Why is gas collected over water not pure

The number to the right of an element's symbol (ex. c-12) identifies the of an isotope.

Which statement is true about this reaction? 14n+1h 15o it is a practical source of energy on earth. it occurs only outside the solar system. its product is heavier than each of its reactants. it shows the critical mass of an element.

The identities of substances are the same before and after which type of change