We live on a planet that is dominated by water. More than 70 % of the Earth's surface is covered with this simple molecule. Scientists estimate that the.

We live on a planet that is dominated by water. More We live on a planet that is dominated by water. More than 70 % of the Earth's surface is covered with this than 70 % of the Earth's surface is covered with this simple molecule. Scientists estimate that the simple molecule. Scientists estimate that the hydrosphere contains about 1.36 billion cubic kilometers of this substance mostly in the form of a liquid (water) that occupies topographic depressions on the Earth. The second most common form of the water molecule on our planet is ice. If our entire planet's ice melted, sea-level would rise by about 70 meters.

Water is also essential for life. Water is the major Water is also essential for life. Water is the major constituent of almost all life forms. Most animals and constituent of almost all life forms. Most animals and plants contain more than 60 % water by volume. Without plants contain more than 60 % water by volume. Without water life would probably never have developed on our water life would probably never have developed on our planet. planet.

The word "water" comes from the Old The word "water" comes from the Old English wæter. The word is derived English wæter. The word is derived from the Proto-Indo-European from the Proto-Indo-European language root *wod-or with other language root *wod-or with other forms including *ud- and *wed-; the forms including *ud- and *wed-; the source of the word wet. This root is the source of the word wet. This root is the source for words about water in many source for words about water in many European languages c.f German European languages c.f German "Wasser", Latvian "udens" or Russian "Wasser", Latvian "udens" or Russian "????" (voda).[3]"????" (voda).[3]

http://en.wikipedia.org/wiki/Water

General

Systematic name Water

Other names

AquaHydrogen oxideDihydrogen monoxideHydrogen hydroxide

Molecular formula H2O

Molar mass 18.02 g/mol

Appearancetransparent, almostcolorless liquid witha slight hint of blue

CAS number [7732-18-5]

http://en.wikipedia.org/wiki/Water_%28molecule%29

Properties

Density and phase1000 kg/m3, liquid917 kg/m3, solid

Melting point 0 °C, 32 °F (273.15 K)

Boiling point 100 °C, 212 °F (373.15 K)

Triple point 273.16 K, 611.73 Pa

Specific heatcapacity (gas)

4186 J/(kg·K)

Specific heatcapacity (liquid)

cp= 1850 J/(kg·K)

cv= 3724 J/(kg.K)

Specific heatcapacity (solid)

2060 J/(kg·K)

Acidity (pKa) 15.74

Basicity (pKb) 15.74

Viscosity 0.001 Pa·s at 20 °C

http://en.wikipedia.org/wiki/Water_%28molecule%29

Temperature (degrees Celsius) Density (grams per cubic centimeter)

0 (solid) 0.9150

0 (liquid) 0.9999

4 1.0000

20 0.9982

40 0.9922

60 0.9832

80 0.9718

100 (gas) 0.0006

Table 1a-1: Density of water molecules at various temperatures

Water has three statesWater has three states

Below freezing water is a solid (ice or Below freezing water is a solid (ice or snowflakes), between freezing and boiling snowflakes), between freezing and boiling water is a liquid, and above its boiling point water is a liquid, and above its boiling point water is a gas. There are words scientists use water is a gas. There are words scientists use to describe water changing from one state to to describe water changing from one state to another. Water changing from solid to liquid another. Water changing from solid to liquid is said to be is said to be meltingmelting. When it changes from . When it changes from liquid to gas it is liquid to gas it is evaporatingevaporating. Water changing . Water changing from gas to liquid is called from gas to liquid is called condensationcondensation (An (An example is the 'dew' that forms on the outside example is the 'dew' that forms on the outside of a glass of cold soda). of a glass of cold soda). Frost formationFrost formation is is when water changes from gas directly to solid when water changes from gas directly to solid form. When water changes directly from solid form. When water changes directly from solid to gas the process is called to gas the process is called sublimationsublimation. .

Most liquids Most liquids contractcontract (get smaller) when they get colder. Water contracts (get smaller) when they get colder. Water contracts until it reach 4 C then it expands until it is solid. Solid water is less until it reach 4 C then it expands until it is solid. Solid water is less densedense than liquid water because of this. If water worked like other liquids, then than liquid water because of this. If water worked like other liquids, then there would be no such thing as an ice berg, the ice in your soft drink there would be no such thing as an ice berg, the ice in your soft drink would sink to the bottom of the glass, and ponds would freeze from the would sink to the bottom of the glass, and ponds would freeze from the bottom up!bottom up!

