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Element · Cl

Chlorine

A green, choking gas that made drinking water safe and was the first chemical weapon used at scale.

Chlorine is a dense green gas, acutely toxic, and one of the more consequential industrial chemicals. Added to water supplies in tiny concentrations it kills the pathogens responsible for cholera and typhoid, and its introduction is among the largest single public-health improvements on record.

The same reactivity made it the first chemical weapon deployed in quantity, at Ypres in 1915. It is a clear case of a material whose character is entirely a matter of how much and where.

Uses

Chlorine's largest use is making PVC, and after that the whole family of chlorinated organic chemicals — solvents, refrigerants, and the intermediates most pharmaceutical synthesis passes through at some stage.

Water treatment is the use with the greatest effect on human health per tonne. Chlorination of drinking water broke the transmission of cholera and typhoid in industrial cities, and it remains the cheapest reliable disinfection available.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

is used as

  • Catalysis — application · chlor-alkali production underpins much of industrial chemistry

is extracted from

  • Halite — mineral · by electrolysis of brine, which yields chlorine and sodium hydroxide together Wikidata

is produced by

is an input to

  • Kroll process — process · converts the ore to a distillable tetrachloride, which is how the purification is done
  • Polymerisation — process · as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil

is found in

  • Polyvinyl chloride — material · more than half the polymer's weight, and the reason PVC is the plastic least exposed to the oil price
  • Seawater — material · the most abundant dissolved ion by mass, and rather more than half the dissolved solids
  • Neoprene — material · one per repeat unit, and the source of the oil resistance and the flame retardancy that distinguish it from natural rubber
  • Salt — compound · sixty per cent by mass, and the part the chemical industry is buying
  • Apatite — mineral · substitutes within a solid solution — an individual specimen may hold little of it
  • Halite — mineral Wikidata
  • Sylvite — mineral Wikidata
  • Vanadinite — mineral Wikidata

is a source for

  • Polyvinyl chloride — material · the chlorine arriving from chlor-alkali electrolysis of salt

is associated with

  • The Montreal Protocol — event · the element the mechanism turns on: released in the stratosphere, it catalyses ozone destruction and is not consumed doing it

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
  • PubChem
    US National Library of Medicine · Mixed — aggregated third-party content, rights retained by depositors
  • NIST — National Institute of Standards and Technology
    United States Department of Commerce · US Government work — public information, credit requested

Questions this page answers

Where it comes from, and what it becomes

Follow Chlorine back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (the largest single use of salt, and the process the whole chlorine and caustic soda industry rests on) → Salt → is produced by (in the solar route — the same process, read from the other end) → Brine evaporation → takes as input (the feedstock for solar salt and, in the salars, for the far stronger brines that lithium comes from) → Seawater → is composed of (about 96.5 per cent by mass — seawater is 3.5 per cent dissolved solids and the rest of it is this) → Water
  • Chlorine → is extracted from (by electrolysis of brine, which yields chlorine and sodium hydroxide together) → Halite
  • Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as the brine the cell electrolyses, and as the source of the hydrogen that comes off the cathode) → Water → is sourced from (by desalination, where energy is cheap and coastline available — the only route that adds fresh water rather than moving it) → Seawater
  • Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (as brine; the salt is the feedstock for both products at once) → Halite
  • Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (the largest single use of salt, and the process the whole chlorine and caustic soda industry rests on) → Salt → is sourced from (by solar evaporation, which needs a dry sunny coast and is the cheapest route there is) → Seawater → is composed of (about 96.5 per cent by mass — seawater is 3.5 per cent dissolved solids and the rest of it is this) → Water
  • Chlorine → is produced by (at the anode) → Chlor-alkali electrolysis → takes as input (the largest single use of salt, and the process the whole chlorine and caustic soda industry rests on) → Salt → is sourced from (mined as rock salt from bedded deposits, or dissolved in place and pumped up as brine — which is what the chemical industry wants anyway) → Evaporite → is composed of (precipitated early, when the water has reduced to roughly a fifth of its volume) → Gypsum

These are the most distinct paths back. Chlorine can be traced through others besides.

Downstream — what it becomes

  • Chlorine → is an input to (converts the ore to a distillable tetrachloride, which is how the purification is done) → Kroll process → produces (as sponge, which must then be crushed, melted and cast before it is usable metal) → Titanium → is a component of (the other half of the strengthening phase) → Nickel superalloy → is associated with (the jet engine created the industry, because no existing material survived the turbine inlet) → The wartime materials programmes complete chain
  • Chlorine → is an input to (as vinyl chloride, which is why more than half of PVC's weight is salt rather than oil) → Polymerisation → produces (from ethylene; chain length and branching decide whether it is a milk bottle or a fibre) → Polyethylene → is associated with (its first significant use was radar cable insulation, and its existence was classified) → The wartime materials programmes complete chain
  • Chlorine → is associated with (the element the mechanism turns on: released in the stratosphere, it catalyses ozone destruction and is not consumed doing it) → The Montreal Protocol complete chain
  • Chlorine → is a source for (the chlorine arriving from chlor-alkali electrolysis of salt) → Polyvinyl chloride → is used in (pipe, window frames, cladding and cable, which is where most of it goes) → Construction
  • Chlorine → is used as (chlor-alkali production underpins much of industrial chemistry) → Catalysis
  • Chlorine → is a source for (the chlorine arriving from chlor-alkali electrolysis of salt) → Polyvinyl chloride → is an input to (pipe and window profile, which is most of what PVC is by tonnage and nothing to do with the packaging it is blamed for) → Extrusion

These are the most distinct paths onward. Chlorine ends up in others besides.