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Acids. Any molecule that can give up (donate) a proton is called an acid. So let’s see how molecules can donate protons. Let’s start with a molecule that looks very much like Sodium Chloride (NaCl). The molecule is Hydrogen Chloride (HCl). Instead of a Sodium atom attached to the Chlorine, there is a Hydrogen atom attached to the Chlorine. So when Hydrogen reacts with Chlorine it makes Hydrogen Chloride. If the Hydrogen Chloride is put in water, the Hydrogen will separate from the Chlorine just like the Sodium separated from the Chlorine in table salt. If we look at the electronegativity of Hydrogen (2.2) and compare it to the electronegativity of Chlorine (3.2), we can tell that Chlorine is stronger than Hydrogen. So Chlorine will win the tug of war and it will keep the electron. The Hydrogen will no longer have its electron. And a proton is a Hydrogen atom without its electron. The proton is now floating in the water all by itself. It was donated to the water by the Hydrogen Chloride. Since the Hydrogen Chloride was able to donate a proton, it is called an acid. Adding HCl to water will turn the pH to acidic.
