Water but Do Not Drink It!
Why does tap water taste different after it has been left to sit out overnight in a glass?
Chlorine in treated water is able to control small amounts of microorganisms but at room temperature they multiply out of control. Once you have had a sip of the water more germs are introduced. This combined with ambient dust particles changes the taste of the water.
However it is carbon dioxide, which changes the waters taste over time. Carbon dioxide mixes with the water molecules so that after approximately 12 hours tap water starts to go flat. This lowers its Ph. level giving it an off taste. Although it tastes off this water is still ok to drink.
If the glass is dirty however then bacteria will multiply at a faster rate. This also occurs if the glass is shared with someone else.
A plastic water bottle however should be left well and truly alone especially if left out in the sun. This is due to BPA leaking into the water from the plastic as it heats up in the sun. BPA is a hormone disruptor, which has been linked to a number of illnesses and medical conditions. The plastic bottles that water is purchased in at your local shop or petrol station has only been designed for a one off use.
As a side note bottled tap water has a shelf life of only 6 months. So be careful when purchasing your water and also check bottled dates. This is because the chlorine dissipates out of the water over that time frame.
So the moral of the story use glasses to drink your water and if it is in a plastic container only use the container once and then dispose of it.