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8.4 Water

8.4.1 Water is distributed on Earth as a solid, liquid and gas

y Define the terms solute, solvent and solutionSolution a homogeneous mixture of two or more substances

Solute a substance that is dissolved in another substance or the component of a solution present

in a lower amount.

Solvent a substance which can dissolve another substance or the component of a solutionpresent in a greater amount.

y Identify the importance of water as a solventWater is essential as a reactant and a solvent in the cycling of C, O, N, P and S in nature. It also

allows biological processes to occur in aqueous solutions and serves as a transport system for

nutrients and waste products in living organisms.

y Compare the state, percentage and distribution of water in the biosphere, lithosphere,hydrosphere and atmosphere

Biosphere Lithosphere Hydrosphere Atmosphere

Percentage ofwater 70% Variable 96-100% 0-5%

State of water Liquid Liquid Liquid gasWater ofcrystallisation

Solid ice

Solid ice

y Outline the significance of the different states of water on Earth in terms of water as: A constituent of cells and its role as both a solvent and a raw material in metabolism

Water is the predominant constituent of cells, functioning as:

A solvent for biochemical reactions that sustain life Photosynthesis : 6CO2 (g) + 6H2O (l) C6H12O6 (aq) + 6O2 (g) Respiration : C6H12O6 (aq) + 6O2 (g)6CO2 (g) + 6H2O (l) A raw material for metabolism (e.g. in plants) A transport medium for nutrients and wastes A thermal buffer that resists large temperature fluctuations

A habitat in which temperature extremes are less than nearby terrestrial habitats Water serves as a natural habitat for many organisms Major advantage: temperatures vary much less in water than on land Therefore marine animals are protected from experiencing temperature extremes

An agent of weathering of rocks As water freezes and thaws repeatedly, the stress due to expansion and contraction

can cause rocks to fragment

Rain, rivers and glaciers erode loose material, carving through mountains andshaping the landscape into its present form.

Weathering is the physical and chemical breakdown and decay of rocks. Erosion refers to the processes by which rock fragments are transported by rivers,

oceans and wind.

Both as liquid and solid a natural resource for humans and other organisms Water is critical to the survival of humans and other organisms.

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Human use water f r drin in , bat in , washin cl thes, cleanin , agricultureandinindustrialprocesses

Water also serves as a source ofentertainment and en oymentfor manypeople (eg.fishing, swimming, sailing and other water sports)

8. . he wide distribution andimportance ofwater on Earth is a consequence ofits molecular structure

and hydrogen bonding

y ConstructLewis electrondot structures ofwater, ammonia and hydrogen sulfide to identifythedistribution ofelectrons

Water Ammonia Hydrogen sulfide

otalnumber ofelectronpairs

Arrangement Bondingpairs

Lone pairs Shape ofmolecule

Examples

2 Linear 2 0 Linear BeCl2,HCN, BeF2

3

rigonalplanar

3 0

rigonal BCl2,CH2O

4

etrahedral 4 0 etrahedral CH4, SiF43 1 Pyramidal NH3,PCl32 2 Bent H2O, H2S

5

rigonalbipyramidal

5 0

rigonalbipyramidal

PCl3

6 octahedral 6 0 Octahedral SF6

y Compare the molecular structure ofwater, ammonia and hydrogen sulfide,the differences intheir molecular shapes andintheir melting and boiling points

S

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y Describe hydrogen bonding between molecules Hydrogen bonding is a special type of dipole-dipole bond only occurring between H

with N, O, or F of a neighbouring molecule

Hydrogen bonding is the strongest out of the intermolecular forces As the electron from the hydrogen is drawn towards and N, O or F atom within the

molecule, the positive hydrogen nucleus is left. This enables the slightly positive

hydrogen to be attracted to the slightly negative N, O or F of a neighbouring molecule.

The size of N, O and F is small enough (i.e. has only 2 electron shells) for the barepositive hydrogen nucleus to interact with the lone electron pairs of a neighbouringmolecule contain N, O or F.

