Water molecules are made from two hydrogen atoms and one oxygen atom. The hydrogen atoms attach to the oxygen atom, forming a shape that resembles a mouse head with ears. The oxygen molecule acts as the negatively charged face, and the hydrogen atoms are the positively charged ears. Since positive and negative particles attract, the hydrogen and oxygen atoms of each water molecule stick together, forming irregular hexagons, which solidify in freezing temperatures.
The hexagonal shape of linked water molecules is apparent in the appearance of snowflakes. Water molecules take on this shape even within ice cubes. Ice freezes from the inside out, causing the molecules to join together gradually. As they do, dissolved gas and particles are pushed to the center since they do not fit into the pattern of linking water molecules. The result is ice that is cloudy at the center. This cloud consists of gas bubbles and dissolved minerals and debris. The more gas and impurities present in the water, the cloudier the ice.
To create clear ice for a science experiment, boil distilled water before freezing. In addition to killing most bacteria and viruses that can be found even in tap water, boiling evaporates the dissolved gasses that are invisible to the naked eye. When you freeze boiled water, fewer air bubbles remain to be pushed to the center, resulting in an ice cube with a smaller cloudy section.
Running water through a reverse osmosis filter will result in ice that is even clearer than that produced by boiled water. A reverse osmosis filter forces water molecules through a tiny opening that removes mineral particles as well as viruses and bacteria. This water yields ice that is clearer than boiled water because boiling only eliminates dissolved gases. Cubes from filtered water should be completely clear, demonstrating that water purity can be indicated by the opacity of ice.