Gas Liquid Solid

Water is found on Earth in all three forms. This is because Earth is a very special planet with just the right range of temperatures and air pressures. Earth is said to be at the triple point for water.

When the water molecule makes a When the water molecule makes a physical physical phase changephase change its molecules its molecules arrange themselves in distinctly arrange themselves in distinctly different patterns (different patterns (Figure 8a-2Figure 8a-2). The ). The molecular arrangement taken by ice molecular arrangement taken by ice (the solid form of the water (the solid form of the water molecule) leads to an increase in molecule) leads to an increase in volume and a decrease in density. volume and a decrease in density. Expansion of the water molecule at Expansion of the water molecule at freezing allows ice to float on top of freezing allows ice to float on top of liquid water.liquid water.

Figure 8a-2Figure 8a-2: The three : The three diagrams above illustrate the diagrams above illustrate the distinct arrangement patterns of distinct arrangement patterns of water molecules as they change water molecules as they change their physical state from ice to their physical state from ice to water to gas. Frozen water water to gas. Frozen water molecules arrange themselves molecules arrange themselves in a particular highly organized in a particular highly organized rigid geometric pattern that rigid geometric pattern that causes the mass of water to causes the mass of water to expand and to decrease in expand and to decrease in density. The diagram above density. The diagram above shows a slice through a mass of shows a slice through a mass of ice that is one molecule wide. In ice that is one molecule wide. In the liquid phase, water the liquid phase, water molecules arrange themselves molecules arrange themselves into small groups of joined into small groups of joined particles. The fact that these particles. The fact that these arrangements are small allows arrangements are small allows liquid water to move and flow. liquid water to move and flow. Water molecules in the form of a Water molecules in the form of a gas are highly charged with gas are highly charged with energy. This high energy state energy. This high energy state causes the molecules to be causes the molecules to be always moving reducing the always moving reducing the likelihood of bonds between likelihood of bonds between individual molecules from individual molecules from forming. forming.

Water has the chemical formula H2O meaning that one molecule of Water has the chemical formula H2O meaning that one molecule of water is composed of two hydrogen atoms and one oxygen atom. It water is composed of two hydrogen atoms and one oxygen atom. It can also be described ionically as HOH, with a hydrogen ion (H+) that can also be described ionically as HOH, with a hydrogen ion (H+) that is bonded to a hydroxide ion (OH-). It is in dynamic equilibrium is bonded to a hydroxide ion (OH-). It is in dynamic equilibrium between the liquid and vapor states at standard temperature and between the liquid and vapor states at standard temperature and pressure. At room temperature, it is a nearly colorless, tasteless, and pressure. At room temperature, it is a nearly colorless, tasteless, and odorless liquid. It is often referred to in the sciences as the universal odorless liquid. It is often referred to in the sciences as the universal solvent and the only pure substance found naturally in all three solvent and the only pure substance found naturally in all three states of matterstates of matter..

• Figure 8a-1: The atomic Figure 8a-1: The atomic structure of a water (or structure of a water (or dihydrogen monoxide) dihydrogen monoxide) molecule consists of two molecule consists of two hydrogen (H) atoms joined to hydrogen (H) atoms joined to one oxygen (O) atom. The one oxygen (O) atom. The unique way in which the unique way in which the hydrogen atoms are attached hydrogen atoms are attached to the oxygen atom causes one to the oxygen atom causes one side of the molecule to have a side of the molecule to have a negative charge and the area negative charge and the area in the opposite direction to in the opposite direction to have a positive charge. The have a positive charge. The resulting polarity of charge resulting polarity of charge causes molecules of water to causes molecules of water to be attracted to each other be attracted to each other forming strong molecular forming strong molecular bonds.bonds.

