Environmental and Atmospheric Chemistry

Raw water carries suspended solids, dissolved minerals and pathogens, and treatment removes them in a fixed order: screening, coagulation/flocculation, sedimentation, filtration, disinfection, then pH adjustment or fluoridation. Screening removes large debris with a mesh grille before anything else happens. Large-scale distillation is too energy-expensive for everyday supply, which is why it's reserved for desalination in water-scarce regions rather than standard treatment.

A coagulant such as $A{l}_2(S{O}_4{)}_3$ or $FeC{l}_3$ is added to neutralise the charge on tiny suspended colloidal particles, so they stop repelling each other and clump into larger 'floc'. This floc is heavy enough to settle out during sedimentation, reducing turbidity before filtration even begins. Naming the process without stating the charge-neutralisation mechanism will only earn partial credit on 'explain' questions.

Sewage treatment mirrors drinking water treatment but in reverse purpose: primary treatment uses physical screening and sedimentation, secondary treatment uses aerobic bacteria to break down organic matter and lower BOD, and tertiary treatment chemically removes nitrates and phosphates before the water is released. Skipping tertiary treatment lets excess nutrients trigger eutrophication — algal blooms that die, decompose, and deplete dissolved oxygen, killing aquatic life.

Dry air is overwhelmingly $N_2$ (~78%) and $O_2$ (~21%), with $Ar$ (~0.93%) and $C{O}_2$ (~0.04%) as trace gases. Composition stays nearly constant up to 80–100 km because production and removal processes balance — nitrogen fixation removes $N_2$ while denitrifying bacteria return it, photosynthesis and respiration balance $O_2$, and photosynthesis plus ocean dissolution keep $C{O}_2$ low. Argon shows essentially zero variation with altitude because it has no biological source or sink at all.

Noble gases like He, Ne and Ar are unreactive because their outer electron shell is already full, leaving no energetic driving force to gain, lose or share electrons — though helium is the exception with only 2 outer electrons, not 8. $N_2$ is far less reactive than $O_2$ at room temperature for a different reason: its triple bond ($N\equiv N$, 945 kJ mol⁻¹) is nearly twice as strong as $O_2$'s double bond ($O=O$, 498 kJ mol⁻¹), so breaking it needs activation energy that isn't available under normal conditions.