Hydrogen bond is the electrostatic attraction between a hydrogen proton and loneelectron pairs.

y Identify the water molecule as a polar molecule In H2O, there are 2 bonded electron pairs and 2 lone electron pairs There are dipole bonds between the oxygen and hydrogen atoms This is due to oxygen being more electronegative than hydrogen Hence the electrons being shared between the oxygen and hydrogens will be pulled more

towards the nucleus of oxygen. This makes the oxygen slightly negative (-) while the hydrogens are slightly positive ( +) Hence the water molecule is a polar molecule with a net dipole.

y Describe the attractive forces between polar molecules as dipole-dipole forces Covalent bonds in which the electrons are unequally shared are called polar covalent

bondsand is due to the differing electronegativities of different atoms.

A dipole is a polar molecule which have a net dipole (imbalance) As polar molecules have a slightly positive and negative end, they are able to line up so

that the positive end of one molecule attracts the negative end of another molecule.

This electrostatic attraction of polar molecules is called dipole-dipole forces.y Explain the following properties of water in terms of its intermolecular forces:

Surface tension In water, intermolecular forces exert different effects on a molecule at the surface

compared to one in the interior.

Interior molecules are attracted equally by other molecules on all sides ( isotropic). Surface molecules are only attracted to others below and to the sides (anisotropic).

As a result, surface molecules experience a net attraction downward This pulls the molecules inward and closer together, making the liquid surface

behave like an elastic skin under tension (with minimised surface area).

To increase surface area, molecules must move to the surface by breaking someinterior attraction and this requires energy.

Surface tension is a measure of energy needed to increase the surface area of aliquid by a unit amount (units=j/m 2).

Water has high surface tension due to strong intermolecular forces (hydrogenbonds).

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In general, the stronger the intermolecular forces, the greater the liquids surfacetension.

Viscosity It is the resistance to flow A liquids viscosity depends on the size/shape of the molecules and strength of its

intermolecular forces.

As water has small and compact molecules that flow readily over each other, it haslow viscosity as compared to highly viscous liquids like motor oil.

The stronger the intermolecular forces between molecules the more resistancethere is to flow.

Water with its strong hydrogen bonding has a much higher resistance to flow thanits small molecular size might suggest.

Boiling and melting pointsDue to the strong hydrogen bonds in between water molecules, greater energy input is

required to break these bonds, hence accounting for its relatively higher melting and

boiling points as compared to other liquids and solvents.

8.4.3 Water is an important solvent

y Explain changes, if any, to particles and account for those changes when the following types ofchemicals interact with water:

o A soluble ionic compound such as sodium chlorideIf the attractive forces between water and the ions are stronger than the attractive forces

between the positive and negative ions, then the ionic solid dissolves in water.

o A soluble molecular compound such as sucroseMost molecular substances such as hexane, kerosene, paraffin wax and chloroform are

insoluble in water. However, some molecular substances like sucrose are soluble. When

soluble molecular compounds are dissolved in water, the crystals of the solid break up and

disperse throughout the solvent, breaking right down to the molecular level. So a solutionof sucrose in water consists of individual sucrose molecules dispersed throughout the

solvent.

A molecular substance only dissolves in water if water can form stronger attachments to

the molecules than the intermolecular forces in the molecular substance. Generally the

only molecular substances which dissolve in water are ones that have very polar

molecules or ones that can form hydrogen bonds with water.

o A soluble or partially soluble molecular element or compound such as iodine, oxygen orhydrogen chloride

Some non-polar molecular substance such as oxygen and nitrogen gases and iodine are

slightly soluble in water. The solvent-solute interactions are weak dispersion forces. Asthese interactions are weak, the solubilities of such substances are quite low.

o A covalent network structure substance such as silicon dioxideCovalent latticesare insoluble in water as the hydrogen bonds in water are not strong

enough to breakstrong covalent bonds between the atoms in these covalent lattices.

