Water: Essential to lifeWater: Essential to life

Chapter 10Chapter 10

A molecule essential to lifeA molecule essential to life

• Water is the most abundant liquid on Water is the most abundant liquid on earth, covering over 70% of the earth, covering over 70% of the planetplanet

The water cycleThe water cycle

The reactions of lifeThe reactions of life

• PhotosynthesisPhotosynthesis6H6H220(l) + 6CO0(l) + 6CO22 (g) C (g) C66HH12120066 (s) + 6O (s) + 6O22

(g)(g)

• RespirationRespiration

CC66HH12120066 (s) + 6O (s) + 6O22 (g) 6H (g) 6H220(l) + 6CO0(l) + 6CO22 (g) (g)

Without these two reactions life on earth could Without these two reactions life on earth could not be sustained. Water is essential for both not be sustained. Water is essential for both of these reactionsof these reactions

Chlorophyll

Sunlight

Other functions of waterOther functions of water• It provides a system to transport nutrients It provides a system to transport nutrients

and soluble wastes.and soluble wastes.– It dissolves a range of substances which are It dissolves a range of substances which are

transported around the body.transported around the body.

• It provides a system to transfer heat.It provides a system to transfer heat.– Water can store large amounts of heat energy, Water can store large amounts of heat energy,

it transports heat energy from cells to the body it transports heat energy from cells to the body surface where it can be removed.surface where it can be removed.

• It cools the body.It cools the body.– When water evaporates from the skin, it When water evaporates from the skin, it

absorbs a large amount of heat energy from absorbs a large amount of heat energy from the body allowing the body to cool down.the body allowing the body to cool down.

Properties of waterProperties of water

• Water is a covalent molecular compound.Water is a covalent molecular compound.

• Molecular formula HMolecular formula H22O. O.

• The O-H bonds are polar, with the O atom The O-H bonds are polar, with the O atom having the larger share of the bonding having the larger share of the bonding electrons.electrons.

• The forces between water molecules are The forces between water molecules are hydrogen bonds, which are strong in hydrogen bonds, which are strong in comparison to other types of comparison to other types of intermolecular bondsintermolecular bonds

Water is uniqueWater is unique• Water is the only substance Water is the only substance

commonly found in all three states commonly found in all three states on earth.on earth.

Looking at this graph can you think of anotherreason that water is so unique.?• Water exists as a liquid over

a temperature range commonlyfound on earth.

• The melting and boiling tempof water are significantly higherthan those of other molecules of similar size.

Why is this so?Why is this so?• In ice each molecule forms In ice each molecule forms

hydrogen bonds to FOUR other hydrogen bonds to FOUR other molecules. molecules.

• A lot of energy is required to break A lot of energy is required to break these four bonds. these four bonds.

• When ice melts enough energy is When ice melts enough energy is added to break added to break somesome of these of these bonds.bonds.

• When water is boiled, When water is boiled, allall the the hydrogen bonds are broken. hydrogen bonds are broken.

• A significant amount of energy to A significant amount of energy to overcome these strong hydrogen overcome these strong hydrogen bonds.bonds.

High latent heat valuesHigh latent heat values• Latent heat measures the energy needed Latent heat measures the energy needed

to change the state of a substance at its to change the state of a substance at its melting or boiling temperature:melting or boiling temperature:– Latent heat of fusionLatent heat of fusion is the amount of is the amount of

energy needed to change a fixed amount of energy needed to change a fixed amount of water from a solid to a liquid at 0water from a solid to a liquid at 0°C°C

– Latent heat of vaporisationLatent heat of vaporisation of water is the of water is the amount of energy needed to change a fixed amount of energy needed to change a fixed amount of water from liquid to a gas at 10amount of water from liquid to a gas at 1000°C°C

Your TurnYour Turn

• Page 194Page 194

• Question 3Question 3

• Question 4 Question 4 – You may have to think about 4cYou may have to think about 4c

4c4c• As it freezes, water expands, unlike most liquids. As it freezes, water expands, unlike most liquids.

