# Why Ice Floats Molecular shape, polarity and hydrogen bonds explain why ice floats, told on a frozen alpine lake at dawn By Numa Topic: Physics & Chemistry [Open the interactive Jig](https://ask.jig.com/j/wmLJFklQt0VL) ## Transcript ### 1. Ice floats, and that is strange Drag the ice block down and let go to see it rise\. Most solids sink in their own liquid, but ice floats because it is less dense than water\. About 92 percent of this block stays below the waterline\. Density is mass divided by volume\. Clear ice with no bubbles floats too\. ### 2. Electron pairs push apart To find out why, we zoom in to a single molecule\. Drag a balloon and watch the others make room\. Around a central atom, groups of electrons, either bonds or lone pairs, repel each other, so they spread as far apart as they can\. Chemists call this VSEPR\. Balloons tied together take the same shapes\. ### 3. Lone pairs bend water Pick a molecule to compare its bond angle\. A lone pair is a pair of electrons that forms no bond\. It pushes harder than a bond and squeezes the bonds together\. Methane has no lone pairs, so its bond angle is 109\.5 degrees\. Ammonia has one: about 107\. Water has two: 104\.5\. A straight drawing of water hides the bend that makes water special\. ### 4. Polar bonds, polar molecules Pick a molecule to compare its dipoles\. Electronegativity is how strongly an atom pulls on shared electrons\. Oxygen pulls harder than hydrogen, so each bond is polar: oxygen is partly negative and hydrogen partly positive\. The arrow showing that pull is a bond dipole\. Because water is bent, its dipoles add up, so the whole molecule is polar\. Ammonia is polar too\. Straight carbon dioxide has polar bonds, but its dipoles cancel, so it is nonpolar\. ### 5. Forces between molecules Drag one water molecule toward another and watch the bonds form\. Intermolecular forces pull whole molecules together\. The strongest is the hydrogen bond, from a hydrogen on one molecule to a lone pair on another\. It is still much weaker than a covalent bond\. Hydrogen bonds need oxygen, nitrogen or fluorine\. They give water its high boiling point of 100 degrees Celsius\. ### 6. Freezing opens the lattice Switch on Freeze to see how much room frozen water needs\. In liquid water, hydrogen bonds keep breaking and forming, so molecules crowd close\. When water freezes, each molecule locks into four hydrogen bonds\. That builds an open lattice with empty channels\. The same mass in 9 percent more volume means lower density, so ice floats\. ### 7. Life under the ice Switch on If ice sank to see what would happen\. Because ice floats, lakes freeze from the top down and the ice shelters the water below\. Water is densest at 4 degrees Celsius, so that water settles on the bottom\. Fish such as perch spend the winter in the 4 degree water under the ice\. ### 8. Water's skin Drag the steel needle onto the water, then keep holding it still until the skin breaks\. Press Pour oil to see oil float\. Cohesion is water sticking to water\. At the surface, hydrogen bonds pull molecules into a stretchy skin called surface tension\. Adhesion is water sticking to other things\. Oil is nonpolar, so water squeezes it out and it floats\.