• Water MoleculeWater Molecule

A water molecule consists of A water molecule consists of an oxygen atom and two an oxygen atom and two hydrogen atoms, which are hydrogen atoms, which are attached at an angle of 105°. attached at an angle of 105°. Not shown are two pairs of Not shown are two pairs of electrons on the bottom that electrons on the bottom that form a similar angle in a form a similar angle in a plane perpendicular to this plane perpendicular to this view. This asymmetrical view. This asymmetrical arrangement accounts for arrangement accounts for the many unusual properties the many unusual properties of water, such as the fact of water, such as the fact that it expands when it that it expands when it freezes.freezes.

This is caused by This is caused by hydrogen bonds that hydrogen bonds that form between the form between the slightly positive and slightly positive and negative ends of negative ends of neighboring neighboring molecules. This is molecules. This is the reason why the reason why water is found in water is found in drops; perfect drops; perfect spheres. spheres. 

Water molecules stick to each Water molecules stick to each other. other. 

http://www.ccs.k12.in.us/chsBS/kons/kons/physical_properties_of_water.htm

• Dew drops adhering to a Dew drops adhering to a spider web. Water sticks to spider web. Water sticks to itself (cohesion) because it itself (cohesion) because it is polar, meaning one end of is polar, meaning one end of the molecule has slightly the molecule has slightly more negative charge than more negative charge than the other, which has slightly the other, which has slightly more positive charge. In more positive charge. In water, this happens because water, this happens because the oxygen atom is more the oxygen atom is more electronegative - that is, it electronegative - that is, it has a stronger "pulling has a stronger "pulling power" on the molecule's power" on the molecule's electrons, drawing them electrons, drawing them closer (along with their closer (along with their negative charge), and negative charge), and making the area around the making the area around the oxygen atom more negative oxygen atom more negative than the area around both than the area around both the hydrogen atoms.the hydrogen atoms.

• The oxygen end of water has a negative The oxygen end of water has a negative charge and the hydrogen end has a positive charge and the hydrogen end has a positive charge. The hydrogen of one water molecule charge. The hydrogen of one water molecule are attracted to the oxygen from other water are attracted to the oxygen from other water molecules. This attractive force is what gives molecules. This attractive force is what gives water its cohesive and adhesive properties.water its cohesive and adhesive properties.

• Water is attracted Water is attracted to other water. This to other water. This is called is called cohesioncohesion. . Water can also be Water can also be attracted to other attracted to other materials. This is materials. This is called called adhesionadhesion..

Figure 8a-4: The adhesive bonding property of water molecules allows for the formation of water droplets (Photo © 2004 Edward Tsang).

Water molecules stick to other Water molecules stick to other surfacessurfaces

http://www.ccs.k12.in.us/chsBS/kons/kons/physical_properties_of_water.htm

Water has the ability to support small objects.  The Water has the ability to support small objects.  The hydrogen bonds between neighboring molecules hydrogen bonds between neighboring molecules cause a “film” to develop at the surface. cause a “film” to develop at the surface. 

The following illustration shows how water molecules are The following illustration shows how water molecules are attracted to each other to create high surface tension. This attracted to each other to create high surface tension. This property can cause water to exist as an extensive thin film property can cause water to exist as an extensive thin film over solid surfaces. In the example above, the film is two over solid surfaces. In the example above, the film is two layers of water molecules thick. layers of water molecules thick.

This surface tension permits water to hold up substances This surface tension permits water to hold up substances heavier and denser than itself. A steel needle carefully placed heavier and denser than itself. A steel needle carefully placed on the surface of a glass of water will float. Some aquatic on the surface of a glass of water will float. Some aquatic insects such as the water strider rely on surface tension to insects such as the water strider rely on surface tension to walk on water.walk on water.

Surface tension is Surface tension is essential for the essential for the transfer of energy from transfer of energy from wind to water to create wind to water to create waves. Waves are waves. Waves are necessary for rapid necessary for rapid oxygen diffusion in oxygen diffusion in lakes and seas. lakes and seas. Nevertheless, surface Nevertheless, surface tension is tension is notnot the force the force that keeps boats that keeps boats floating. floating.

