The only difference between ice cubes made of tap water and ice cubes made of distilled water is that ice cubes of tap water contain impurities in the water that distilled water does not have. Distilled water contains H2
O molecules and very little else. Tap water is not as highly purified, so it may contain (small amounts of) a number of different substances such as salts (especially calcium salts), nitrates, chloramines, fluorides, and sometimes some very tiny microorganisms. The actual composition of your tap water depends where the water is drawn from and on how the water treatment plant filters it. Tap water varies from location to location, and it may even be treated within the home to change its properties. For example, water "softeners" replace calcium salts with sodium chloride (the same as table salt) to improve the way soap works with the water. The levels of impurities in tap water are tightly controlled by government regulations and are usually quite low.
When the water freezes, most of these impurities are not actually included in the ice crystals, but they usually do get stuck somewhere in little pockets in the ice cubes. Thatís why, with our very sensitive sense of taste, we can tell the diference between different types of ice cubes, the same as you can taste the diference between water from different sources. These small differences between tap water and distilled water may not produce any obvious visible difference in ice cubes, however.
A less subtle difference might arise from the fact that many taps "aerate" the water by passing it through a fine mesh with an air intake. Tap water out of most taps has quite a bit of air dissolved in it. If you leave a glass of tap water out for a while you may notice tiny air bubbles forming on the side. If you freeze the water quickly, the ice will form before the air has a chance to bubble out. The bubbles will form anyway inside of the ice cubes because the dissolved air does not fit into the ice crystal lattice. So if you look at ice cubes made from tap water compared with distilled water, you might find that the tap water ones are not as clear and transparent as the distilled water ones because of all the air bubbles inside. But this isnít a fixed property of tap water or distilled water, just in their handling (you can dissolve air in the distilled water too!).
(published on 10/22/2007)