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Name: Paul
Status: other
Grade: 12+
Location: NV
Country: USA
Date: Summer 2011

When I pour filtered tap water into a glass beaker, the water is clear and there is an absence of any particles floating about. Then I pour the same water into an ice cube tray and place the tray into a freezer.

When these ice cubes are placed back into the glass beaker and allowed to melt, the result is water that is no longer clear but now contains tiny white particles floating about. These particles resemble toilet paper, if you will, that has been placed in a blender filled with water and pureed. What has happened to the water after the freezing and thawing process and what are these white particles?

Hi Paul,

If you have hard water (which means there are minerals dissolved in it), you might be observing the precipitation (coming out of solution) of those minerals. Lime (calcium carbonate) is often found in water, and when in precipitates, it can looks like you are describing.

For many solids, the warmer the water, the more solid it can dissolve. For example, hot tea can dissolve more sugar than iced tea. The same is true for lime (mineral) and water. If the water is "saturated" with lime (which means it has the maximum amount of lime dissolved in it) at room temperature, then as it cools, there will be an excess of lime in the water, because the water can dissolve less lime at lower temperatures. Once the lime drops out of solution, and then the water freezes and thaws, it will take some time for the lime to re-dissolve in the water. Depending on how much excess lime, the 'floating bits' could last a very long time before they redissolve. And, they may never redissolve, because the water evaporates -- even faster in a dry place like Nevada -- but the lime does not.

There are other impurities that could explain your behavior as well, including other minerals, or even microbial growth, but I think lime is the most likely candidate.

I hope this helps,

Burr Zimmerman

Hi Paul,

Tap water contains many different dissolved substances (metal ions such as calcium, nonmetal ions such as fluorides, organic substances, etc). Some of these substances can only remain dissolved in water at certain concentrations. When this maximum concentration is exceeded, the substances will precipitate out of the water.

When water is gently frozen (not stirred or otherwise disturbed) much of the water will freeze without incorporating these substances in the crystal form of water. As such the concentrations of these substances will steadily increase as the water solidifies - the substances only being able to remain in the still liquid form of water. At a certain point, when much of the water has solidified, the solubility limit of the particular substance is exceeded and the substance precipitates out.

When the water melts again, the substances may not go back into solution fast enough - sometimes agitation is necessary for the substance to redissolve, sometimes heat is needed. So the precipitated substances remain for you to see.

You can verify this by (a) rinsing the ice cube with warm water before allowing it to melt (most of the whitish substance will be on the surface of the ice cube) - in this case you will see less of the substance when the ice melts, (b) continuously agitate the ice-water as it melts - this will allow the substances to redissolve and you should see much less of the substance when the water is fully melted, (c) heat the ice-water (but do not boil, as the steam escapes and you will end up with a concentrated solution anyway) - again the heating (and stirring) will allow the substance to redissolve, and you should, again, see less of the precipitated substance.

Greg (Roberto Gregorius)
Canisius College

Hi Paul -

Tap water is by no means pure.

It commonly has about 100-200 ppmw (parts-per-million, by weight) of dissolved minerals and often some dissolved carbon-based substances too. Almost all of these impurities are not smoothly accepted into the ice as the water freezes. Crystallization of the water as it freezes tends to reject most of the impurities, forcing them into the remaining liquid adjacent to the ice/water interface. The increased concentration in the liquid near the ice drifts away from the ice by diffusion and/or convection, and so in the long run it gets pushed forwards until the last remaining water and then the last formed ice is stuck with it.

This can be a means of purifying water, if you take only the first 80-90% of the water frozen and melt it, or of concentrating the impurities, it you take only the last 1-5%.

It is plausible that the concentrated impurity-front during freezing will somehow conglomerate into the white wisps you eventually see. They could be made of metal hydroxides, or of starches and proteins, or a biofilm of bacteria which had fed on the concentrated impurity front while the ice grew, or the dead remains of bacteria and algae that used to be invisibly dispersed throughout the clear tap water.

It sounds a little difficult to gather enough of this stuff to analyze it by amateur means. But you might try evaporating away all the water and scraping the remaining solids together. The first thing to check is it's weight compared to the weight of water that was evaporated.

You might want to be aware of how your re-melting container was last cleaned, too. It's not too difficult to leave enough hard-water deposit or soap-scum on it to float up visibly in the next water that wets it. Rinsing last with distilled or bottled water would prevent that, and filling first with such purified water would be the test for it.

Jim Swenson

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