Water – The Unique Water – The Unique SubstanceSubstance

VCE Chemistry VCE Chemistry

Unit 2: Environmental ChemistryUnit 2: Environmental Chemistry

Area of Study 1 – WaterArea of Study 1 – Water

• Water is unique!

• Unlike other natural substances all three states can co-exist.

• Liquid water has a higher surface tension than other natural liquids

and expands when it freezes.

• It is the ultimate heat bank - it can both store and transfer heat.

• In addition, it can dissolve and transport many substances, and even

purify itself by constant cycling and recycling through the atmosphere.

Water – An IntroductionWater – An Introduction

• Water is essential to life.

• It is a key reactant in many biochemical reactions.

• These include the breakdown of starch and protein molecules in the

digestive systems of animals, and the production of glucose during

photosynthesis in plants.

• It plays a key role in controlling the temperature of body cells so that

these vital processes can occur.

• Water also carries nutrients to our cells, enables substances within

cells to collide so they can react and carries away the waste products

of these cell reactions.

Water – An IntroductionWater – An Introduction

The Water CycleThe Water Cycle

• The water on Earth exists in solid, liquid and gaseous states and

readily changes from one state into another.

• The water cycle could be said to begin with evaporation of water from

the oceans, solar energy being the major energy source.

• Water vapour in the air is transported around the globe until it

condenses to form clouds.

• It precipitates as rain water, or occasionally as ice crystals in hail or

snow, and falls to the ground.

The Water CycleThe Water Cycle

• To understand why water is a special compound, we need to examine

its structure and bonding.

• Water is described as a covalent molecular compound, with a

molecular formula of H2O.

• The oxygen–hydrogen bonds are polar, with the oxygen atom

having the larger share of the bonding electrons, due to its much

higher electronegativity.

• The forces between water molecules are hydrogen bonds, which are

relatively strong in comparison to other types of intermolecular

bonds.

Properties of WaterProperties of Water

Properties of WaterProperties of Water

• It is the polarity of the oxygen–hydrogen bonds and the subsequent

hydrogen bonding between water molecules that are responsible for

the unique properties of water.

Properties of Water – Relatively High BP and MPProperties of Water – Relatively High BP and MP

Properties of Water – Relatively High BP and MPProperties of Water – Relatively High BP and MP

Properties of Water – Relatively High BP and MPProperties of Water – Relatively High BP and MP

• Latent heat measures the energy needed to change the state of a

substance at its melting or boiling temperature:

• The latent heat of fusion of water is the amount of energy needed to

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

• The latent heat of vaporisation of water is the amount of energy

needed to change a fixed amount of water from a liquid to a gas at

100°C.

• The latent heat values of water are considerably higher than those of

most other common molecular substances.

• This means much more heat is needed to melt or boil water than for

equivalent amounts of other substances.

Properties of Water – Latent HeatProperties of Water – Latent Heat

• Oceans account for 71 per cent of the Earth’s surface.

• Their average depth is 6 km.

• These vast expanses of water play a very large part in controlling the

climate of the Earth.

• One reason for the influence of the oceans on our climate is a property

known as heat capacity.

• The specific heat capacity of a substance is the amount of

energy, in joules, required to raise the temperature of 1 g of that

substance by 1 °C.

Properties of Water – Heat CapacityProperties of Water – Heat Capacity

• The specific heat capacity of liquid water is 4.19 J °C-1 g-1.

• This means that it takes 4.19 J to raise the temperature of 1 g of water

by 1 °C.

• This means that water is able to store a great deal of heat energy

without undergoing the huge temperature rise that other materials

would exhibit.

• This capacity of water to store heat so well is used in the chemical

industry to cool down reaction vessels and in many engines to cool

down the engine.

Heat Capacity = Mass x Specific Heat x Change in Temperature

Q = m . C . DT

Properties of Water – Heat CapacityProperties of Water – Heat Capacity

• A solvent is substance in which other

substances dissolve.

• A solute is the substance dissolved in a

solvent.

• A solution is a mixture in which a solute

is dispersed through a solvent.

• Water is the most widely used solvent.

• In every aspect of our lives we rely on the

ability of water to dissolve substances.

Water as a SolventWater as a Solvent

• The process by which a solid substance completely dissolves in

water could be considered to occur in stages.