Organisms like the water strider can be seen taking advantage of this property.

http://www.ccs.k12.in.us/chsBS/kons/kons/physical_properties_of_water.htm

Try this at homeTry this at home: :

• Place a drop of water onto a piece of wax paper. Place a drop of water onto a piece of wax paper. Look closely at the drop. What shape is it? Why Look closely at the drop. What shape is it? Why do you think it is this shape? What is happening? do you think it is this shape? What is happening? Water is not attracted to wax paper (there is no Water is not attracted to wax paper (there is no adhesion between the drop and the wax paper). adhesion between the drop and the wax paper). Each molecule in the water drop is attracted to Each molecule in the water drop is attracted to the other water molecules in the drop. This the other water molecules in the drop. This causes the water to pull itself into a shape with causes the water to pull itself into a shape with the smallest amount of surface area, a bead the smallest amount of surface area, a bead (sphere). All the water molecules on the surface (sphere). All the water molecules on the surface of the bead are 'holding' each other together or of the bead are 'holding' each other together or creating surface tension.creating surface tension.

Liquid Water Molecules with hydrogen bonds

Water Vapor Molecules

Thermal propertiesThermal properties::

http://www.ccs.k12.in.us/chsBS/kons/kons/physical_properties_of_water.htm

Water has a high boiling pointWater has a high boiling point

WaterWater is one of the few is one of the few substances that remain a liquid substances that remain a liquid at such a large range of at such a large range of temperatures (O-100 °C).  A temperatures (O-100 °C).  A large amount of energy must be large amount of energy must be invested to overcome the invested to overcome the hydrogen bonds in liquid water hydrogen bonds in liquid water to change it to the gas phase. to change it to the gas phase. Water absorbs or releases more Water absorbs or releases more heat than many substances for heat than many substances for each degree of temperature each degree of temperature increase or decrease. Because increase or decrease. Because of this, it is widely used for of this, it is widely used for cooling and for transferring heat cooling and for transferring heat in thermal and chemical in thermal and chemical processes.processes.

Specific heatSpecific heat is the is the amount of energy required amount of energy required to change the temperature to change the temperature of a substance. Because of a substance. Because water has a high specific water has a high specific heat, it can absorb large heat, it can absorb large amounts of heat energy amounts of heat energy before it begins to get hot. It before it begins to get hot. It also means that water also means that water releases heat energy slowly releases heat energy slowly when situations cause it to when situations cause it to cool. Water's high specific cool. Water's high specific heat allows for the heat allows for the moderation of the Earth's moderation of the Earth's climate and helps organisms climate and helps organisms regulate their body regulate their body temperature more temperature more effectively.effectively.

Mpemba effectMpemba effect

• The The Mpemba effectMpemba effect is the surprising phenomenon is the surprising phenomenon whereby hot water can, under certain conditions, whereby hot water can, under certain conditions, freeze faster than cold water, even though it must freeze faster than cold water, even though it must pass the lower temperature on the way to freezing. pass the lower temperature on the way to freezing. However, this can be explained with evaporation, However, this can be explained with evaporation, convection, supercooling, and the insulating effect of convection, supercooling, and the insulating effect of frost.frost.

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/imgheat/hfrez.gif

Hot iceHot ice• Hot ice is the name given to Hot ice is the name given to

another surprising another surprising phenomenon in which water phenomenon in which water at room temperature can be at room temperature can be turned into ice that remains turned into ice that remains at room temperature by at room temperature by supplying an electric field of supplying an electric field of the order of 106 volts per the order of 106 volts per meter. (Choi 2005)meter. (Choi 2005) The effect of such electric The effect of such electric

fields has been suggested fields has been suggested as an explanation of cloud as an explanation of cloud formation. The first time formation. The first time cloud ice forms around a cloud ice forms around a clay particle, it requires a clay particle, it requires a temperature of -10 °C, but temperature of -10 °C, but subsequent freezing subsequent freezing around the same clay around the same clay particle requires a particle requires a temperature of just -5 °C, temperature of just -5 °C, suggesting some kind of suggesting some kind of "ice memory" (Connolly, P.J, "ice memory" (Connolly, P.J, et al, 2005)et al, 2005)

Surface tension is related to the Surface tension is related to the cohesive properties of water. cohesive properties of water. Capillary actionCapillary action however, is however, is related to the adhesive properties related to the adhesive properties of water. You can see capillary of water. You can see capillary action 'in action' by placing a action 'in action' by placing a straw into a glass of water. The straw into a glass of water. The water 'climbs' up the straw. What water 'climbs' up the straw. What is happening is that the water is happening is that the water molecules are attracted to the molecules are attracted to the straw molecules. When one water straw molecules. When one water molecule moves closer to a the molecule moves closer to a the straw molecules the other water straw molecules the other water molecules (which are cohesively molecules (which are cohesively attracted to that water molecule) attracted to that water molecule) also move up into the straw. also move up into the straw. Capillary action is limited by Capillary action is limited by gravity and the size of the straw. gravity and the size of the straw. The thinner the straw or tube the The thinner the straw or tube the higher up capillary action will pull higher up capillary action will pull the water (Can you make up an the water (Can you make up an experiment to test this?).experiment to test this?).

