Water

موضوع: Water الخميس نوفمبر 26, 2009 9:59 am

Water
Water is a ubiquitous chemical substance that is
composed of hydrogen and oxygen and is essential for 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. 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.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.

Water on Earth moves continually through a cycle of
evaporation or transpiration (evapotranspiration), precipitation, and runoff,
usually reaching the sea. Over land, evaporation and transpiration contribute
to the precipitation over land.

Clean, fresh drinking water is essential to human and other life forms. Access to safe drinking water has improved steadily and substantially over the last decades in almost every part of the world. There is a clear correlation between access to safe water and GDP per capita. However, some observers have estimated that by 2025 more than half of the world population will be facing water-based vulnerability. 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% of freshwater is consumed by agriculture.

Chemical and
physical properties

Water is the chemical substance with chemical formula
H2O: one molecule of water has two hydrogen atoms covalently bonded to a single oxygen atom .Water appears in nature in all three common states of
matter and may take many different forms on Earth: water vapor and clouds in
the sky; seawater and icebergs in the polar oceans; glaciers and rivers in the
mountains; and the liquid in aquifers in the ground.

he major chemical and
physical properties of water are:

Water is a tasteless, odorless liquid at standard
temperature and pressure. The color of water and ice is, intrinsically, a very
light blue hue, although water appears colorless in small quantities. Ice also
appears colorless, and water vapor is essentially invisible as a gas.

Water is transparent, and thus aquatic plants can live
within the water because sunlight can reach them. Only strong UV light is
slightly absorbed.

Since the water molecule is not linear and the oxygen
atom has a higher electronegativity than hydrogen atoms, it carries a slight
negative charge, whereas the hydrogen atoms are slightly positive. As a result,
water is a polar molecule.

Pure water has a low electrical conductivity, but this
increases significantly with the dissolution of a small amount of ionic
material such as sodium chloride.

The boiling point of water (and all other liquids) is
dependent on the barometric pressure. For example, on the top of Mt. Everest
water boils at about 68 °C (154 °F), compared to 100 °C (212 °F) at sea level.
Conversely, water deep in the ocean near geothermal vents can reach
temperatures of hundreds of degrees and remain liquid.

Taste and odor
Water can dissolve many different substances, giving it varying tastes and odors. Humans and other animals have developed senses which (more or less) enable them to evaluate the potability of water by avoiding water that is too salty or putrid. Humans also tend to prefer cold water to lukewarm water since cold water is likely to contain fewer microbes.
The taste advertised in spring water or mineral water derives from the minerals
dissolved in it: Pure H2O is tasteless and odorless. The advertised purity of
spring and mineral water refers to absence of toxins, pollutants and microbes.

copied

موضوع: رد: Water الخميس نوفمبر 26, 2009 8:51 pm

thanks
الدرس دة اللي هو تعلم ذاتي
self-learning
ياااااااااا رب ما يجيش في الامتحان
ادعي يا سارة

وشكرا علي الدرس

موضوع: رد: Water الخميس نوفمبر 26, 2009 9:38 pm

رخامة بقى أنا عايزاه ييجي
على فكرة هو مش صعب أوي بالنسبة لأسئلة الامتحانات
هو عايز فهم زي مثلا
hydrogen bonds
عمرنا ما أخدنا حتى فكرة عنها ولقيناها موجودة في الدرس!!

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