A chemical change involves the transformation of substances into new substances with distinct chemical properties.
Let's analyze each option:
a. C₃H₈ + 5O₂ -> 3CO₂ + 3H₂O: This is a combustion reaction where propane (C₃H₈) reacts with oxygen (O₂) to form carbon dioxide (CO₂) and water (H₂O). This is a chemical change because new substances are formed.
b. NH₃(g) -> NH₃(liquid): This is a phase change (gas to liquid) of ammonia. It does not involve a change in the chemical structure of the substance, so it is not a chemical change.
c. H₂O(s) -> H₂O(g): This represents the melting (solid water turning into vapor) and is also a phase change (solid to gas). Again, this does not involve a change in chemical structure, so it is not a chemical change.
d. C₂H₅OH(liquid) -> C₂H₅OH(g): This represents a phase change from liquid ethanol to gaseous ethanol, which does not change the chemical structure of ethanol and therefore is not a chemical change.
Based on this analysis, the correct option that shows a chemical change is:
a. C₃H₈ + 5O₂ -> 3CO₂ + 3H₂O.