Water has the ability to “climb” structures.

http://www.ccs.k12.in.us/chsBS/kons/kons/physical_properties_of_water.htm

• Water molecules as well as binding to each other, Water molecules as well as binding to each other, bind to many other substances such as glass, cotton, bind to many other substances such as glass, cotton, plant tissues, and soils. This is called adhesion. For plant tissues, and soils. This is called adhesion. For example, in a thin glass tube, when the molecules at example, in a thin glass tube, when the molecules at the edge reach for and adhere to the molecules of the edge reach for and adhere to the molecules of glass just above them, they at the same time tow glass just above them, they at the same time tow other water molecules along with them. The water other water molecules along with them. The water surface, in turn, pulls the entire body of water to a surface, in turn, pulls the entire body of water to a new level until the downward force of gravity is too new level until the downward force of gravity is too great to be overcome. This process is called great to be overcome. This process is called capillary capillary actionaction..

• Thus water readily wets many materials. Capillary Thus water readily wets many materials. Capillary action allows a paper towel or a sponge to be used to action allows a paper towel or a sponge to be used to soak up spilled water. Without this property, the soak up spilled water. Without this property, the nutrients needed by plants and trees would remain in nutrients needed by plants and trees would remain in the soil.the soil.

Capillary ActionCapillary Action

Liquids display a behavior called Liquids display a behavior called capillary action because their capillary action because their molecules are more or less molecules are more or less attracted to the surface they attracted to the surface they contact than they are to contact than they are to themselves. Water molecules, for themselves. Water molecules, for instance, are more attracted to instance, are more attracted to glass than they are to one glass than they are to one another. Water will therefore another. Water will therefore climb up a narrow glass tube that climb up a narrow glass tube that is dipped into a beaker of water, is dipped into a beaker of water, because the water would rather because the water would rather be in contact with the glass than be in contact with the glass than with itself. Mercury molecules, on with itself. Mercury molecules, on the other hand, are more the other hand, are more attracted to each other than they attracted to each other than they are to glass. Mercury will avoid are to glass. Mercury will avoid contact with a narrow glass tube contact with a narrow glass tube that is dipped into a beaker of that is dipped into a beaker of mercury.mercury.

© Microsoft Corporation. All Rights © Microsoft Corporation. All Rights Reserved.Reserved.

Microsoft ® Encarta ® 2006.Microsoft ® Encarta ® 2006. © 1993- © 1993-2005 Microsoft Corporation. All rights 2005 Microsoft Corporation. All rights

reserved.reserved.

More substances will dissolve in More substances will dissolve in water than any other liquid.  This water than any other liquid.  This includes other polar substances includes other polar substances (such as sugar) and ionic compounds (such as sugar) and ionic compounds (such as salt).  When a substance (such as salt).  When a substance dissolves in a liquid, the mixture is dissolves in a liquid, the mixture is termed a solution. The dissolved termed a solution. The dissolved substance (in this case salt or sugar) substance (in this case salt or sugar) is the solute, and the liquid that does is the solute, and the liquid that does the dissolving (in this case water) is the dissolving (in this case water) is the solvent. Water is an excellent the solvent. Water is an excellent solvent for many substances solvent for many substances because of its polar bonds. because of its polar bonds.

Water is considered to be the universal Water is considered to be the universal solvent. solvent. 

