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This may sound like a silly question, but I've always wondered:

If I boil some water and use some of it and leave the remainder in the kettle, and then a few days later boil that same water again, will it taste the same as if I'd emptied the old water and boiled fresh new water?

I've got a habit of emptying the kettle water and starting with fresh water to boil when I prepare my coffee (using a French press), and I'm wondering if there's no good reason to do that.

For what it's worth, the kettle has a top (so I'm ruling out dust as a concern), and the water I'm boiling is tap water that's been through a Brita filter in a jug.

If the taste is different, what's the chemical or physical justification?

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up vote 14 down vote accepted

Yes, it is different. Two things happen: the dissolved oxygen boils out, and whatever mineral solids are in there become concentrated as steam evaporates.

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Not that it's necessarily a bad thing. A little extra minerals in water can significantly affect coffee flavour, but depending on the specific combination, it can be for the better. There is a town near where I live where coffee made from well water on one side of the river tastes amazing. Coffee made from well water on the other side is unpalatable. – Eclipse Aug 26 '10 at 18:48
The effect of losing dissolved oxygen is also well known in tea-making. Thoroughly boiling the water very quickly extracts the dissolved oxygen. For this reason, enthusiasts of green and oolong tea never bring the water to a full, rolling boil in the kettle. Instead, they wait until the water in the kettle makes a distinctive rumbling sound prior to actually boiling. – Angelo Aug 29 '11 at 14:18

If it tastes different, it is either evaporating and you're getting a stronger flavor of the same water you drink.

The other thing is that it may be picking up flavor from the kettle.

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Dissolved air is eliminated when you boil the water. However this is only really an issue immediately after the water is boiled. Waiting for a long enough time will allow the air to reabsorb into the water.

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And this matters why? – SAJ14SAJ Jul 20 '13 at 2:04

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