Density, measuring it, and evaporation

Evaporation is a liquid turning into a gas; it happens at the liquid’s surface at any temperature and speeds up when the liquid gains heat. Boiling is when vapour bubbles form throughout the liquid and rise; water boils at 100 °C at normal atmospheric pressure. Condensation is water vapour turning into tiny water droplets: this is how drops form on a window or on a pot lid. In nature, water evaporates from seas, lakes, rivers and soil warmed by the Sun, cools higher up and condenses into clouds, and falls as rain or snow. Oceans cover more than 70 percent of the Earth, so the ocean is the main source of water vapour in the atmosphere. Boiling water kills microbes in it, but boiling water scalds skin.