        When salt crystals are placed in water, the When salt crystals are placed in water, the slightly positive and negative ends of the water slightly positive and negative ends of the water molecules attract the ions in the crystal.  The ionic molecules attract the ions in the crystal.  The ionic bonds holding the sodium and chlorine ions bonds holding the sodium and chlorine ions together are broken and the ions are pulled into together are broken and the ions are pulled into solution.solution.

http://www.ccs.k12.in.us/chsBS/kons/kons/physical_properties_of_water.htm

• When a sugar crystal is placed in water, the When a sugar crystal is placed in water, the slightly positive and negative ends of the water slightly positive and negative ends of the water molecule attract the sugar molecules in the molecule attract the sugar molecules in the crystal (they are also polar) and pull them into crystal (they are also polar) and pull them into solution.solution.

http://www.ccs.k12.in.us/chsBS/kons/kons/physical_properties_of_water.htm

Water – the universal Water – the universal solventsolvent

• Evaporation: Evaporation: As water is heated by the sun, it's surface molecules become As water is heated by the sun, it's surface molecules become sufficiently energized to break free of the attractive force sufficiently energized to break free of the attractive force binding them together, and then binding them together, and then evaporateevaporate and rise as and rise as invisible vapour in the atmosphere.invisible vapour in the atmosphere.

• Transpiration: Transpiration: Water vapour is also emitted from plant leaves by a process Water vapour is also emitted from plant leaves by a process called transpiration. Every day an actively growing plant called transpiration. Every day an actively growing plant transpirestranspires 5 to 10 times as much water as it can hold at once. 5 to 10 times as much water as it can hold at once.

• Condensation: Condensation: As water vapour rises, it cools and eventually As water vapour rises, it cools and eventually condensescondenses, , usually on tiny particles of dust in the air. When it condenses it usually on tiny particles of dust in the air. When it condenses it becomes a liquid again or turns directly into a solid (ice, hail or becomes a liquid again or turns directly into a solid (ice, hail or snow). These water particles then collect and form clouds.snow). These water particles then collect and form clouds.

• Precipitation: Precipitation: Precipitation in the form of rain, snow and hail comes Precipitation in the form of rain, snow and hail comes from clouds. Clouds move around the world, propelled from clouds. Clouds move around the world, propelled by air currents. For instance, when they rise over by air currents. For instance, when they rise over mountain ranges, they cool, becoming so saturated mountain ranges, they cool, becoming so saturated with water that water begins to fall as rain, snow or with water that water begins to fall as rain, snow or hail, depending on the temperature of the surrounding hail, depending on the temperature of the surrounding air.air.

• Runoff: Runoff: Excessive rain or snowmelt can produce overland flow Excessive rain or snowmelt can produce overland flow to creeks and ditches. Runoff is visible flow of water in to creeks and ditches. Runoff is visible flow of water in rivers, creeks and lakes as the water stored in the rivers, creeks and lakes as the water stored in the basin drains out.basin drains out.

• Percolation: Percolation: Some of the precipitation and snow melt moves Some of the precipitation and snow melt moves downwards downwards percolatespercolates or or infiltratesinfiltrates through cracks, through cracks, joints and pores in soil and rocks until it reaches the joints and pores in soil and rocks until it reaches the water table where it becomes groundwater.water table where it becomes groundwater.

• Groundwater: Groundwater: Subterranean water is held in cracks and Subterranean water is held in cracks and pore spaces. Depending on the geology, pore spaces. Depending on the geology, the groundwater can flow to support the groundwater can flow to support streams. It can also be tapped by wells. streams. It can also be tapped by wells. Some groundwater is very old and may Some groundwater is very old and may have been there for thousands of years.have been there for thousands of years.

• Water table:Water table:The water table is the level at which water The water table is the level at which water stands in a shallow well.stands in a shallow well.

Hydrogen Ion exchangeHydrogen Ion exchange

Positively charged hydrogen ions (H+) Positively charged hydrogen ions (H+) can spontaneously move from one can spontaneously move from one water molecule to another, thereby water molecule to another, thereby creating two ionic species. Since the creating two ionic species. Since the process is rapidly reversible, process is rapidly reversible, hydrogen ions are continually hydrogen ions are continually shuttling between water molecules. shuttling between water molecules. Pure water contains a steady state Pure water contains a steady state concentration of hydrogen ions and concentration of hydrogen ions and hydroxyl ions (both 10-7 M).hydroxyl ions (both 10-7 M).

