WATER A guide for GCSE students 2010 SPECIFICATIONS KNOCKHARDY PUBLISHING.

WATERWATERA guide for GCSE studentsA guide for GCSE students

2010 2010 SPECIFICATIONSSPECIFICATIONSKNOCKHARDY PUBLISHINGKNOCKHARDY PUBLISHING

WATERWATER

INTRODUCTION

This Powerpoint show is one of several produced to help students understand selected GCSE Chemistry topics. It is based on the requirements of the AQA specification but is suitable for other examination boards.

Individual students may use the material at home for revision purposes and it can also prove useful for classroom teaching with an interactive white board.

Accompanying notes on this, and the full range of AS and A2 Chemistry topics, are available from the KNOCKHARDY WEBSITE at...

www.knockhardy.org.uk

All diagrams, photographs and any animations in this Powerpoint are original and created by Jonathan Hopton. Permission must be obtained for their use in any work that is distributed for financial gain.

All diagrams, photographs and any animations in this Powerpoint are original and created by Jonathan Hopton. Permission must be obtained for their use in any work that is distributed for financial gain.

CONTENTSCONTENTS

• Properties of water

• Occurrence of water – The water cycle

• Hardness of water – the causes

• Temporary hardness

• Permanent hardness

• Removing permanent hardness in water

• Hard water – advantages and disadvantages

• Water pollution and its treatment

WATERWATER

PROPERTIES OF WATERPROPERTIES OF WATER

GENERAL INFORMATION

Structure Water consists of covalent molecules of formula H2O.

Physical colourless, odourless liquid.properties boils at 100°C freezes at 0°C

(if pure and at atmospheric pressure)

Chemical Reacts with some metals to produce hydrogen.properties

Uses Essential for life.An important solvent.A coolant for many industrial processes (e.g. power

stations)Raw material in the manufacture of ammonia.Raw material in the conversion of ethene to ethanol.

Test Turns blue cobalt chloride pink... orTurns white anhydrous copper(II) sulphate blue.

COMPOSITION OF WATERCOMPOSITION OF WATER

Fill both limbs of a Hofmann Voltameter withwater which has been acidified with a smallamount of dilute sulphuric acid. Pass sufficientdirect current through the apparatus to enablemeasurable amounts of gas to be collected.

ANODE (+) - a colourless gas collected - the gas re-lit a glowing splint

CATHODE (-) - a colourless gas collected - its volume was twice that at

anode- gas exploded with a squeaky pop when a lighted splint was applied

Conclusion Water can be split into its elementsby passing electricity through it.

water ——> hydrogen (2 vols) + oxygen (1 vol)

Dilute sulphuric acid is added to improve the conductivity of the water.

OCCURRENCEOCCURRENCE

Water is the most abundant substance on the surface of our planet.



Importance LIVING SYSTEMSRESOURCE FOR INDUSTRIAL

PROCESSES




PROCESSES

Occurrence OCEANSLAKESRIVERSATMOSPHERE - clouds and water vapour




PROCESSES

Occurrence OCEANSLAKESRIVERSATMOSPHERE - clouds and water vapour

Water Cycle Shows the inter- relationship between waterin different environments.

THE WATER CYCLETHE WATER CYCLE

Shows the inter- relationship between water in different environments.

Water evaporates fromseas, lakes and rivers andfrom the leaves of plants.

The water vapour producedcondenses to form cloudsand later falls as rain, hail and snow.

The rain and snow completethe cycle by flowing downstreams and rivers into lakesor seas.

Some water seeps throughrocks and soils to joinunderground reservoirs.

PURITY OF WATERPURITY OF WATER

Natural water is never pure. It is such a good solvent that it contains dissolvedsubstances whatever its origin.

RAIN dissolved gases from the air e.g. CO2, SO2

RIVERS dissolved salts from rocks and soils; also oxygen.

SEA dissolved sodium and magnesium salts (e.g. NaCl) and CO2

water alsocontains man-made chemicals such as detergents, acids,

fertilizers and other pollutants.

WHAT IS HARDNESS?WHAT IS HARDNESS?

“Hard water is water which does not readily form a lather with soap”

WHICH IONS CAUSE HARDNESS?WHICH IONS CAUSE HARDNESS?


MethodPlace a sample of one of the solutions in a test-tube to a depth of about 2cm.Using a dropping pipette, place a measured amount of soap solution into thewater and shake vigorously for five seconds. Record the height of the lather. Repeat with another sample; decide which ions are responsible for hardness.




A sodium sulphateB magnesium sulphateC potassium chlorideD calcium chlorideE sodium nitrateF magnesium nitrateG calcium nitrateH sodium chloride

A B C D E F G H




A sodium sulphateB magnesium sulphateC potassium chlorideD calcium chlorideE sodium nitrateF magnesium nitrateG calcium nitrateH sodium chloride

A B C D E F G H




Solution used ions present lather + ive - ive

A sodium sulphate Na+ SO42- YES

B magnesium sulphate Mg2+ SO42- NO

C potassium chloride K+ Cl- YESD calcium chloride Ca2+ Cl- NOE sodium nitrate Na+ NO3

- YESF magnesium nitrate Mg2+ NO3

- NOG calcium nitrate Ca2+ NO3

- NOH sodium chloride Na+ Cl- YES

Conclusions The ion(s) responsible for hardness is / are...




