Now we're going to say fundamentally when looking at any balanced equation. Here we have a simple one: A is changing into B. A could be any type of compound, and B could be any type of compound, but fundamentally, this is a balanced chemical equation. When looking at any chemical equation, and if we take a look at the time elapsed, when we start out at 0 seconds, right? Initially, all we have are the white balls. Those are A. So in the beginning, before a reaction is even allowed to start, we have only reactants. And take a look, over time, what begins to happen. Over time, you're going to get more and more of compound B, our product, forming. Now, the more our product forms, the less reactants we're going to have around because remember, the products are made from our reactants breaking down. Eventually, we'll say that our reaction reaches completion. We're going to say when our reaction reaches completion, basically almost all of the reactants are gone, and all we have are products. For reactions that go to completion, we have one single arrow going forward.

Later on, we're going to learn about reactions that don't reach completion but actually reach equilibrium. At those states, we're not going to have a single arrow going forward. We're going to have two arrows: one going forward and one going backwards. When we have double arrows, we're at equilibrium. When we get to states of equilibrium, we don't get rid of our reactants. Eventually, we're going to have some reactants and products, basically living in harmony with each other, living in equilibrium. For now, we're just going to be worried about reactions going to completion. But eventually, we're going to get to the point where we should realize that sometimes chemical reactions go to equilibrium.