What is a main difference between a mixture and a pure substance? a mixture is only a liquid, but a pure substance can be in any state. a mixture looks the same throughout, but a pure substance does not. a mixture can vary in composition, but a pure substance has a set composition. a mixture can be made up of a single compound, but a pure substance cannot.

The correct answer is option 3: A mixture can vary in composition, but apure substance has a set composition.

Explanation:

the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.