Pure water self-ionizes into equal tiny amounts of H+ and OH- ions giving pH 7, and the product of these concentrations always equals 1.0 times 10 to the negative 14 at 25 degrees Celsius. When you add acid the H+ concentration shoots up and pH drops below 7, while adding base increases OH- concentration and pH rises above 7. This is the real knowscape from knowhere, not a picture of one. Drag it. Watch what actually changes.
Acid and base aren't labels a substance carries — they're roles it plays in one exchange, defined by which way the proton moves. Every question here is proton bookkeeping: track the H⁺ from donor to acceptor, and pH just measures where it landed. Even water plays both roles depending on its partner.
Water molecules naturally split into H+ and OH- ions at equilibrium where their concentrations multiply to give Kw equals 1.0 times 10^-14, making neutral pH equal to 7.
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