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.
2. Phases of Water Gas, liquid, and solid are known as the three states of matter or material, but each of solid and liquid states may exist in one or more forms. Thus, another term is required to describe the various forms, and the term phase is used. Each distinct form is called a phase, but the concept of phase defined as a homogeneous portion of a system, extends beyond a single material, because a phase may also involve several materials. For example, a homogeneous solution of any number of substances is a one-phase system. Phase is a concept used to explain many physical and chemical changes (reactions). A solid has a definite shape and volume. A liquid has a definite volume but it takes the shape of a container whereas a gas fills the entire volume of a container. You already know that diamond and graphite are solids made up of the element carbon. They are two phases of carbon, but both are solids. Solids are divided into subclasses of amorphous (or glassy) solids and crystalline solids. Arrangements of atoms or molecules in crystalline solids are repeated regularly over a very long range of millions of atoms, but their arrangements in amorphous solids are somewhat random or short range of say some tens or hundreds of atoms. In general, there is only one liquid phase of a material. However, there are two forms of liquid helium, each have some unique properties. Thus, the two forms are different (liquid) phases of helium. At a definite temperature and pressure, the two phases co-exist. So far, all gases behave alike as do mixtures of gases. Thus, a gas is usually considered as a phase.
3. Why water is important for life. Every known form of life on earth, from the largest mammals to the smallest microbes, relies on water. Why? Because water is an extraordinarily versatile molecule - it's the perfect liquid medium in which to dissolve nutrients for ingestion or wastes for excretion, to transport important chemicals or even be used as one. Water has two particular physical properties that are unique among natural molecules: it remains liquid over an extremely broad range of temperatures, and it decreases in density when converted to solid phase (frozen). While this may seem a relatively minor point, its consequences (that ice floats) are critical to the evolution of life. If ice were more dense than water and the earth cooled slightly, ice formed on the oceans would sink and push the already cold water from the bottom to the surface, where it too would freeze and sink, repeating the cycle until all water on the planet was frozen. Not all scientists believe that the presence of water is "concrete" evidence of life, but liquid water certainly improves the likelihood of life taking hold and finding a hospitable environment. This should not be confused with ice, however, which we know is present in many planets and moons in the solar system. Remember that ice may not be only frozen water, but perhaps vapor from other gases - in either case not as conducive to life
