dilution

Meaning: Dilution is lowering a solution's concentration by adding more solvent (usually water). The amount of solute, in moles, does not change. Only the volume increases, so concentration (molarity) decreases. Doubling the volume halves the molarity; a tenfold volume increase gives a tenfold lower molarity.

Equation: M₁V₁ = M₂V₂, where M₁ and V₁ are the initial molarity (mol/L) and volume (L, or m³ in strict SI), and M₂ and V₂ are the final molarity and volume. Any volume unit works if both sides use the same one. V₂ is the total final volume, not the volume of solvent added.

On the MCAT: Expect questions asking what volume of water to add to reach a target concentration, or how many mL of stock solution are needed. Dilution also appears in acid-base problems: diluting a strong acid by 10 lowers [H⁺] tenfold and raises pH by 1. Diluting a weak acid raises its percent dissociation (Le Châtelier), but pH rises by less than 1. Dilution also lowers the colligative effect, such as the van't Hoff factor times molarity.

Common confusion: Students add the volumes wrongly or think moles of solute change. Moles of solute stay constant, so n = MV is the same before and after. Also, diluting an acid or base never pushes pH past 7 at MCAT level; it approaches 7 as a limit.