01Why water is polar
Oxygen is far more electronegative than hydrogen, so it pulls the shared electrons in each O–H bond toward itself. The bonds are polar covalent: oxygen ends up with a partial negative charge (δ−) and each hydrogen with a partial positive charge (δ+).
Water is also bent (about 104.5°), not straight. Because of that shape the charges don't cancel out — one end of the molecule is negative, the other positive. That permanent imbalance is what makes water a polar molecule.
02Hydrogen bonds: the attraction between molecules
The δ+ hydrogen of one water molecule is attracted to the δ− oxygen of a neighbor. That attraction is a hydrogen bond. Each water molecule can hydrogen-bond to up to four neighbors.
Individually a hydrogen bond is weak (about 1/20 the strength of a covalent bond) and constantly breaks and reforms. But billions of them together give water its unusual, life-supporting behavior.
03The properties that come from hydrogen bonding
Cohesion — water sticks to water. This creates surface tension (insects walk on ponds) and pulls water up plant xylem as a continuous column.
Adhesion — water sticks to other polar surfaces, like the cellulose walls of xylem. Adhesion + cohesion = capillary action.
High specific heat — heat must break hydrogen bonds before molecules move faster, so water resists temperature change. Oceans and bodies stay thermally stable.
High heat of vaporization — evaporating water carries a lot of heat away, which is why sweating cools you.
Ice floats — in ice, hydrogen bonds lock molecules into a spaced-out lattice that is less dense than liquid water. Lakes freeze from the top, insulating life below.
Universal solvent — water surrounds ions and polar molecules (hydration shells), dissolving them. That's why polar molecules are hydrophilic and nonpolar ones are hydrophobic.
04Notebook box
Structure → function chain: electronegativity → polar O–H bonds → bent shape → polar molecule → hydrogen bonds → cohesion, adhesion, high specific heat, high heat of vaporization, less-dense ice, solvent.
Worked example (AP style): A student notices a pond's temperature barely changes from day to night while nearby sand swings 20 °C. Explain. → Water's hydrogen bonds absorb heat energy before molecular motion (temperature) increases, giving water a high specific heat; sand has no such network.