We all know that water is tasteless. But it happens from time to time: you pick up the bottle of water you didn't finish yesterday，and it tastes strange. Most often，you tend to drop the bottle in the trash bin，believing that the water has gone bad. But is it true?
According to Time ， of course not，taste has little to do with quality of water.
According to Time ， when water is exposed to the air for 12 hours，carbon dioxide interacts with the H2O in the water，and the pH value lowers slightly. As a result，the water has a different taste.
"But it's most likely safe to drink，" Norwegian expert Truls Krogh told Science Nordic. "If the water is covered and of good quality to start with，in principle it can last a thousand years. That's because when water is fresh，it contains little organic matter. As long as water is held in clean glasses or bottles，no pollutants will enter it to harm our health. "
People in countries like the US，the UK and Australia usually drink tap water. According to
Time ， if tap water is drunk within six months，the chlorine(氯气) in the water will be enough to kill any bacteria and keep it safe to drink.
However，there are also some exceptions. If you accidentally put your fingers into water or store water in unclean containers day after day，microorganism(微生物) will enter the water.
With the help of surrounding temperature，and sunlight streaming through windows，these microorganisms multiple quickly. Sooner or later，the water will be in the charge of the unfriendly bacteria. And if you drink the water too often，then you're more likely to be ill.
And what about water in plastic bottles? Heat and plastic are a bad combination， US researcher Kellogg Schwab stresses. When plastic bottles are used at high temperatures， they produce a chemical called BPA. BPA is something that affects hormones(荷尔蒙) and research has tentatively linked it to "several health damage，including heart disease and cancer"，Time reported.
Schwab suggests replacing disposable(一次性的) plastic bottles with the refillable containers made of metal or glass to deal with BPA.
