Water (chemical formula: H ₂ O) is composed ofhydrogen、oxygentwo typeselementConstituentInorganic, non-toxic, drinkable.Colorless, odorless and transparent under normal temperature and pressureliquid, known as the source of human life, is an important material to maintain life, also known as hydrogen oxide.
Water is one of the most common substances on the earth. About 71% of the earth's surface is covered by water.It is an important resource for the survival of all life, including inorganic compounds and human beings. It is also the most important part of organisms.Although its content in the air is small, it is an important part of the air.[1]
pure waterVery weak conductivityelectrolyte。The water in daily life has more anion and cation due to the dissolution of other electrolytesConductivity。[1]
Chinese name
water
Foreign name
Water
Alias
Hydrogen oxide、Oxidane 、Dihydrogen monoxide
chemical formula
HtwoO[7]
molecular weight
eighteen point zero one five two
CAS login number
7732-18-5[7]
EINECS login number
231-791-2
Melting point
0 ℃
Boiling point
100 ℃(standard atmospheric pressure)
Density
1g/cm³,10³kg/m³（t=4℃）
Appearance
Colorless transparent liquid at normal temperature
There are many different understandings about the source of water on the earth, and each has its own reason, but the truth is still up to scientists to collect more objective evidence to uncover the mystery.As we all know, 72% of the earth's surface is covered by water.However, where the water on the earth comes from has always been an unsolved mystery.[4-5]
According to an article published in Nature,University of Munster, GermanyBased on isotope analysis of meteorite and mantle rock samples from Lake Tagesh, British Columbia, Canada, scientists of, showed that water appeared on the earth much later than previously expected.This, to a large extent, refutes the argument held by many scientists that "water was transformed from the ice layer on the surface of meteorites during the formation stage of the Earth".The origin and formation time of water on the earth determine the direction of the earth's evolution and the time of the origin of life.For the source of water on the earth“Exogenous theory”And "endogenous theory".[5]
Until the 1870s, people still regarded water as an element.Since then, Cavendish, especially Lavoisier, has confirmed that water is a compound and that water is composed of hydrogen and oxygen.
In 1805, through the quantitative research of Gay Lussac and Humbolt, it was further determined that hydrogen and oxygen were combined in the ratio of two to one, and the composition of water was HtwoO has been preliminarily established.
In September 2022, the scientific research team of the Institute of Geochemistry of the Chinese Academy of Sciences carried out a study on Chang'e-5 lunar soil samples. Through infrared spectroscopy and nano ion probe analysis, it was found that there was a large amount of solar wind generated water in the mineral surface of Chang'e-5, and it was estimated that the solar wind proton injection contributed at least 170 ppm to the water content of Chang'e-5 lunar soil.The research results were published in the international academic journal Nature Communication。[6]
Physical and chemical properties
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physical property
Structure diagram of water molecule
Although water is the standard of many physical constants, it has some special physical properties.Unlike most substances, when they solidify, their volume decreases and their density increases. When water freezes, its volume increases and its density decreases;The density of water has a maximum value at 277.14K, which is different from that of most substances when the temperature decreases;Among all solid and liquid substances, water has the largest specific heat capacity;Although the molecular weight of water is small, its boiling point and evaporation heat are quite high;The boiling point and freezing point of compounds of the same family and type generally increase with the increase of molecular weight, while the boiling point and freezing point of water and its congener with larger molecular weight are higher;Among many substances, water has a very large dielectric constant, so it is also a very good polar solvent.All these "abnormal" phenomena are closely related to the formation of hydrogen bonds and association of water.[8]
Structure of water molecule
Condensation of water vapor in quartz capillary tube without air and under the condition of less than saturated vapor pressure can obtain so-called "abnormal water" or "poly water" which is thicker, more viscous, harder to volatilize and has higher thermal expansion coefficient than ordinary water.The structure and even composition of this abnormal water are uncertain.As for its abnormal nature, it is almost certain through research that it is caused by impurities, while pure "abnormal" water does not exist.[8]
