Chlorine and chloramine
Most municipal water carries a disinfectant to keep it safe between the treatment plant and your tap, and it is a common reason tap water tastes wrong in a cup of tea. There are two chemicals a utility is likely to use, chlorine and chloramine, and they behave very differently once a kettle is involved. Knowing which one is in your water decides whether a jug filter and a few hours of standing will fix the problem or do almost nothing.
What it is
Chlorine is the disinfectant most people picture: volatile, and it does not stay in water long. Chloramine is chlorine bonded to ammonia, and utilities choose it deliberately, because that bond makes it far more stable across a whole distribution network. The same stability that keeps chloramine working over miles of pipe is exactly what makes it stick around in a glass or a kettle once it reaches you. The swimming pool smell people know is chloramine rather than chlorine, which is why it is the harder of the two to get rid of.
What it does to your tea
Either disinfectant can carry straight into the cup, because tea's own aroma is faint enough to be overpowered by almost anything competing with it. Chloramine is the more persistent about it: standing or boiling that would clear chlorine barely touches it, so a kettle of it keeps tasting the same way, brew after brew. Both mask the tea rather than changing its chemistry, which is why removing the disinfectant before brewing, rather than trying to brew around it, is the fix that works.
What you can do about it
Chlorine dissipates on its own: leave water standing uncovered for a few hours or boil it, and most of it is gone. Ordinary activated carbon, the kind in a basic filter jug, also removes it easily. Chloramine does not behave the same way. Standing and boiling barely touch it, and it needs catalytic activated carbon, a filter media manufactured specifically to break the chlorine-ammonia bond rather than just adsorb it. Not every filter jug uses catalytic carbon, so check the cartridge specification rather than assuming any carbon filter will do the job, and replace cartridges on schedule since catalytic capacity runs out like any other filter media.
What people get wrong
The advice to let your water stand overnight or just boil it comes from a chlorine-only world, and a lot of tea writing still repeats it unchanged. Many utilities have since switched to chloramine specifically because it is more persistent in the pipe network, which is exactly the property that makes the old advice stop working: standing and boiling do little to it. If a fix that used to clear up your tap water has quietly stopped working, a utility switch to chloramine is a likely reason, and checking your water quality report before buying a new filter is faster than guessing.
Sources
- chloramine is deliberately more stable across a distribution network than chlorine Established municipal water-treatment practice; see the design research note, section 5
- chloramine needs catalytic activated carbon, ordinary activated carbon is not enough Established water-treatment practice, consistent across filter-media specifications and the patent literature; see the design research note, section 5