Treatment depends on the source and intended use of water. Fine clay particles often carry negative surface charge and repel one another. Coagulation reduces this repulsion, while flocculation joins small particles into larger flocs. Aluminium-containing coagulants and suitable polymers help this process. Sedimentation and filtration can then remove suspended material; ordinary filtration does not remove every dissolved salt.
Hard water contains appreciable Ca2+ and Mg2+, which can form precipitates with soap ions. Cation-exchange materials such as zeolites replace these ions with other cations, often Na+. Reverse osmosis applies pressure to drive water through a membrane while retaining most salts. Desalination can also use distillation. Both require energy and produce a concentrated waste stream whose management matters.
Example 1
A zeolite exchanges Na+ for 0.0030 mol Ca2+. Find released Na+ moles.
One Ca2+ balances the same negative-site charge as two Na+. Released Na+=2×0.0030=0.0060 mol.
Example 2
Which stage removes sand, and which can remove dissolved NaCl?
Filtration can retain sand particles. Dissolved Na+ and Cl− pass ordinary filters but can be largely separated from water by a suitable reverse-osmosis membrane or distillation.
Watch out:Clarity concerns suspended particles; dissolved ions can remain in visually clear water.
Water clarification, softening and desalination practice | Year 12 Chemistry | Ascent Arcade