This is because of hydrogen bonding. Each This is because of hydrogen bonding. Each molecule is surrounded by four others in what is molecule is surrounded by four others in what is almost a crystal-type situation. (See graph below, almost a crystal-type situation. (See graph below, which shows the variation in density of water with which shows the variation in density of water with temperature.) Therefore, ice is less dense than temperature.) Therefore, ice is less dense than liquid water, and it floats on liquid water. (For liquid water, and it floats on liquid water. (For most liquids, the solid is denser than the liquid.) most liquids, the solid is denser than the liquid.) This is good news for fish, but not good news for This is good news for fish, but not good news for travellers on the travellers on the TitanicTitanic!!

Solutes and solventsSolutes and solvents

• When a solid, liquid or gas dissolves When a solid, liquid or gas dissolves in water, an aqueous solution is in water, an aqueous solution is formed.formed.

• The dissolved substance is the solute The dissolved substance is the solute and the water is the solvent.and the water is the solvent.

SolutionsSolutions

• Solutions have the following Solutions have the following important characteristics:important characteristics:– They are homogenous; that is, the solute They are homogenous; that is, the solute

and the solvent can not be distinguished and the solvent can not be distinguished from one another.from one another.

– The dissolved particles are too small to The dissolved particles are too small to see.see.

– The proportion of dissolved solutes varies The proportion of dissolved solutes varies from one solution to another.from one solution to another.

Water as a solventWater as a solvent

• The polar nature of water molecules The polar nature of water molecules enables water to dissolved a large enables water to dissolved a large number of substances.number of substances.

• Due to this water is not found pure in Due to this water is not found pure in nature.nature.

• Not all substances dissolve in water, Not all substances dissolve in water, however. Oils, fats and other non-however. Oils, fats and other non-polar substances will not dissolve in polar substances will not dissolve in water. Also many gases have low water. Also many gases have low solubility in water.solubility in water.

What dissolves and what What dissolves and what doesn’tdoesn’t• When one substance dissolves in another, the When one substance dissolves in another, the

following process occur:following process occur:– The particles of the solute are separated from one The particles of the solute are separated from one

another.another.– The particles of the solvent are separated from The particles of the solvent are separated from

one another.one another.– The solute and solvent particles attract each other.The solute and solvent particles attract each other.

• A solute will dissolve if the attraction between A solute will dissolve if the attraction between the solute and solvent particles is strong the solute and solvent particles is strong enough to compete wit the solute-solute and enough to compete wit the solute-solute and solvent-solvent forces of attraction.solvent-solvent forces of attraction.

Substances that dissolveSubstances that dissolve

• Polar covalent Polar covalent compounds that compounds that can form hydrogen can form hydrogen bonds with water.bonds with water.

• Polar covalent Polar covalent molecular molecular compounds that compounds that ionise.ionise.

• Ionic compounds.Ionic compounds.

Polar covalent compounds that Polar covalent compounds that can form hydrogen bonds with can form hydrogen bonds with waterwater

• When ethanol dissolves in water:When ethanol dissolves in water:– Hydrogen bonds between water molecules are brokenHydrogen bonds between water molecules are broken– Hydrogen bonds between ethanol molecules are brokenHydrogen bonds between ethanol molecules are broken– Hydrogen bonds form between ethanol and water Hydrogen bonds form between ethanol and water

molecules.molecules.

CC22HH55OH(l) OH(l) CC22HH55OH(aq) OH(aq) H2O

Non-Polar moleculesNon-Polar molecules

• Non-polar substances are insoluble in Non-polar substances are insoluble in water because the water molecules form water because the water molecules form hydrogen bonds with each other in hydrogen bonds with each other in preference to the weak attractions with preference to the weak attractions with non-polar molecules.non-polar molecules.

• The larger the non-polar molecule is the The larger the non-polar molecule is the less soluble it is in water.less soluble it is in water.

Comparing VitaminsComparing Vitamins

Polar covalent molecular Polar covalent molecular compounds that ionisecompounds that ionise• Some compounds have one of more Some compounds have one of more

covalent bonds that are so highly polarised covalent bonds that are so highly polarised that they break when the compounds is that they break when the compounds is placed in water.placed in water.

• Such bonds break as a result of the Such bonds break as a result of the electrostatic forces of attraction between electrostatic forces of attraction between the solute molecules and the water the solute molecules and the water molecules.molecules.

• This attraction results in the H-Cl bond This attraction results in the H-Cl bond breaking, both electrons go with the breaking, both electrons go with the chlorine atom and a Hchlorine atom and a H++ ion forms a ion forms a covalent bond with water.covalent bond with water.

