Water can exist in three phases: gas, liquid, and solid. It has unique properties in each phase. As a liquid, water is able to dissolve nutrients and transport chemicals, making it perfect for life. If ice were denser than water, it would sink to the bottom of oceans when frozen and could freeze the entire planet, preventing the evolution of life. Water's ability to remain liquid over a wide range of temperatures and decrease in density when frozen allows life to thrive by keeping oceans from completely freezing.