Scientific theories are always (a. testable b. true c. unreliable). a theory may be changed as a result of (a. competing theories b. new observations c. untested hypotheses) complete the sentences with those options.

1. A. testable

2.new observations

Explanation:

A scientific theory is the one which explains the cause of a natural phenomena through observatory and experimentation approach. It is valid, accurate and acceptable worldwide.

1. It is testable by implementation of suitable methodology for the purpose of proving the cause of the natural phenomena occurring in nature.

2. A the explanation of the theory can be changes on the basis of the new observations obtained after implementation of new technology and methodology.

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.