Asingle replacement reaction is a reaction in which one element replaces a similar element within a compound

As a part of an experiment a student burns propane to produce carbon dioxide and water she remembers that she must follow the law conservation of matter when writing a balanced chemical equation which of these equation adheres to the law of conservation of matter

In which phase(s) do the molecules take the shape of the container?

How are isotopes of the same chemical element alike? how are they different?