how is an economics a positive science or normative science?

Economics can be defined in positive as well as normative sense. 

Positive Economics are the factual statements and describe what was, what is and what would be. These statements can be tested, proven or disproven. These statements do not involve any personal value judgment. For example, if one says that it is raining outside, then the truth of this statement can be checked out by going outside.

Normative Economics describe what should be or what ought to be. These statements cannot be tested and verified. Unlike positive statements, normative statements involve personal value judgments. Usually, these statements are debatable in nature. For example, if one say that he/she prefers tea over coffee, then such statement reflects his/her personal ideas, preferences, thereby, cannot be proven or disproven.

Thus, it can be said that while the positive statements are objective statements, the normative statements are subjective statements. For example, Indian Stock Market has boomed in recent years, is an example of positive statements; whereas, Indian government should make the legal formalities for the foreign investors less-stringent is an example of normative statements.