o A substance with large molecules, such as cellulose or polyethyleneSome molecules of substances are so large and held to one another in such orderly

fashions by hydrogen bonds that water is unable to separate them from one another. (e.g.

cellulose, polyethylene)

However, there are many proteins (including enzymes) and some carbohydrates (amylase,

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glycogen) are soluble in water despite having large molecular weights. These molecules

are very complex structures and do not pack together neatly into crystals, so water is able

to separate molecules and form solutions.

y Analyse the relationship between the solubility of substances in water and the polar nature of thewater molecule

The most important factors for causing solubility in water are the highly polar nature of the

water molecule and the ability of water to form hydrogen bonds with other molecules. Polar

substances dissolve in polar solvents and not in non-polar solvents while non-polar substancesdissolve in non-polar solvents and dont dissolve polar ones. As water is a polar solvent, it will

dissolve substances which have polar molecules.

8.4.4 The concentration of salts in water will vary according to their solubility, and precipitation can

occur when the ions of an insoluble salt are in solution together

y Identify some combinations of solutions which will produce precipitates, using solubility dataBa(NO3)2 (aq)+ ZnSO4 (aq) ZN(NO3)2 (aq) +BaSO4 (S)

Ba2+ (aq) + SO42-

(aq) BaSO4 (s) (BaSO4 (s) is a white precipitate)

AgNO3 (aq) + NaCl (aq) NaNO3 (aq) + AgCl(s)

Ag+ (aq) + Cl-

(aq) AgCl (S) (AgCl (S) is a white precipitate)

CuSO4 (aq) + 2NaOH (aq) Na2SO4 (aq) + Cu(OH)2 (s)

Cu2+ (aq) + (OH)-(aq) Cu(OH)2 (s) (Cu(OH)2 (s) is a blue precipitate)

Pb(NO3)2 (aq) + 2KI (aq) 2KNO3 (aq) + PbI2 (s)

Pb2+ (aq) + 2I-

(aq) PBI2 (s) (PBI2 (s) is a yellow precipitate)

Na2So4 (aq) + Ba(NO3)2 (aq) 2NaNO3 (aq) + BaSO4 (s)

Ba2+ (aq) + SO42-

(aq)) BaSO4 (s) (BaSO4 (s) is a white precipitate)

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y Describe a model that traces the movement of ions when solution and precipitation occur

Before the two solutions are mixed, they remain as free moving electrons where the electrostatic

attraction between the water molecules and the ions is greater than the electrostatic attraction

between the ions. When mixed, a precipitate may form. A precipitation reaction only occurs

when the force of attraction between two ions is stronger than the electrostatic attraction

between the water molecules and the ions. In the above diagram, the electrostatic attraction

between Ag+ (aq) and Cl-

(aq) ions is stronger than the electrostatic pull of the polar water

molecules. Hence the precipitate AgCl is formed.

y Identify the dynamic nature of ion movement in a saturated dissolution

Saturated solution: solution in which no more of a particular solute will dissolve in a particular

solvent

In a saturated solution, a dynamic equilibrium exists between dissolution and precipitation. As 2

ions break off the ionic crystal and dissolve in the solution, another 2 ions will precipitate out.These processes occur at the same rate such that there is no net change in concentration.

y Describe the molarity of a solution as the moles of solute per litre of solution using

Molarity is one type of measurement for concentration. The molarity of a solution is equal to the

number of moles of solute per litre of solution.

y Explain why different measurements of concentration are importantA variety of ways of expressing concentration is used because each method has advantages forparticular situations. In commerce and industry and in shopping where the main concern is withhow much solute is present, then mass per unit volume is very convenient. In environmentalcontexts concentrations are usually very low. Masses per unit volume or percent compositions

generally lead to very small numbers so parts per million (ppm)gives more manageable numbers.