chemical property
thermal stability
Because water has a great heat of formation, it must be a very stable compound.In fact, the decomposition rate of water at 3000K is only 11.1%, and the reaction is reversible.[9]
Obviously, water absorbs a lot of heat when it dissociates.At 1000K and 202.65kPa, 2 moles of gaseous water need to absorb 495.8kJ of heat to decompose into H2 and O2;At 3000K, 572.04kJ is required.[9]
Generally, ordinary water is evaporated into steam first and then condensed to obtain pure distilled water.At this time, the soluble salt in the water cannot be volatilized and remains in the remaining water;However, some gases and volatile substances may still exist in the condensed water. If part of the steam is condensed and part of it escapes, the gases and volatile substances dissolved in the water will be removed with the escaping part.[10]
For more pure water, KMnO ₄ and Ba (OH) ₂ can be added into ordinary distilled water and then distilled, which can remove trace organic compounds and volatile acid oxides (such as CO ₂);Then add nonvolatile acid (such as H ₂ SO ₄ or H ∨ PO ₄) into the distilled water and distill it again to remove volatile alkali such as NH ∨.Since steam condenses in air and gas may dissolve into water, the final distillation should be carried out in vacuum. Since the purity of this water can no longer be tested by general chemical methods, it is often measured by specific conductivity, so it is called conductivity water;hisSpecific conductanceUsually 10-6S/cm。Because sodium salt and other impurities in glass will slowly dissolve in water and reduce the purity of conductive water, quartz (or tin or silver) utensils or plastic containers are often used to store conductive water.[10]
Ion exchange method
Although the purity of conductive water is high, it is only suitable for small-scale preparation and difficult for mass production.High purity water can be produced in large quantities by using synthetic ion exchange resin.If water is first allowed to pass through the cation exchange resin (such as RSO ∨ H), all cations in the water will exchange with H ions in the ion exchange resin.[10]
application area
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Water has many different uses, including but not limited to the following:
one
Drinking water: water is one of the most basic survival needs of human beings.We need water to meet our daily drinking needs and keep healthy and normal physiological functions.
two
Agricultural water: water is one of the important resources for agricultural production.Water is used for irrigation of crops, drainage of farmland, animal husbandry in aquaculture, plant growth in greenhouse, etc. Water is crucial for agricultural development and food production.
three
Industrial water: Many industrial processes require a large amount of water, such as manufacturing, cooling, cleaning, lubrication and chemical reaction.Industrial water is widely used in manufacturing, energy production, chemical, textile, pharmaceutical and other industries.
four
Business water: business places need water to meet the daily activity needs of offices, shops, restaurants and entertainment places, such as drinking water, clean water, toilet flushing, etc. for employees and customers.
five
Power generation and energy production: Water can be used for the hydropower generation process in power plants, which generates electricity by driving turbine generators through water flow.In addition, water can also be used as a coolant in nuclear power and thermal power generation.
six
Sanitation and cleaning: water is used for personal hygiene, including hand washing, bathing, brushing teeth, washing clothes, etc.In addition, water is also used to clean homes, public facilities, roads and vehicles.
seven
Ecosystem maintenance: water is the basis and support for maintaining ecosystems such as wetlands, rivers, lakes and oceans.Water provides habitat for many organisms and plays an important role in the food chain.
Physiological significance
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Water is normal for human bodymetabolizeNecessary substances[2]Under normal circumstances, the body should pass throughskin、Viscera、lungas well askidneyDrain about 1.5L of water to ensure toxin is discharged from the body.[3]There is 80% water in children, 50%~60% in the elderly and 70% in normal middle-aged people.
Water has many important functions in the body[2]：
(1) Water is an important component of cell protoplasm;[2]
(2) Water acts as a solvent in the body and dissolves various substanceselectrolyte；[2]
(3) Water plays a transport role in the body, which can transfer nutrients, metabolic wastes and endocrine substances (such as hormones);[2]
(4) Water has high thermal conductivity and specific heat. It can be used as a "heat carrier" to transfer heat between the body and the skin surface, helping the body regulate its temperature.[2]
