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transparent vs. opaque ice

Its hot summer here in israel,
and I like freezing bottles of mineral water.
Some times the ice comes out opaque and other times its crystal clear.

can any one tell me why is this ?
5 Solutions
Water from the tap is generally aerated.  If you want really clear ice you have to get the air out of the water, either by letting it stand for a while or boiling it.

You may also need to confine the water while it is freezing.  One time I tried to make a large flat sheet of ice.  I put a rimmed cookie sheet full of water in the freezer.  I got more of a mountain than a flat sheet.  The ice started to freeze in the middle of the pan, floated up and started freezing faster will the increased exposure.
Mineral content can also cause ice to be cloudy, but I would expect that to be less of a problem than gasses.
It is usually the trapped airbubbles in the ice that makes it opaque as d-glitch mentioned. This is the white ice. The dissolved air in the water is forced out as a gas as it freezes and these form bubbles. These bubbles are trapped in the ice, making it look opaque.The bubbles cause scattering of light. This is  in contrast to the black ice that has no air bubbles and which therefore is transparent as the light passes through this without scattering. We all have seen waterfalls, where the transparent water becomes opaque when it strikes against the bottom and spreads as small misty bubbles.
Hardwater has different chemicals in it like calcium which can also give opaque appearance.
Another factor that can cause opaque appearance is fine cracks within the crystal. Water freezes from outside which leads to such faults in the centre.
Obviously you have frozen different kinds of water to achieve the effects.
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SnowFlakeAuthor Commented:
well, actually its all the same water (as far as I know) - bought from the same company of mineral water, it is also always bottles that have been opened and some water removed and in some cases water "residue" from several bottles that have been put into a single bottole.
maybe in the later case I am getting more air mixed in the water ?
might it also relate to wather I am handling the bottles while freezing ?
does the amout of "free" air in the bottle affect this (causing the ice to freeze under different levels of pressure) ?
maybe having the bottles sideways or upright also matters...
One factor may be how fast the water freezes. If it takes some time, because of initial temp or due to the freezer labouring with cooling a lot of other stuff, my guess would be that the water would be more thoroughly "degassed" than if it freezes quickly (the crystals form more slowly and gasses can escape during freezing, rather than get trapped inside the crystal framework). The surface of the water body may also have an impact (larger surface = more efficient degassing) as well as pressure inside the bottle.
I live in Colorado, and there are lots of ice climbers here.  They say the best ice that is the most firm is the blue kind.  This ice has so few air bubbles and is so clear that it somehow refracts blue light.  It is formed by very slow freezing by water residue trickling.  The layers are so small that there isn't room for air bubbles, so it is super hard and clear to the point that it refracts blue light.
SnowFlakeAuthor Commented:
thank you everybody for the interesting and enlightening information.

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