Solution used ions present lather + ive - ive

A sodium sulphate Na+ SO42- YES

B magnesium sulphate Mg2+ SO42- NO

C potassium chloride K+ Cl- YESD calcium chloride Ca2+ Cl- NOE sodium nitrate Na+ NO3

- YESF magnesium nitrate Mg2+ NO3

- NOG calcium nitrate Ca2+ NO3

- NOH sodium chloride Na+ Cl- YES

Conclusions The ion(s) responsible for hardness is / are... Ca2+ and Mg2+

HARD WATER – HOW DOES IT ARISE?HARD WATER – HOW DOES IT ARISE?

Definition “Water that does not readily form a lather with soap”. Instead, it forms an unsightly scum.

Causes Arises from the solvation of compounds out of rocks / soils.

The compounds are SOLUBLE magnesium or calcium salts.





LIMESTONELIMESTONE(CALCIUM CARBONATE)

WATER

ROCKS CONTAININGSOLUBLESOLUBLE CALCIUM OR

MAGNESIUM COMPOUNDS







MAGNESIUM COMPOUNDS

WATER CONTAINING SOLUBLE CALCIUM OR MAGNESIUM IONS

RAIN DISSOLVES THE SOLUBLE

SALTS IN ROCKS







LIMESTONE IS INSOLUBLE IN WATER

BUT DOES DISSOLVE IN RAIN WHICH IS ACIDIC

DUE TO DISSOLVED CO2


MAGNESIUM COMPOUNDS








SALTS IN ROCKS





MAGNESIUM COMPOUNDS








SALTS IN ROCKS





MAGNESIUM COMPOUNDS





There are TWO main types of hardness…





There are TWO main types of hardness…

PERMANENT HARDNESS

TEMPORARY HARDNESS

TEMPORARY HARDNESSTEMPORARY HARDNESS


Source Atmospheric carbon dioxide dissolves in rain water making

an acidic solution which reacts with limestone (calciumcarbonate). Soluble calcium hydrogencarbonate is

formed.

CaCO3(s) + CO2(g) + H2O(l) ——> Ca(HCO3)2(aq)







MAGNESIUM COMPOUNDS




formed.


Removal Boiling the water reverses the above reaction. A white precipitate of insoluble calcium carbonate isproduced. The calcium is thus removed from the water.

Ca(HCO3)2(aq) ——> CaCO3(s) + CO2(g) + H2O(l)




formed.




plus Any method used for removing permanent hardness.

REMEMBEROnly water containing SOLUBLE calcium and magnesium ions causes hardness.





formed.




plus Any method used for removing permanent hardness.



PERMANENT HARDNESSPERMANENT HARDNESS


Source Soluble calcium and magnesium compounds which dissolve

in streams and rivers as they pass over and through rocks

and soils.



MAGNESIUM COMPOUNDS



SALTS IN ROCKS




and soils.

SOLUBLE calcium chloride CaCl2magnesium chloride MgCl2magnesium sulphate MgSO4

INSOLUBLE calcium carbonate CaCO3

magnesium carbonate MgCO3




and soils.

SOLUBLE calcium chloride CaCl2magnesium chloride MgCl2magnesium sulphate MgSO4

INSOLUBLE calcium carbonate CaCO3

magnesium carbonate MgCO3



PERMANENT HARDNESS - REMOVALPERMANENT HARDNESS - REMOVAL

DISTILLATION


DISTILLATION

Boil the water and collect pure water as the distillate. All the dissolved solidswill be left behind. This is the only method to produce pure water.

However, it is an expensive methodbecause of the energy requirements.

HARD WATER

PURE WATER

CONDENSER


ION-EXCHANGE RESIN


ION-EXCHANGE RESIN

• Ion-exchange resins are special compounds containing Na+ or H+ ions• Hard water contains soluble calcium and/or magnesium ions

RESIN


ION-EXCHANGE RESIN

• Ion-exchange resins are special compounds containing Na+ or H+ ions

• Ca2+and Mg2+ ions in the hard water swap with those on the resin• The water comes out with Na+ ions in it• Ca2+and Mg2+ ions remain attached to the resin• Na+ and H+ ions do not cause hardness.

Ca2+(aq) + 2Na+ ——> Ca2+ + 2Na+(aq) in hard water on resin on resin in softened water

RESIN


ION-EXCHANGE RESIN

• Ion-exchange resins are special compounds containing Na+ or H+ ions

• Eventually, all the Na+ ions get used up so the resin must be replaced or flushed through with sodium chloride solution to replace the calcium.

