properties of water
/ WAW-ter /
Pour a glass of water and you are holding the most important substance in any living thing. A cell is mostly water — roughly two-thirds to three-quarters of its weight. Water is not just the empty space that the interesting molecules float in; its own behavior shapes how everything else in the cell works.
Water is a small molecule made of one oxygen atom bonded to two hydrogen atoms (H2O). Because oxygen pulls on the shared electrons more strongly than hydrogen does, one end of the molecule carries a slight negative charge and the other a slight positive charge. This lopsidedness is called polarity, and it lets water molecules cling to one another and to many other substances through weak attractions called hydrogen bonds. From that single fact flow water's famous talents: it dissolves a huge range of substances (a universal solvent for life), resists temperature swings, sticks to itself and to surfaces, and is denser as a liquid than as ice.
These properties are not trivia — they are the rules every cell plays by. Salts and sugars dissolve so they can be transported; the watery interior stays at a stable temperature so reactions run reliably; water's stickiness pulls fluids up through plants. Without water's peculiar behavior, the chemistry of life as we know it simply would not happen.
Ice floats on a pond, so fish survive winter under a frozen lid instead of being trapped in solid ice from the bottom up — a direct consequence of water being less dense as a solid.
Most substances sink when they freeze; water expands and floats, which keeps lakes liveable.
Calling water the universal solvent is a useful slogan, not a literal truth: oils, fats, and many gases dissolve poorly or not at all in water, and that very fact is what lets cells build membranes.