Now, what exactly does the second law of thermodynamics tell us? Well, what it tells us is every time a real reaction takes place, be it physical or chemical, the change in entropy of the universe is always positive. So the entropy of the Universe increases in any real reaction. So the second law of thermodynamic states that the entropy of the universe always increases. Now, that doesn't mean that the change in entropy of a system cannot be negative. So the entropy of a system can still decrease as long as the entropy of the surroundings increases by greater amount, so that this is always true.