Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71% of the Earth's surface[1

Water in three states: liquid, solid (ice), and (invisible) vapor in air. Clouds are droplets of liquid, condensed from water vapor.

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71% of the Earth's surface^[1]. On Earth, it is found mostly in oceans and other large water bodies, with 1.6% of water below ground in aquifers and 0.001% in the air as vapor, clouds (formed of solid and liquid water particles suspended in air), and precipitation.^[2] Saltwater oceans hold 97% of surface water, glaciers and polar ice caps 2.4%, and other land surface water such as rivers, lakes and ponds 0.6%. A very small amount of the Earth's water is contained within biological bodies and manufactured products. Other water is trapped in ice caps, glaciers, aquifers, or in lakes, sometimes providing fresh water for life on land.

Water moves continually through a cycle of evaporation or transpiration (evapotranspiration), precipitation, and runoff, usually reaching the sea. Winds carry water vapor over land at the same rate as runoff into the sea, about 36 Tt (10¹²kilograms) per year. Over land, evaporation and transpiration contribute another 71 Tt per year to the precipitation of 107 Tt per year over land.

Clean, fresh drinking water is essential to human and other life. Access to safe drinking water has improved steadily and substantially over the last decades in almost every part of the world.^[3][4] However, some observers have estimated that by 2025 more than half of the world population will be facing water-based vulnerability,^[5] a situation which has been called a water crisis by the United Nations. Water plays an important role in the world economy, as it functions as a solvent for a wide variety of chemical substances and facilitates industrial cooling and transportation. Approximately 70 percent of freshwater is consumed by agriculture.^[6]

Water is such a common material that many people overlook that water has many astonishing qualities. The properties of water fits into its role on earth like a jigsaw puzzle. The source of these properties all came from the formation of its molecular structure. It was this simple H2O that started the life on Earth.

The first thing water reminds us of is the clear liquid in our water bottles. However, water is more than a normal substance. Water is the beginning and the continual of life on earth. Because of its many unique properties, water was able to start life on our planet.

The simple structure of H2O is the source of all water's properties. With two hydrogen atoms sticking to an oxygen atom in a tetrahedron shape, water is considered as a polar molecule. Because oxygen is very electronegative, it pulls hydrogen's electrons towards it, causing oxygen to become partial negative. The hydrogen atoms then will have fewer electrons towards its own nucleus, therefore making it partial positive. Since positive charge and negative charge attracts each other, the molecular formation of H2O will result hydrogen bonding.

Cohesion is an important property of water that greatly helps organisms. One of the significant contributions is the transportation of water in plants against gravity. When water evaporates from a leaf, other water molecules from further down the vessel will be tugged up due to the hydrogen bonding. The way water molecules attach to each other is called cohesion. Adhesion, the clinging of different substances, also plays a role. Water can hold on to the walls of the vessels to oppose gravity. Because of cohesion and adhesion, water became the prime transportation in the ecological society.

Air temperature can be stabilized by water because water can absorb and release heat. When the temperature is high, water absorbs heat to break the hydrogen bonds; thus, cooling the hot air. When the hydrogen bonds form, heat is released, causing the surrounding air to warm up. This property of water has many advantages because it keeps Earth's temperature shifts within limits that permit life. Also, organisms that are generally made out of water are more able to resist changes in their own temperature.

Water's high heat of vaporization also helps moderate temperature. When water molecules move fast enough to break the hydrogen bonds between the molecules, water molecules escape into the air. This process, called evaporation, will result warmer air and cooler water. The evaporation will stabilize climate for the reason that tropical seas absorb a large amount of solar energy and the moist air will move poleward. Then, the moist air will release heat as it condenses into rain.

When the hot water molecules turn into gas, evaporative cooling occurs. For example, water evaporates from the leaves of a plant will help keeping the tissues from getting too hot. Also, in human's perspective, when sweat departs as gas, the leaving of the hot molecules will cool down our skin.

Many liquids will expand when heated and shrink when froze, but water does the opposite. At the temperature of 4oC or above, water molecules act just like other liquids. On the other hand, when the temperature drops to or below 0oC, water becomes locked in a crystalline structure. The hydrogen bonds will keep the molecules at "arm's length", making ice less dense than water. Because ice floats, it keeps the water underneath insulated to allow life to continue through extremely cold weather.

Many things dissolve in water. Substances are able to dissolve into water due to the hydrogen bonding. Suppose salt were placed in water. Since oxygen is positive, it will stick to the negative chlorine while the negative hydrogen will stick to the positive sodium. This way, H2O will slowly break the sodium chloride crystals apart. Moreover, there are substances that don't dissolve in water but are hydrophilic. Cotton is a good example. Cotton towels absorb water very well, but it does not dissolve. However, there are some that are neither dissolvable nor "water loving". These substances are considered as hydrophobic. This is the reason why oil, a hydrophobic matter, and water don't mix. Hydrophobic is important because all membranes are primarily made out of oil. If oil dissolves in water, then cells won't be able to hold any water.

Water is such a common material that many people overlook that water has many astonishing qualities. The properties of water fits into its role on earth like a jigsaw puzzle. The source of these properties all came from the formation of its molecular structure. It was this simple H2O that started the life on Earth.