pHpHChemically, water is Chemically, water is amphotericamphoteric — — i.e., it is able to act as either an acid or i.e., it is able to act as either an acid or a base. Occasionally the term a base. Occasionally the term hydroxic acid is used when water acts hydroxic acid is used when water acts as an acid in a chemical reaction. At a as an acid in a chemical reaction. At a pH of 7 (neutral), the concentration of pH of 7 (neutral), the concentration of hydroxide ions (OH-) is equal to that hydroxide ions (OH-) is equal to that of the hydronium (H3O+) or hydrogen of the hydronium (H3O+) or hydrogen (H+) ions. If the equilibrium is (H+) ions. If the equilibrium is disturbed, the solution becomes disturbed, the solution becomes acidic (higher concentration of acidic (higher concentration of hydronium ions) or basic (higher hydronium ions) or basic (higher concentration of hydroxide ions). The concentration of hydroxide ions). The acidity of a solution is defined by the acidity of a solution is defined by the concentration of H+ ions it possesses. concentration of H+ ions it possesses. For convenience we use the pH scale, For convenience we use the pH scale, where pH = _log10[H+]. For pure water where pH = _log10[H+]. For pure water [H+] = 10_7 moles/liter.[H+] = 10_7 moles/liter.

http://www.accessexcellence.org/RC/VL/GG/waterProps2.html

AcidsAcidsSubstances that release hydrogen Substances that release hydrogen ions into solution are called acids. ions into solution are called acids. Many of the acids important in the Many of the acids important in the cell are only partially dissociated, cell are only partially dissociated, and they are therefore weak acids-and they are therefore weak acids-for example, the carboxyl group (-for example, the carboxyl group (-COOH), which dissociates to give a COOH), which dissociates to give a hydrogen ion in solution. This is a hydrogen ion in solution. This is a reversible reaction.reversible reaction.

BasesBasesSubstances that reduce the number Substances that reduce the number of hydrogen ions in solution are of hydrogen ions in solution are called bases. Some bases, such as called bases. Some bases, such as ammonia, combine directly with ammonia, combine directly with hydrogen ions.hydrogen ions. Other bases, such as Other bases, such as sodium hydroxide, reduce the sodium hydroxide, reduce the number of H+ ions indirectly, by number of H+ ions indirectly, by making OH- ions that then combine making OH- ions that then combine directly with H+ ions to make H2O. directly with H+ ions to make H2O. Many bases found in cells are Many bases found in cells are partially dissociated and are termed partially dissociated and are termed weak bases. This is true of weak bases. This is true of compounds that contain an amino compounds that contain an amino group (-NH2), which has a weak group (-NH2), which has a weak tendency to reversibly accept an H+ tendency to reversibly accept an H+ ion from water, increasing the ion from water, increasing the quantity of free OH- ions.quantity of free OH- ions.http://www.accessexcellence.org/RC/VL/GG/

waterProps2.html

A common misconception about water is that it is a powerful A common misconception about water is that it is a powerful conductor of electricity, with risks of electrocution explaining this conductor of electricity, with risks of electrocution explaining this popular belief. Any electrical properties observable in water are due popular belief. Any electrical properties observable in water are due to the ions of mineral salts and carbon dioxide dissolved in it. Water to the ions of mineral salts and carbon dioxide dissolved in it. Water does self-ionize where two water molecules become one hydroxide does self-ionize where two water molecules become one hydroxide anion and one hydronium cation, but not enough to carry enough anion and one hydronium cation, but not enough to carry enough electric current to do any work or harm for most operations. Pure electric current to do any work or harm for most operations. Pure water can also be electrolyzed into oxygen and hydrogen gases but water can also be electrolyzed into oxygen and hydrogen gases but without any dissolved ions, this is a very slow process and thus very without any dissolved ions, this is a very slow process and thus very little current is conducted. The taste advertised in spring water or little current is conducted. The taste advertised in spring water or mineral water derives from the minerals present, while pure H2O is mineral water derives from the minerals present, while pure H2O is tasteless. As such, purity in spring and mineral water refers to tasteless. As such, purity in spring and mineral water refers to purity from toxins, pollutants, and microbes.purity from toxins, pollutants, and microbes.

Pure water (de-ionised) is an excellent insulator as it Pure water (de-ionised) is an excellent insulator as it does not conduct electricity at all.does not conduct electricity at all.