 You've probably all already heard talking to somebody about the pH value. But what is the pH value? The pH value is a number in 0 and 14 that tells you how acidic a solution is. Everything higher than 7 is basic and everything lower than 7 is acidic with 7 being neutral. But to really understand this, we have to go a couple of steps back. First of all, what is water? Water has the formula H2O, meaning each molecule of water is formed by an oxygen atom and two hydrogen atoms. Sometimes, however, you can also find this molecule here, one oxygen and three hydrogen. This one is called H3O and the name of it is hydronium. Then we have another molecule that looks very similar, which is this one here. One oxygen and one hydrogen, so HO, which is called hydroxide. Now, what is an acid? If you dump an acid into water, then the amount of hydronium molecules inside the water will increase. Usually what is happening is that the acid brings some additional hydrogen in there. What is a base? A base is a substance that increases the hydroxide count if you dump it into water. The concentration of how much hydronium or how much hydroxide that there is inside a solution is usually measured by molarity, or can be measured by molarity, which is the number of solute. If we want to calculate the hydronium molarity, which is going to be the important one for the pH, that would mean the number of H3O molecules counted in mol. I'm going to get to that in a second, divided by the volume in liter. Molarity M is the number of moles of a substance per meter. One mole is equal to 6.0 to 2 times 10 to the 23 units. Now we use mol, because there is a lot of hydronium in a liter of water. If you would have to write numbers here with 10 to the 23, that's a number with 23 zeros. That would be an extremely big number. So usually we're using this unit mol, 1 mol is 6.0 to 2 times 10 to the 23 moles per unit. Now back to our pH. What is pH? pH is related to the molarity in the sense that the pH value, the number between zero and 14, is calculated as minus the logarithm to the base of 10 of the H3O molarity, or the H plus molarity. Some books call it the H plus molarity, some books call it the hydronium molarity. For example, if I had a concentration, remember concentration of molarity of 1 times 10 to minus 2, that would give me a pH of 2. You can type this in the calculator, minus log of H3O plus molarity, you get 2. If I have 1 times 10 to minus 5 moles of hydronium in my solution, then the 5 will become a pH, and if I have 1 times 10 to minus 12, then I will get a pH of 12, and so on and so on. Sometimes you also want to calculate the other way around. The other way around is even simpler, you can get your H3O molarity by simply taking 10 to minus the pH value, so this is the other calculation. So if you have a pH of 12, 10 to minus 12 is your molarity, if you have a pH of 5, 10 to minus 5 is your molarity, and if you have a pH of 2, 10 to minus 2 is your molarity. So that's a short introduction to pH. The whole chapter with acids and basics is actually quite big, so this is just a really quick simplified overview.