RESIN


SOAP


SOAP

Soap molecules have two distinctly different ends;

HYDROPHOBIC HYDROPHILIC

“water hating” “water liking”

attracts oil and grease attracts water

CHAIN OF CARBON ATOMS


SOAP





When soap is placed in hard water, it reacts with the calcium andmagnesium ions to produce an unsightly, insoluble grey scum.



SOAP





When soap is placed in hard water, it reacts with the calcium andmagnesium ions to produce an unsightly, insoluble grey scum.

The scum is a calcium compound and is thus removed from the water.When all the hardness has been removed, the soap can act in the normal way.



SOAP






In HARD WATER AREAS it is better to use SOAPLESS DETERGENTS which have structures similar to soap.

Detergents form a lather without forming a scum.

Because of this they DO NOT REMOVE HARDNESS.

In HARD WATER AREAS it is better to use SOAPLESS DETERGENTS which have structures similar to soap.

Detergents form a lather without forming a scum.

Because of this they DO NOT REMOVE HARDNESS.


WASHING SODA (sodium carbonate)



Sodium carbonateis one of the fewsoluble carbonates -it is a good sourceof soluble carbonateions.



Hard water containssoluble calcium(or magnesium)

ions.




Add the two solutions inorder to mix the ions.

Hard water containssoluble calcium(or magnesium)

ions.




The Ca2+ and CO32- ions

come together to form aprecipitate of insolublecalcium carbonate.

The calcium ions areremoved from the waterso it is now soft.


WASHING SODA (sodium carbonate) - SUMMARY

Sodium carbonate is one of the few soluble carbonates- it is a good source of soluble carbonate ions.

Addition of washing soda to a solution containing magnesiumions or calcium ions results in the precipitation of insoluble carbonates.

Once the magnesium, or calcium, has been removed from the water,the water is no longer hard - it is soft.

equation CaCl2(aq) + Na2CO3(aq) ——> 2NaCl(aq) + CaCO3(s)

ionic equation Ca2+(aq) + CO32-(aq) ——> Ca2+CO3

2-(s)

HARD WATERHARD WATER

Disadvantages

Furring up of pipes and boilers; the heating is not as effectiveblockages may lead to an explosion


Disadvantages


Furring up of kettle elements; wastes electricity


Disadvantages



Wastes soap; as some is needed to remove the hardness

unsightly scum is formed during washing


Disadvantages



Wastes soap; as some is needed to remove the hardness

unsightly scum is formed during washing

Advantages calcium strengthens teeth and bones.

better for preventing heart diseases

WATER POLLUTIONWATER POLLUTION


Origin nitrates and phosphates in farm fertilizersanimal waste productsindustrial chemical wastelead ions (Pb2+) from old pipes



Effects Nitrates from fertilizers encourage plant and algae growth.

When algae die they are decomposed by aerobic bacteriawhich need oxygen. The oxygen is thus removed from

thewater making the river “dead” and incapable of

supportingfish populations.

Nitrates can also be converted to nitrites (which affecthaemoglobin in blood) and into carcinogenic

nitrosamines.

Industrial waste can be in many forms including cyanide,detergents and heavy metals and radioactive products.

A build up of lead can lead to brain damage.



Effects Nitrates from fertilizers encourage plant and algae growth.

When algae die they are decomposed by aerobic bacteriawhich need oxygen. The oxygen is thus removed from

thewater making the river “dead” and incapable of

supportingfish populations.

Nitrates can also be converted to nitrites (which affecthaemoglobin in blood) and into carcinogenic

nitrosamines.

Industrial waste can be in many forms including cyanide,detergents, heavy metals and radioactive products.

A build up of lead can lead to brain damage.

WATER PURIFICATIONWATER PURIFICATION


Water of the correct quality is essential for life. For humans, drinkingwater should have sufficiently low levels of dissolved salts and microbes.



Filtration removes insoluble organic matter;the sludge is digested to produce methane.

Aeration removes organic waste using oxygen-requiring bacteria.

Chlorinationkills bacteria to allow water to be used for drinking purposes.






IT CAN THEN BE SENT TO HOUSES THROUGH THE MAINS






OTHER THINGS THAT CAN BE DONE

Fluoridation Fluoride compounds are added to water in some areas to aidthe prevention of tooth decay. The fluoride is only helpful tochildren and can be given in tablet form. Many people objectto the addition of fluoride to drinking water.








Water filters These contain carbon, silver and ion exchange resins andcan remove some dissolved substances from tap water to

improve the taste and quality.








Water filters These contain carbon, silver and ion exchange resins andcan remove some dissolved substances from tap water to

improve the taste and quality.

Distillation Needs a lot of energy so is expensive but makes the purest water.

© 2011 © 2011 JONATHAN HOPTON & KNOCKHARDY PUBLISHINGJONATHAN HOPTON & KNOCKHARDY PUBLISHING

WATERWATER

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