Dilution
Definition
Adding solvent to make a solution less concentrated: the moles of solute stay the same, only the volume grows.
Technical definition
Dilution is the reduction of concentration by adding solvent, governed by M1V1 = M2V2 (moles before equal moles after). It is the standard method for preparing working solutions from concentrated stocks.
Example
To make 500 mL of 0.10 M HCl from 1.0 M stock: (1.0)(V1) = (0.10)(0.500), so measure 50 mL of stock and dilute to 500 mL.
Common confusion
Add the stock TO water and dilute TO the final volume. Pouring 50 mL of stock into 450 mL of water is only approximate; proper technique dilutes to exactly 500 mL in a volumetric flask.