1 Answers

Vasanth SR
askIITians Faculty 1330 Points
4 years ago
Big question! I'll give a go at an answer that is a little more descriptive than some of the others. H2O as a molecule is very asymmetrical. It has oxygen with two hydrogens sticking off from the oxygen on the same side to form a "V" shape. The O loves electrons, and tends to concentrate the electron cloud on its end of the molecule. The H's, being deprived of electrons will snuggle up to the O of another water molecule, and this "hydrogen bonding" is how water molecules interact with each other and most other things that dissolve in water.

Now, since the O's concentrate the electron cloud and become relatively negative, and the poor H's are now relatively positive, it is pretty easy for a water molecule to steal an H from another water molecule, resulting in the formation of an H3O+ (hydronium, also often just called H+ for simplicity) and an OH- (hydroxide), and these two things (surrounded by water molecules) exist in pure water. They have measurable concentrations of about 1e-7 molar for each of them. In pure water they have equal concentrations since one comes from the other, but this changes as we add things that are acidic or basic to the water.

Okay, put something in the water that has a weakly bound hydrogen atom, like hydrogen chloride gas. HCl interacts with the water the way I described water interacting with water, but stronger. The chlorine atom REALLY wants electrons and immediately allows a water molecule to take the hydrogen to form H3O+, leaving a Cl- (chloride). Notice that what we have done is dump an H+ into the water, but no hydroxides. That is what acids do, and that behavior is one definition for an acid. This unbalances the concentrations of H+ and OH- in favor of the H+ and the solution becomes acidic.

Put something in the water that wants to strongly bind one of the H+'s, like something that already has hydroxide in it, and the hydroxide will react with the H+ already in the water and reduce its concentration, leaving more hydroxide than hydronium. This is the opposite of what happens in an acid, so the H+ concentration goes down and you get what is defined as a base.

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