Question

Why the density of water decreases when it is heated above or below 4 degrees

Answer 1

Actually ice has a defined crystalline structural arrangement and the molecules of freezing water start to arrange themselves in that order below 4C as their kinetic energy becomes less and the attraction interaction between them due to the polarity of molecules start to act. In structure of ice, each oxygen molecule is attached with 6 hydrogen atoms, 2 via covalent bonds and 4 via hydrogen bonds which makes a matrix of cubes all around the lattice. Now as cubes have empty space between them, that is air pockets, the volume occupied becomes larger than it would have been when the molecules had more freedom in getting close to each other.