What occurs when energy is added or removed from an object or a system by an outside agent? plz help

Describe in detail the melting point behavior of the 80: 20 benzoic acid-mandelic acid mixture

How are the terms group and period used on the periodic table

Which best describes the activation energy of a chemical reaction? a. the combined energy of all the reactants b. the amount of energy required for a reaction to occur c. the difference in energy between products and reactants d. the potential energy stored in the bonds of reactants and products