What is water from the tap?

[revlaz]

If water is = H20 (2xhydrogen+1oxygen).

What does water become when calcium & fluorine (or fluoride) are added?

[Azalian]

It stays as water with additional dissolved salts.

The "calcium" may be present as calcium hydroxide or carbonate or chalk if you prefer.
There are also magnesium, sodium, potassium and other compounds in tap water

[Teddio]

Water from the tap is mainly H2O with some dissolved oxygen and nitrogen. Dissolved minerals are in the form of positive metal ions such as: sodium(Na+), potassium(K+), calcium(Ca++) and magnesium(Mg++) and negative ions such as chloride(Cl-), fluoride(F-), oxychloride(OCl-), sulphate(SO4- -), carbonate(CO3- -) and hydrogencarbonate(HCO3- )
There is also present the naturally occurring isotope of hydrogen called deuterium giving D2O and HDO.

[newtron]

The two posts above mention dissolved minerals. By that they mean that there are contituents (that are not water molecules) that are fairly evenly distributed among the water molecules. In simple terms, these dissolved constituents do not react with the water molecules. As Teddio mentioned, many of the dissolved minerals are in the form of ions. In tap water, there are also small amount of dissolved contituents that are not present as ions, such as dissolved oxygen, nitrogen, and carbon dioxide , which comes from the atmosphere.

As for deuterium, it is not considered a dissolved constituent, as it is part of the water molecule.

[revlaz]

thanks for your responses, very much appreciated.