Chemistry - Any idea why tap water goes stale overnight?

Solution 1:

Exclude reasons one by one

Surface biological contamination: use two vessels, wipe the one of them with H2O2, hydrogen peroxide solution, leave another untreated. Check if time till smell appears is different by a few days. Bacterias can survive 20% vinegar and 100*C for a short time, some of them, if they are in a sleeping stage, it is not quite reliable method i would say.

Tap water biological contamination: use two vessels with water, add a few tablespoons H2O2 with water in one, and just water in other. Check if time till smell appears is different by a few days.

Tap water chemical contamination: buy some bottled water, use two vessels, one with tap water, another with bottled water. Check if it will be different in a few days.

Bulk material of the vessel releasing chemicals: leave water in different material vessels, one is glass, another one is your preferred one. Check if glass vessel has less smell adter a few days. Glass releases the least amount of chemicals into the water.

Dust accumulation: use two vessels, cover one and leave another one open, check if smell will appear faster in one.

Psychological factor: use two identical vessels, ask your friend to give you one or another, randomly, blindfold yourself, do a test 10 times, check if your score is above 8 correct guesses out of 10 tries.

After completing this tests you will narrow down your search significantly to give you some idea about what is going on.

Solution 2:

A few more thoughts:

• fresh water taste is AFAIK associated with come $\ce{CO_2}$ being dissolved in the water. This can get lost, and make the water taste stale.
  Here's an easy experiment to check this: compare the taste of:

  • fresh tap water,
  • water gone stale "your way" (cooled to the temp of the fresh tap water), and
  • de-gassed fresh tap water: take water, boil it to remove dissolved $\ce{CO_2}$, cool to same temp as the other 2.

  If the boiled water tastes stale like the stale water, loss of $\ce{CO_2}$ is likely the cause.

  I wouldn't describe this as anything like paint-thinner or plastic or the like. But for me it is the textbook meaning of "stale" for water.

• Sidethought about "chlorine" smell. What we usually describe as chlorine smell in water are chloramines (mono-, di-, tri-). In first approximation, we can say that they form from e.g. from chlorine used for water treatment with amino groups from organic matter. AFAIK, our nose is more sensitive to them than to chlorine itself. I.e. the smell indicates that some organic contamination occured, but also that chlorine was there and did work - but it does not indicate whether the water is now safe from a microbiological point of view, the amount of chlorine may not have been sufficient.
  So if your tap water arrives fine and with some residual chlorine, but you then get organic matter into the water, chloramine smell may develop later on.