8.4.5 Water has a higher heat capacity than many other liquids

y Explain what is meant by the specific heat capacity of a substanceThe specific heat capacity of a substance is the amount of heat required to raise the temperature

of 1 gram of the substance by 1C (or 1K).

y Compare the specific heat capacity of water with a range of other solvents

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Liquid Specific heat capacity (J/C/g)

Water 4.18

Ethanol 2.44

Ethylene glycol 2.39

Glycerol 2.38

Acetone 2.17

Chloroform 0.96

Hexane 2.26

Mercury 0.14

y Explain and use the equation This equation is used to measure heat energy changes.

H: change in heat energy, in joules (J)

m: mass of substance, in grams (g)

C: specific heat capacity, in J/C/g

T: temperature change, in digress Celsius (C)

o If the temperature goes up (+T), the energy change is considered negative (-H)o

If the temperature drops (-T), the energy change is considered positive (+H)

y Explain how waters ability to absorb heat is used to measure energy changes in chemicalreactionsCalorimeter: equipment used to measure heat energy.

Since many chemical processes occur in water and due to waters high

specific heat capacity, it is often used in calorimeters as the working

fluid or the medium used to absorb the heat energy.

Limitations of calorimeter:

o It is assumed that the calorimeter itself does not absorb a significantamount of heat energy of the reaction

o It is assumed that there is no heat lost or gained between the calorimeter and itssurroundings.

oIt is assumed that the specific heat capacity of the solution reacting in the calorimeter isthe same as water (i.e. 4.18 J/C/g)

Heat of solution: energy change that occurs when 1 mole of solute dissolves in water.

Hsol is negative if energy is released. (exothermic: the calorimeter temperature rises)

e.g. soluble hydroxides like NaOH, KOH

Hsol is positive if energy is absorbed. (endothermic: the calorimeter temperature falls)

e.g. ammonium nitrate NH4NO3 (aq), ammonium chloride NH4Cl (aq)

y Describe dissolutions which release heat as exothermic and give examplesExothermic reactions are reactions that produce and release energy

When chemicals lose energy, the temperature in the calorimeter rises because the energy release

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heats up the water in the calorimeter. When the temperature rises, the energy quantity is

considered negative.

e.g. freezing water, precipitation

y Describe dissolutions which absorb heat as endothermic and give examplesEndothermic reactions are reactions that absorb energy where energy must be supplied in order

to make the reaction occur.

When chemicals absorb energy, the temperature in the calorimeter drops because the energy

absorbed from its surroundings cools down the water in the calorimeter. When the temperaturedrops, the energy quantity is considered positive.

e.g. melting ice cubes, evaporating liquid water

y Explain why waters ability to absorb heat is important to aquatic organisms and to life on earthgenerally

Water plays a significant role to weather, climate and life on Earth due to its high specific heat

capacity. Water is able to absorb a large amount of energy from the Sun without much

temperature change, keeping water habitats at a very stable temperature. This means that aquatic

organisms do not require complex temperature control mechanisms because their habitat remains

quite stable. More importantly, the oceans absorb and transport huge quantities of heat from the

tropics towards the poles via ocean currents.y Explain what is meant by thermal pollution and discuss the implications for life if a body of

water is affected by thermal pollution

o Many aquatic organisms rely on a fairly constant water temperature to aid propermetabolism, survival and reproduction.

o Thermal pollution is the discharge of large quantities of hot water into a river or lakesufficient to cause a significant increase in the waters temperature (2-5C).

o River or lake water is used in the industry for cooling and when generating electricity.o When the cooling water is discharged back into a body of water it has absorbed

substantial amounts of heat energy.

oThermal pollution has adverse consequences for aquatic life.

o In particular, the solubility of oxygen decreases as water temperature increases. Lessdissolved oxygen causes stress to aquatic organisms.

o Other detrimental effects of rising water temperature: Increased metabolic rates further demand of oxygen Fish eggs do to develop properly or hatch with a sudden change in temperature False temperature cues given to aquatic life