• This process is called ionisation and can be This process is called ionisation and can be represented by the following equation.represented by the following equation.

HCl(aq) + HHCl(aq) + H22O(l) O(l) HH33OO++(aq) + Cl(aq) + Cl--(aq)(aq)

• A HA H33OO++ ion is called a hydronium ion. ion is called a hydronium ion.

Polar covalent molecular Polar covalent molecular compounds that ionisecompounds that ionise

In summary when polar covalent In summary when polar covalent molecules ionise in water.molecules ionise in water.

• Polar covalent bonds within molecules are Polar covalent bonds within molecules are broken, producing hydrogen ions and anions.broken, producing hydrogen ions and anions.

• A covalent bond forms between each HA covalent bond forms between each H++ and and a Ha H22O molecule giving HO molecule giving H33OO++ ions. Ion-dipole ions. Ion-dipole attractions between the newly formed ions attractions between the newly formed ions and the polar water molecules are formed.and the polar water molecules are formed.

• Other polar covalent molecular compounds Other polar covalent molecular compounds that ionise in water include the common acids that ionise in water include the common acids nitric acid, sulfuric acid and ethanoic acid.nitric acid, sulfuric acid and ethanoic acid.

Ionic compoundsIonic compounds• Ionic solids contain +ve and –ve ions held Ionic solids contain +ve and –ve ions held

in a 3D lattice by strong electrostatic in a 3D lattice by strong electrostatic forces.forces.

• When an ionic solid such as NaCl is placed When an ionic solid such as NaCl is placed in water, the +ve ends of the water in water, the +ve ends of the water molecule are attracted to the –ve chloride molecule are attracted to the –ve chloride ions. The –ve ends of the water molecule ions. The –ve ends of the water molecule are attracted to the +ve sodium ions. are attracted to the +ve sodium ions.

Ionic CompoundsIonic Compounds

• The partially charged water molecules The partially charged water molecules start pulling the sodium and chloride ions start pulling the sodium and chloride ions on the outer part of the ionic lattice apart on the outer part of the ionic lattice apart and dragging them into the surrounding and dragging them into the surrounding solution.solution.

DissociationDissociation

• When an ionic compound dissolves in When an ionic compound dissolves in water, positive and negative ions in an water, positive and negative ions in an ionic lattice are separated from one ionic lattice are separated from one another. another.

• This process is known as dissociation.This process is known as dissociation.

• It can be represented by the equation:It can be represented by the equation:

NaCl(s) NaNaCl(s) Na++(aq) + Cl(aq) + Cl--(aq)(aq)

H2

0

Dissociation EquationsDissociation Equations

• Dissociation equations must balance. Dissociation equations must balance.

• The number of elements on each side The number of elements on each side must balance.must balance.

• The charges on each side must balance.The charges on each side must balance.

• The equation breaks into its cation and The equation breaks into its cation and anion in the presence of water.anion in the presence of water.

• For example:For example:

KK22COCO33(s) 2K(s) 2K++(aq) + CO(aq) + CO332-2-(aq)(aq)

H20

Insoluble ionic substancesInsoluble ionic substances

• Not all ionic substances are soluble, Not all ionic substances are soluble, limestone (CaCOlimestone (CaCO33) is not soluble. ) is not soluble.

• For insoluble ionic substances, the For insoluble ionic substances, the energy required to separate the ions energy required to separate the ions from the lattice is much greater than from the lattice is much greater than the energy released when the ions the energy released when the ions are dissolved in water. are dissolved in water.

• The ions tend to stay in the lattice.The ions tend to stay in the lattice.

So what is soluble?So what is soluble?

All group 1 metals are soluble.

Worked example 10.4aWorked example 10.4a

• Predict, giving reason, whether each of Predict, giving reason, whether each of the following substances is likely to be the following substances is likely to be water soluble.water soluble.

(a)(a)Copper(II) nitrate (Cu(NOCopper(II) nitrate (Cu(NO33))22))

(b)(b)Glycerol (CHGlycerol (CH22(OH)CH(OH)CH(OH)CH(OH)CH22(OH)(OH)

(c)(c)Octane (COctane (C88HH1818))

(d)(d)Nitric acid (HNONitric acid (HNO33))

(e)(e)Barium carbonate (BaCOBarium carbonate (BaCO33))