A pure substance that is present before a reaction but not after
Super Confused

Explanation:

Pure subsance is a substance that is made up of only one type of particle - each piece is the same throughout.

Being present before the reaction but not after means it's no the same (it couldve evaporated)

definition

                    entropy is a measure of the randomness or the extent of disorder of a chemical process.

                      enthalpy is a measure of the heat change of a reaction occurring at a constant pressure.

measurement units:

                        entropy is measured in joule per kelvin (jk⁻¹)   whereas enthalpy is measured in joule per mole (jmol⁻¹)

requirements:

                            entropy has no requirements or limits, and its change is measured by the division between the heat change of the chemical process and the temperature.

                              in contrast, enthalpy can only be related to the heat change of the reaction under standard conditions.