Friction. It transforms other forms of energy into thermal energy
A system that releases heat to the surroundings, an exothermic reaction, has a negative ΔH by convention, because the enthalpy of the products is lower than the enthalpy of the reactants of the system. The enthalpies of these reactions are less than zero, and are therefore exothermic reactions
It is two or more objects and different things that can be removed from each other.
Answer:
A pure substance has a definite chemical composition
