Why is "pure" water not really pure? What about distilled water?

Answer 1

Distilled water approaches being pure, but ultrapure water (passed through osmotic filters and a UV light at the end) is actually as pure as it will ever be as it is dropping into the container is will be held in. Water doesn't like being empty. It will tend to leach small amounts of whatever container it is being held in. This is the reason that even regular water that is held in a metal canteen tastes like metal. It can happen even with glass if you are talking about ultrapure water.

I work in a lab where we use a device that requires HPLC pure water (about as clean as is commercially available), and it's more expensive than the hoity-toitiest bottled water out there. So we went to a lab across the way who had one of the distillation rigs I described above, and collected water in old clean glass bottles. Our device started malfunctioning soon after, and we were told to use their premade solutions, because glass was being leached off the bottles by the ultrapure water and wreaking havoc with the device. (*sigh* the premade solutions were more expensive than the water)

Answer 2

First, the word pure has no single meaning and each person will give you a different answer. It is like the words safe and comfortable. The definition of absolutely nothing but water molecules is a physical impossibility so we don't have to talk about that. When people say pure, they mean below a certain level of contaminants. This could mean many things depending on the application for the water. Pure drinking water is different than general pure lab water which is different than pure water for a fish which is different than pure water for medicines. See the links for info on distilled and deionized water.

Answer 3

The previous answers have some interesting points, but have not captured the basic reason that this is the case for water in particular. The essential answer to your question is actually that 'pure' water will always have a small concentration of H+ (hydrogen ion) and OH- (hydroxide ion) in it. This is because water spontaneously dissociates into hydrogen and hydroxide ions. These ions simultaneously reform water, but the balance of these rates is such to maintain a small concentraion of these ions. Essentially, 'pure water' is at pH 7, where pH is the log of the hydrogen ion concentration and 1-pH is the log of the hydroxide ion concentration. 'Pure water' will thus always be in equilibrium with 10^-7 molarity H+ and 10^-7 molarity OH-. If you were to try to separate out these ions, more water would simply dissciate to preserve this concentration.

Answer 4

Purity is often relative, not absolute. Distilled water will have impurities. Organic compouds may carry over during distillation. Trace impurities will be found on the glass it is collected in. It takes many steps to produce "ultra pure" water that is almost devoid of impurities (parts per billion).

If water is exposed to room air, gases will dissolve in it.

Answer 5

Distilled water is water that has had virtually all of its impurities as well as electrolytes removed through distillation (that is, boiling the water and re-condensing the steam into a clean container, thus leaving contaminants behind). It is widely used in chemical and biological laboratories.

Distilled water, however, lacks many ionic particles that are somewhat essential for human beings, such as fluoride ions.

Answer 6

not really distilled!?

Pure water is water that has been unmixed, so therefore it really isn't pure. Why call it pure water if it hasn't been mixed or untainted?

Distilled water is water where it's essence has been extracted.

Simply Water.

Answer 7

'Pure' water is stripped of most of it's natural minerals, sort of like melted ice left out for an hour :P.
Distilled water, while with many minerals, could carry bacteria and germs.

Answer 8

Pure water is clean water. But in Chemistry, it is not. That's because Pure substances are Elements, not Compounds!