What rules should you follow when adding and subtracting ?

Adding and subtracting with scientific notation may require more care, because the rule for adding and subtracting exponential expressions is that the expressions must havelike terms. Remember that to be like terms, two expressions must have exactly the same base numbers to exactly the same powers. Thinking about decimal arithmetic, the requirement that we have the same powers makes sense, because that guarantees that all of the place values are lined up properly.

Negative - Negative = Negative.
Positive - Negative = Positive
Negative - Positive = Negative
Negative + Negative = Negative
Positive + Positive = Positive

Cause its being chemically applied?