why must you cut and clean magnesium pieces to be exact the same in a reaction rate and temperature lab.?

Answer 1

The pure Magnesium will react with the water vapor in the air to convert it into Magnesium Oxide on the surface of the strips. So when you remove the MgO, it leaves only the pure Mg for your experiment.

Answer 2

You are trying to control all of the things in the reaction except for the one thing you are testing. That is why the Mg must be as close to exactly alike as you can possibly get. Your variable will be the temperature, and if the Mg is not consistant it will give you two different variables. You would not know which variable - Mg or temp - caused the changes noted in your data.