So we can say we can determine the sign of the entropy change, which is Δs, in a system by examining physical and chemical changes. Now we're going to say that our entropy change, which is Δs, is a measure of increase or decrease in order due to chemical or physical changes. So here, let's take a look at entropy and physical changes first. We're going to say in this first image, we're transitioning from our solid to liquid to gas. We're heading in a direction where the space between the molecules is increasing. And if you think about it, which is more chaotic? A solid object is sitting there on a table or a container filled with gases that are bouncing everywhere. We'd say that gases have the most entropy. So as we're heading from solids towards the gases, we're gonna say that our entropy is increasing. That means that our Δs value is going to be positive. There's a positive change in entropy. We're going to say that this move from solid to gas is usually accompanied by an increase in our temperature. Think about it. Increasing temperature on a solid would cause it to melt, increasing it further will cause that liquid to vaporize into gases. Now let's think about the opposite direction. If we were to decrease the temperature, molecules would come closer and closer together, our gases would condense down into liquid, and if we lower the temperature even more, they would freeze into a solid. As we head from gas to solid, we expect a decrease in entropy, so a negative change in entropy. Alright. So now that's physical changes.
Let's look at chemical changes. Here we can say that we can have entropy increasing, so positive Δs if we are breaking bonds. Right? Remember, chaos, chaotic, disorder. The more things we can break apart, the more chaotic things become. Breaking bonds is a chaotic action. We can also say, if we look at this chemical reaction, we have calcium carbonate solid, it breaks apart into calcium oxide and carbon dioxide gas. It's an increase in chaos because we are breaking bonds, but it's also an increase in chaos because we just increased the moles of gas. So there's an increase in moles of gas. Remember, gases have the most entropy. Their molecules are further spread apart, bouncing everywhere. Now entropy decreasing, so a negative change in Δs is if we did the opposite, if we're forming bonds or if we decreased the number of moles of gas. Here we have 1 mole of nitrogen gas plus 3 moles of hydrogen gas for a total of 4 moles of gas. They combine together to give me 2 moles of gas here. So not only am I forming bonds, but I've decreased the number of moles that I have. You don't have to have both happen, but as long as one of these is happening that tells us if there's an increase or decrease in entropy. If you're breaking bonds, it's an increase in entropy. If you're decreasing the number of moles of gas, it's a decrease in entropy. Both don't need to happen for this to be an increase. Alright. So keep in mind the different physical and chemical changes and how they can affect the increasing or decreasing of entropy.