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?
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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
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