Problem 1: for which of the following activities might you want to hire a chemist?
a. building a garage
b. testing a sample of water from a well
c. finding a comet
d. examining the structure of plant cells
problem 2: which of the following procedures involves a physical change in one of the substances?
a. creating salt by reacting sodium metal and chlorine gas
b. allowing a nail to rust
c. building a model rocket propelled by mixing baking soda and vinegar
d. grinding chalk into a fine powder

Problem 1) From the given options the one that is proper of a chemist is the option B. testing a sample of water form a well. This activity belongs to the field called analytical chemistry which uses special techniques (instruments and processes) to study (separate, identify and quantify matter).
Problem 2) The only of the given procedures that involves a physical change in one substance is the option D. grinding chalk into a fine powder. In this process the molecules remain being the same (same atoms bonded in the number and way).