Which is the best example and explanation that a physical change has occurred?

Define dew point. i am writing this part to be able to ask the question

Achemistry student needs to standardize a fresh solution of sodium hydroxide. he carefully weighs out of oxalic acid , a diprotic acid that can be purchased inexpensively in high purity, and dissolves it in of distilled water. the student then titrates the oxalic acid solution with his sodium hydroxide solution. when the titration reaches the equivalence point, the student finds he has used of sodium hydroxide solution.calculate the molarity of the student's sodium hydroxide solution. be sure your answer has the correct number of significant digits.

Why is it possible for different microorganisms to extract energy not only from carbohydrates and other biological molecules but from a large variety of substances?