Objectives: Students will be able to describe the
techniques in the purification of water.
Natural fresh waters are not pure, but contain dissolved minerals, suspended matter, and sometimes harmful bacteria.
The sources for water are often drawn from rivers and lakes for cities; and from groundwater wells in more rural areas. Such water is generally unsafe to drink without treatment. To make such water potable (that is, safe to drink), it is treated by some or all of the following processes:
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1. The first process is
Screening
, which is the removal of relatively large objects, such as trash, fish and so on.
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2. Coagulation: a chemical such as aluminum sulfate (Alum) is added to create small gelatinous particles (floc) which gather dirt and other solids.
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This can be demonstrated by adding alum to water that comes from ocean or add some dirt and india ink to some tap water. You must adjust the pH to 8-10.
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Coagulation is a very useful process for removing colloidal particles. The colloidal particles carry a negative charge and the aluminum sulfate surrounds each colloidal particle to overcome the negative charge.
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3. Flocculation: is the gentle mixing of the water which causes floc particles to join and form larger particles.
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This can be demonstrated if a stirrer and stirring bar is available.
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4. Settling: floc and sediment fall to the bottom and are eventually removed as sludge.
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5. Filtration: the water is passed through granular material such as sand and crushed anthracite coal.
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A filter made of charcoal sand and gravel has been made and donated to the institute.
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(figure available in print form)
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To the left is a figure of the filter made for the institute. Filter water from many different sources and compare before and after filtering.
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It is suggested that after use you attach the tubing to the nozzle of a faucet and force water up through the filter to clean the filter.
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6. Disinfection: Chlorine gas is bubbled into the water and dissolves in it to kill bacteria and other microbes.
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7. Corrosion Control: Chemicals such as quicklime, CaO, can be added to reduce acidity in water, which can be attributed by the addition of chlorine, and to also prevent corrosion in city and household pipes.
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8. Fluoridation: fluoride is added in many municipal treatment facilities to help prevent tooth decay.
Water Hardness:
Although not a problem in New Haven, many water sources contains dissolved calcium and magnesium salts which makes the water
hard
. One drawback of hard water is that ordinary soap does no lather well in it; the soap reacts with the calcium and magnesium ions to form an insoluble greasy scum. However, synthetic soaps, known as detergents or syndets, are available; they have excellent cleaning qualities and do not form precipitates with hard water. Hard water is also undesirable because it causes “boiler scale” to form on the walls of water heaters and steam boilers, which generally reduces their efficiency.
There are four techniques used to “soften” hard water: distillation, chemical precipitation, ion exchange and demineralization. In distillation, the water is boiled off and the steam is condensed back to a liquid again, leaving the minerals behind in the distilling apparatus.
Calcium and magnesium are precipitated from hard water by adding lime and sodium carbonate. The insoluble calcium carbonate and magnesium hydroxide are precipitated and are removed by filtration or sedimentation.
In the ion-exchange method, used in many households, hard water is effectively softened as it is passed through a bed or tank of zeolite. Zeolite is a complex of sodium aluminum silicate. In this process sodium ions replace objectionable calcium and magnesium ions, and the water is thereby softened:
Na2 Zeolite(s) + Ca
+2
(ag) ————-CaZeolite(s) + 2Na
+
(aq)
The zeolite is regenerated by back-flushing with concentrate sodium chloride solution, reversing the foregoing reaction. The sodium ions that are present in water softened either by chemical precipitation or by the zeolite process are not objectionable to most users of soft water.
In demineralization both cations and anions are removed by a two-stage ion exchange system. Special synthetic organic resins are used in the ion-exchange beds. In the first stage metal cations are replaced by hydrogen ions. In the second stage anions are replaced by hydroxide ions. The hydrogen and hydroxide ions react, and essentially pure, mineral free water is produced.
LAB ACTIVITY:.
Purpose: To determine the sudsing action in samples of hard and soft water.
Materials: 4 250m1 flasks, a 10 ml graduated cylinder and a 100 ml graduated cylinder, soap solution ( use 100 ml ivory liquid soap and dilute to one liter BE SURE TO DO THIS SLOWLY TO MINIMIZE SUDSING) water samples: distilled water (control), tap water, Ca
+2
(aq) water (calcium dilution 200 mg/L) Mg
+2
(aq) water (magnesium dilution 200 mg/L)
Procedure:
-
1. Measure exactly 100 ml of each water sample into each of the flasks and label.
-
2. Add 10 ml of soap solution to the distilled water Cover flask with hand and shake
gently
for 5 seconds
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3. Measure the height of bubbles + water in centimeters Measure the height of the water and then subtract to get the actual height of the suds. In measuring bubbles use judgement to determine height, disregard an odd bubble that may be a lot higher then the majority
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4. Repeat steps 2 & 3 for the other water samples, be sure to shake the flasks as similarly as the first.
Data Table:
|
Sample
|
height of suds
|
-
|
height of water
|
=
|
height of suds
|
|
distilled
|
|
-
|
|
=
|
|
tap
|
|
-
|
|
=
|
|
Ca
+2
|
|
-
|
|
=
|
|
Mg
+2
|
|
-
|
|
=
|
Questions:
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1. Why is distilled water used in this experiment?
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2. How did the different samples compare in sudsing action?
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3. What effect do you think hard water, or water with a high concentration of dissolved minerals, has on washing clothes? Bathing?
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4. What is often done to water containing a large amount of dissolved minerals before it is used by people?
Answers:
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1. The distilled water acts as a control because the tap water may have some minerals dissolved in it.
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2. The distilled water should have the most sudsing action followed by the tap and then the Ca+2 and Mg+2 water samples showing that the last to are the hard water samples.
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3. Hard water does not wash clothes as well as soft. Extra soap is needed to produce lather in the hard water.
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4. Softeners can be added to water or an ion-exchanger can be installed in a house where there is hard water present. Water treatment plants can also soften the water
before sending the water into the system.
LAB ACTIVITY 2
Purpose: To determine the amount of extra soap needed for hard water samples.
Materials: four 250 ml flasks, a buret and buret stand, soap solution (see instruction in previous lab) water samples: distilled, ta[, Ca
+2
(aq) and Mg
+2
(aq)
Procedure:
-
1. Measure exactly 100 ml of each water sample into a flask and label.
-
2. Fill Buret with soap solution to zero.
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3. Titrate soap into flask containing distilled water while continually swirling the flask consistently. When soap bubbles begin to appear, stop titrating and measure the amount of soap delivered into the flask.
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4. Repeat steps 2 & 3 and be sure to swirl the flask at the same rate as you did previously to minimize error.
|
Data Table
|
|
Questions:
|
|
Sample
|
amount of Soap (ml)
|
Same as first lab.
|
distilled
tap
Ca+2(a )
Mg
+2
(aq
)
ACTIVITY; Taste Test
Give students samples of tap water, bottled water, boiled water and distilled water and ask them which tastes the best and water are some of the subtle differences between the samples.
