Water can exist in 3 different physical states – solid, liquid, and gaseous. In the solid state, water exists in the form of ice or snow. In the gaseous state, water exists as water vapour or steam. Finally, the liquid state of water (which is the state in which it is consumed by living organisms) is simply referred to as ‘water’.
The Solid State of Water – Ice
The freezing point of water under standard conditions for temperature and pressure (STP) corresponds to 0 oC. When liquid water is cooled to this temperature, the substance freezes and enters the solid state. When this occurs, the molecules of water occupy certain designated points in a lattice. Due to the fixed intermolecular distances in the solid state of water, the density of ice is lower than the density of liquid water. This is the reason why ice floats on water. Therefore, when the weights of equal volumes of ice and water are compared, the weight of the water will almost always be higher.
When solid water is heated to temperatures above 0 degrees celsius, it melts and reverts to its liquid state. This temperature point on the celsius scale corresponds to a value of 32 degrees on the Farenheit scale and 273.15 on the Kelvin scale. The solid form of water has numerous cooling-based applications. For example, ice is often added to cold-drinks and other beverages to reduce the temperature of the beverage.
The Liquid State of Water
This state of water has a very low viscosity and, therefore, rapidly undergoes deformation when taken out of its container and placed in an open environment (this state of water has the ability to ‘flow’ without much resistance from viscous stress). Under standard conditions for temperature and pressure (STP), water exists in the liquid state in the temperature range of 0-100 degrees celsius.
Liquid water covers over 70% of the Earth’s surface. Water, in its liquid state, is an essential requirement for the metabolisms of all living organisms. This is because liquid water functions as a universal solvent and provides the environment for many biochemical reactions. Furthermore, liquid water is also used for washing clothes and for other cleaning activities (such as bathing). Liquid water is also used industrially as a solvent for the manufacture of several commercially important goods.
The Gaseous State of Water – Water Vapour
Water vapour (sometimes referred to as steam, especially when it holds temperatures above 100oC) is the gaseous state of water. Liquid water can enter the gaseous phase via two different processes – evaporation and boiling. The former involves the escape of water molecules from the surface into the atmosphere whereas the latter involves the transfer of thermal energy to the water molecules at the bulk in order to facilitate a phase transition.
The opposite of this process is known as condensation (when water vapour reverts to the liquid state). It is important to note that evaporation, boiling, and condensation are all physical changes. They do not alter the chemical properties of the water.
The Water Cycle
The biogeochemical cycle that involves various changes in the states of water as it moves through different ecosystems on the planet is commonly referred to as the water cycle. The water cycle, which is also known as the hydrological cycle or hydrologic cycle, details the constant water movement on, above and below the Earth’s surface. The Earth’s water mass tends to stay relatively constant over time, although the breakdown of water into the big ice, salt water, fresh water, and surface water reservoirs varies based on a broad variety of climate variables. The water travels from certain reservoirs to others through the physical processes of erosion, evaporation, precipitation, surface drainage, condensation, and subsurface movement (such as from rivers to oceans or from oceans into the atmosphere).
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