constant of 0.031 m? 1? min? 1, calculate the time at which the concentration of reactant a will be 0.175m .

t (min) [a]t(m)
0.00 0.500
20.0 0.382
40.0 0.310
60.0 0.260
80.0 0.2

Which of the following is an indication that a substance has undergone a chemical change? a. no new product has been formed. b. the color of the substance has not changed. c. the original constitute has not changed. d. the molecular structure has changed.