leveling effect

Pour HCl, HBr, HClO4, and HNO3 into water and try to rank them by strength, and you hit a wall: they all seem equally strong. Every one of them ionizes essentially completely. The water is hiding the differences. This is the leveling effect — a solvent can only let you distinguish acids that are weaker than the solvent's own conjugate acid, and bases weaker than its own conjugate base.

Here is why. In water, the strongest acid that can actually exist is H3O+ (the conjugate acid of water). Any acid stronger than H3O+ — HCl, H2SO4, HClO4 — simply hands its proton straight to water and becomes H3O+. So in water, all super-strong acids are flattened down, or leveled, to the strength of H3O+; you cannot tell them apart. Likewise the strongest base water can hold is OH-: any base stronger than OH- (such as the oxide ion O2-, or H-) just rips a proton off water to make OH-. To see the real differences between very strong acids, you must move to a less basic solvent that does not so readily accept their proton. In pure (glacial) acetic acid, which is far less willing to take protons than water, HClO4 turns out to be a markedly stronger acid than HCl — a ranking water could never reveal.

Leveling explains why we use specific solvents to study extreme acids and bases. Anhydrous sulfuric acid or fluorosulfonic acid (poor proton acceptors) let superacids stand out; liquid ammonia, an excellent proton acceptor, levels acids even harder than water does but lets very strong bases be compared. The practical takeaway: an acidity or basicity ranking is meaningful only relative to the solvent you measure it in. The window of distinguishable strengths is set by the solvent's own two conjugate species.