# The pH scale

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I know of the 1 – 10 scale and pure water is a ‘neutral 7’ or something but what decides the number? What’s main difference between a substance at 1 and another at 10 on the scale?

In: 6 It determines how much acid/how acidic (or not) a liquid is.

This is determined mostly of how many H+ ions and OH- ion molecules are present in the liquid. When a liquid is at neutral 7, it has about the same amount of each ion The number of hydrogen ions in the solution. IIRC pH stands for power of hydrogen or something. As you add more, the pH decreases. Every time you go down 1 on the scale you need vastly more H+ ions to get to the next step. If you plotted the pH number on a scale of 1 to 14 it would be at a maximum at one, then fall quickly at first as the pH increases until the fall becomes less at higher numbers. Others will step in with more, but that should get you started. The pH scale goes from 0-14, so 7 is right in the middle being neutral makes sense. It describes the concentration of H+ and OH- ions logarithmically, 7 means the concentration of H+ is 10^-7 mol/L , 6=10^-6, 5=10^-5, and so on – for OH- ion concentration it is the reverse 7 is the same but the concentration changes in the opposite direction pH=8 means it is 10^-6, 9=10^-5 and so on. So the lower the pH the more acidic it is / the higher the pH the more basic it is. As you may know, water is H2O, two hydrogens, one oxygen. Water does something called autoprotolysis, where the H2O molecules will sometimes randomly split into H^+ and OH^- ions, and conversely these ions will sometimes randomly rejoin into H2O molecules. These reactions balance out such that the concentration of ions is around 10^-^7 mol/L. Thus water has a pH of 7 (pH ≈ – log base 10 of this concentration). When you add acids to water, they give off H^+ ions, so they will increase the H^+ concentration and thus decrease the pH, for example a 10^-^3 mol/L concentration of H^+ means a pH of 3. Conversely, basic chemicals can capture H^+ ions, decreasing their concentration and increasing the pH, so when you have a concentration of 10^-^1^0 mol/L of H^+ the pH would be 10.

So to conclude, pH = 7 = neutral = normal water, pH < 7 = acidic solution, pH > 7 = basic solution