Diapositiva 1

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Transcript Diapositiva 1

SOFT AND HARD WATER

M.Morelli I.T.I. Malignani - Udine

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what’s the stuff you get from the tap ?

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serious problems can stem if unsuitable kind of water is used;

M.Morelli I.T.I. Malignani - Udine

what you should know is that water must undergo special treatments before it’s supplied to towns :

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first water flows through a screen that stops gross rubbish M.Morelli I.T.I. Malignani - Udine

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then it is pumped on a bed of coarse sand which eliminates the most parts of solid material;

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hence, in a sedimentation tank, special chemicals make small solids stick together, so that they settle on the bottom;

M.Morelli I.T.I. Malignani - Udine

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from the top of the tank water flows through a sand filter where also the smallest solid particles are trapped;

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finally chlorine gas is added to kill bacteria and give clean sterilized water, fit to drink, that can be stored.

but what kind of water is this ?

M.Morelli I.T.I. Malignani - Udine

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depending on its source, water contains various dissolved compounds, that have not been removed by previous treatments, and especially : calcium sulphate, calcium hydrogen carbonate, magnesium sulphate and magnesium hydrogen carbonate.

M.Morelli I.T.I. Malignani - Udine

these compounds may cause serious drawbacks, actually :

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dissolved solids require more energy to reach the boiling point ( especially expensive in power stations); scale deposition in boilers and pipes; stones precipitation in bladder and kidney ( not completely known ); M.Morelli I.T.I. Malignani - Udine

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all these seem to be good reasons to be into water hardness;

• water hardness

results when water is particularly rich in compounds containing:

Ca

and

Mg

M.Morelli I.T.I. Malignani - Udine

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hard water is mainly caused by carbonate, which is produced when rain falls on limestone and chalk rocks;

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these are not soluble in pure water, but dissolve in rain water that contains CO 2 as in the following reaction : M.Morelli I.T.I. Malignani - Udine

H 2 O (l) + CO 2 (g) + CaCO 3 (s) Ca(HCO 3 ) 2 (aq) water + carbon dioxide + calcium carbonate calcium hydrogen carbonate

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the same process concerns also other kinds of rocks, as: - dolomite CaCO 3 , MgCO 3 - gypsum CaSO 4 ·2H 2 O M.Morelli I.T.I. Malignani - Udine

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is everything clear ?

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is everyone aware of the importance of the topic?

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for these special people our chemistry department offers a “

free

“ hardness test

and now, to work

M.Morelli I.T.I. Malignani - Udine

Determination of hardness of water

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by soap solution the hardness can be gauged by its abilitity to form a thick lather with soap

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by EDTA titration is an analytical measurement using a polydentate ligand or a chelating ( claw like ) ligand EDTA M.Morelli I.T.I. Malignani - Udine

First method: 1. Fill the test tube to about 1/3 the capacity with sea, tap, rain, and hard water samples.

2. place soap solution in a buret 3. add 2mL of soap solution from the buret to each sample 4. cap the test tube and shake vigorously 5. put each test tube in the rack and wait 20 seconds if the lather remains, we can say 2mL of soap solution was required 6. if a lather does not remain, add another 2 mL and shake again and wait 20 seconds 7. go on in this way and record the volume of soap solution required to produce a permanent lather in each sample of water; can you now arrange the samples in order of increasing hardness ?

conclusion: water that lathers easily is called soft, whereas water that produces very little lather and a scum, is termed hard.

M.Morelli I.T.I. Malignani - Udine

Second method : EDTA titration

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Ethylenediamine tetraacetic acid and its sodium salts (abbreviated EDTA) form a chelated soluble complex when added to a solution of certain metal cations. If a small amount of a dye such as Eriochrome Black T is added to an aqueous solution containing calcium and magnesium ions at a pH of 10.0 ± 0.1, the solution will become wine red .

If EDTA is then added as titrant, the calcium and magnesium will be complexed. After sufficient EDTA has been added to complex all the magnesium and calcium the solution will turn from wine red to blue . This is the end point of the titration.

M.Morelli I.T.I. Malignani - Udine

laboratory equipment

materials apparatus

•buret •beaker •Erlenmayer flask •pipette filler bulb • funnel • graduated cylinder • volumetryc flask

reagents

•EDTA •EBT •ammonia buffer

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experimental procedure • • • •

100 cm³ of sample (V sample) are transferred to a titration vessel 5 cm³ of the ammonia buffer solution and a bit of indicator EBT are added. The obtained violet solution is titrated with the EDTA standard solution until the colour of the solution is turned to a stable blue. titrations is carried out for the determination of water hardness.

M.Morelli I.T.I. Malignani - Udine

Chemists express the amount of hardness in :

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ppm ( parts per million ): mg of CaCO 3 in 1 litre of water ° F = 1g of CaCO 3 in 100 litre of water different scales of "degrees" of water hardness exist : 1 German degree = 17.9 ppm (as CaCO 3 ) 1 American degree = 17.1 ppm 1 English degree = 14.3 ppm 1 French degree = 10.0 ppm M.Morelli I.T.I. Malignani - Udine

Grains Per Gallon

(gpg)

less than 1.0 1.0 – 3.5

3.5 – 7.0

7.0 – 10.5

over 10.5

Milligrams Per Litre (mg/l) °F less than 17.1 17.1 – 60 less than 7 7-15 60 – 120 15-22 120 – 180 over 180 22-35 Over 35 M.Morelli I.T.I. Malignani - Udine Rating Soft Slightly Hard Moderately Hard Hard Very Hard

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Water Softening

These mineral ions are removed using ion exchange technology.

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This technology is based on removing certain ions from treated water, using resin that have the property of attracting specific types of loose ions in the water, while releasing other ions that were originally attached to it.

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When hard water passes through a column filled with small granules of resin, the Ca and Mg ions are exchanged for sodium ions RNa 2 + Ca 2+ RCa + 2 Na M.Morelli I.T.I. Malignani - Udine +

Questions 1- Do all kinds of water behave the same way when soap is added ?

2- What about hard water in plumbings and boilers?

3- Chalk is insoluble in water. Why does it dissolve in rainwater? Write an equation for the reaction.

4- Why are EDTA solutions used to clean conctact lenses ?

5- Is rain water hard or soft ?

M.Morelli I.T.I. Malignani - Udine

Glossary dump verb [transitive always + adv/prep] = to put something such as a load, bag etc somewhere in a careless, untidy way lather noun [singular, uncountable] = a white mass of bubbles produced by mixing soap in water scale water pipes [uncountable] = a white substance that forms around the inside of hot water pipes or containers in which water is boiled scum noun [singular, uncountable] = an unpleasant substance that forms on the surface of a liquid stem = to stop something from spreading or growing; stem from :to develop as a result of something stuff noun [uncountable] = a kind of substance or material suds noun [plural] = the mass of bubbles formed on the top of water with soap in it; tap noun [countable] = a piece of equipment for controlling the flow of water, gas etc from a pipe or container; waterworks noun [plural] = the system of pipes and artificial lakes used to clean and store water before it is supplied to a town M.Morelli I.T.I. Malignani - Udine

Finally let’s have a little fun: insert the correct names in the figure below choosing among the

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following : groundwater

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treatment plant

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water tower

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rain

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ridge

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distribution home and factories

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spring

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well

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reservoir

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diverted for irrigation use M.Morelli I.T.I. Malignani - Udine