Is the water from the tap/faucet potable in Istanbul or must I buy bottled water? If not, what are the specific contaminants in it?
I asked my Turkish friends and here's what they had to say: although major cities do have water filtration plants and water is considered drinkable at the source, the quality of water pipes that supply it within a building or in specific localities might be suspect. You are, thus, better off not drinking tap water straight unless you're using it after boiling (or in foods/drinks where water is boiled in the process) or filtering.
I live in Izmir and have travelled in Istanbul many times. Tap water all over Turkey including big cities is a hit-or-miss affair. It's not guaranteed to kill you, but it doesn't generally taste that good and drinking it is considered a bad idea just in case. Most cities try to treat it enough to make it non-dangerous, but that doesn't make it good water.
The local custom is to drink bottled water. Water is always available on the street and for the home 20 litre water jugs are delivered usually within minutes of ordering, for 3-6 Lira, and you just keep those bottles on hand for drinking.
In most places you could cook / wash with tap water, but if your location has particularly bad tasting stuff you may even prefer cooking with bottled water.
Edit: Big city water systems are not monolithic, there is usually not one right answer. In Izmir the water at my house is safe to drink, but at an office I am often at it is so rank we don't even use it for dishes or handwashing.
Istanbul in particular spans two continents. You should ask people in your local district about the water quality and generally do as they do. If your hosts wash the veggies with tap water, it's probably ok. It's always nice to play it one step safter than the locals when traveling, but if they tell you not to, then definitely listen!
Absolutely agree with Ozgur. I was born and raised in Istanbul. I have been washing the vegetables and fruits with tap water.
I do not advise to drink it because the infrastructure is not like in Europe and may cause some health issues. I drink it several times and had no regret but again do not advise :)
It is better to buy bottled waters from the supermarkets because it is cheaper than cafes or vending machines.
Tap water is very safe to drink. There's a widespread brief among locals that this isn't so, which apparently was the result of problems in 1987-1991. TLDR: Tap water is (1) better regulated and (2) is of better quality than commercial alternatives.
What hasn't been mentioned in the previous answers: The water smells of chlorine, so it is very unpleasant to drink. The chlorine is added as a disinfectant. While this is done in many cities around the globe, in central europe this smell is not noticable while in Istanbul it is so I assume they just put a bit more chlorine into the water.