• The mobile water molecules next to the surface of the solid attract

surface particles away from the solid (via ion-dipole interactions).

• Once these particles are pulled away, they are surrounded by water

molecules and are able to move through the water.

• Over time, the whole solid crystal dissolves, layer by layer, from the

outside in.

• This process is known as dissociation.

Dissolving Ionic SolidsDissolving Ionic Solids

Dissolving Ionic SolidsDissolving Ionic Solids

Dissolving Ionic SolidsDissolving Ionic Solids

• The process by which a molecular solid dissolves is very similar to

that by which an ionic solid such as sodium chloride dissolves.

• Water generally only dissolves polar substances.

• “Like dissolves like” (Polar – Polar, Non-polar - Non-polar)

• In these cases the force of attraction between the particles of the

solid and the water molecules that draw them away and then

surround them is likely to be dipole–dipole attraction.

• Some hydrogen bonding may also occur, depending on the structure

of the particular substance.

Dissolving Molecular SolidsDissolving Molecular Solids

• The solubility of a substance refers to the maximum amount of that

substance that can be dissolved in a given quantity of solvent at a

certain temperature.

• A solution in which no more solute can be dissolved at that

temperature is described as a saturated solution.

• The measure of solubility of a substance at a given temperature is

how many grams dissolve in each 100 g of solvent to form a

saturated solution.

• The units of solubility are written as g/100 g solvent.

SolubilitySolubility

• Solubility values allow us to compare the extent to which different

solutes dissolve.

• Substances such as calcium carbonate, that have very, very small

solubilities, are said to be insoluble.

SolubilitySolubility

Solubility TablesSolubility Tables

• The relationship between solubility and temperature can be

represented by a solubility curve.

• Each point on the solubility curve represents a saturated solution - a

solution in which no more solute can be dissolved at the particular

temperature.

• Any point below a curve represents an unsaturated solution for that

solute.

• An unsaturated solution contains less solute than is needed to make

a solution saturated at that temperature.

Solubility CurvesSolubility Curves

Solubility CurvesSolubility Curves

• Honey often crystallises if you keep it in the refrigerator or if the

weather is very cold.

• The sugar becomes less soluble as the honey cools.

• The sugar that will no longer stay dissolved comes out of solution as

crystals in a process known as crystallisation.

• If a hot saturated solution of potassium nitrate is cooled, crystals of

the solute will appear.

• This happens because potassium nitrate also becomes less soluble

as the temperature falls.

• The potassium nitrate crystallises from solution.

CrystallisationCrystallisation

• A supersaturated solution can be

prepared by slowly cooling a saturated

solution.

• When this is done, the solution contains

more solute than it should at that

temperature.

• The addition of a small crystal seed or

even some dust will cause the excess

solute to crystallise.

SupersaturationSupersaturation

• Human activities have introduced many unwanted substances into

our rivers and water supplies.

• These include paints, cleaners, insecticides, fertilisers, industrial

wastes and other contaminants, including disease organisms.

• These substances can be spread far beyond their point of entry,

contaminating the water supply and even the soils of an entire

country or region.

• Some contaminants have migrated thousands of kilometres through

the oceans.

• Whole ecosystems can die and species become extinct as a result of

contaminated water systems.

Water PurificationWater Purification

• One method of obtaining clean water from an aqueous solution such

as salty water is to distil it.

• This separation method mimics the purifying action of nature in the

water cycle.

DistillationDistillation

• In a distillation, some or all of the components of a solution are

separated on the basis of their different boiling points.

• The mixture is heated, and the substance with the lowest boiling

point will boil out of the mixture first.

• Its vapours rise and are passed down through a tube in which they

are cooled and condensed.

• The condensed liquid is then collected.

• The mixture is further heated and the substance with the next lowest

boiling point distils over and is collected in a fresh collection vessel.

• This process is continued until the required components are

collected.

DistillationDistillation

• Osmosis is the natural tendency of water to move through a semi-

permeable membrane from a region of low salt concentration to one

of higher salt concentration.

Reverse OsmosisReverse Osmosis

• This natural tendency can be

reversed if pressure is applied

to the salt water side of the

semi-permeable membrane.

• This process is called reverse

osmosis.

• The difficulty and associated

cost of reverse osmosis is in

making suitable membranes

that will not rupture under the

high pressures used.

Reverse OsmosisReverse Osmosis