Question

please read the material underneath is not clear to me how to aproach this am not a chem major so its kind of hard. thanks for the help

In Part I, when the solution containing aluminum and sodium hydroxide is heated under the hood, why should you wait until the “fizzing” stops?

In Part I, item #8, what is the purpose of the centrifuge?

To speed the process of settling the aluminum hydroxide gel.

Would it be safe for you to drink the water from today’s experiment?

Why or Why not?

CLARIFICATION OF WATER

One of the steps that must be performed on water to make it suitable for drinking is that any suspended solid material must be removed from the water. Suspended solids are materials such as very fine dirt particles that are carried along by, but not truly dissolved in the water. One way to remove suspended particles is to use a type of chemical called a flocculant that combines with the suspended solids to form a mass or floc that is solid enough to be filtered out of the water. Water treatment plants use several different flocculants to remove suspended solids. In this experiment, you will prepare one f1occulant, aluminum hydroxide [Al(OH) ₃] from aluminum metal and then use the aluminum hydroxide to clarify (remove the suspended solids from) some dirty water. Taking used aluminum cans and preparing aluminum hydroxide from them is one possible use for the old cans, but turning the cans into new aluminum cans is a more economical way to recycle the aluminum.

The reactions involved in this experiment are:

Dissolving aluminum metal with sodium hydroxide solution:

Al +      2 Na⁺         +      2 OH^-      +    6 H₂O →    2 Na⁺    +   2 Al(OH)₄^-          +       3 H₂ (g)

Converting the aluminum ion into aluminum sulfate (aluminum and sulfate ions in solution):

Al(OH)₄^- +      4 H₂SO_{4         →}        2 Al³⁺     +     4 SO₄^2-      +     8 H₂O

Converting the aluminum sulfate solution into solid aluminum hydroxide:

Al³⁺ +    6 OH^-       _→        2 Al(OH)₃ (s)

The hydroxide ions used in step 3 come from the reaction of sodium hydrogen carbonate (also known as sodium bicarbonate or baking soda) when it is dissolved in water:

                                NaHCO₃ (s) +            H₂O _→   Na⁺    +    OH^-    +     CO₂ (g)   +     H₂O

Another type of material that sometimes must be removed from water before it can be used for drinking is a dissolved chemical. If the dissolved molecules are large enough, they can be removed by passing the water over a form of carbon called activated charcoal. Activated charcoal has a very large surface area and this surface is able to latch onto (absorb) certain dissolved materials and hold onto them. The charcoal, along with the chemicals it has caught, can, then be filtered out of the water. After the charcoal has been filtered from the water, it can be made ready to be used again by heating it with steam (500- 1000 °C). This causes the charcoal to release the absorbed chemicals leave clean charcoal that can be used again.

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PROCEDURE:

PREPARATION AND USE OF ALUMINUM HYDROXIDE

Weigh out 0.20 g of aluminum foil and put it into a 250 mL beaker.

Measure out 15 mL of 6 M sodium hydroxide (NaOH) solution and pour it over the aluminum foil in the beaker. Make sure that the foil is completely covered by the solution.

Put the beaker on a hot plate in the fume hood and GENTLY heat the mixture. Do not boil. Stir the mixture to keep it from foaming over.

Continue warming the solution until most of the fizzing stops (about five minutes). Remove the beaker from the hot plate and allow it to cool.

Add 20 mL of 3 M sulfuric acid (H₂S04) to the beaker and stir the mixture well to dissolve as much of the gel-like material as possible.

Suction filter the mixture using a Büchner funnel as your instructor will demonstrate. Be sure that the filter paper lies flat on the bottom of the Büchner funnel. Wet the filter paper slightly with deionized water to help flatten the filter paper. Take the filtrate (the liquid that went through the filter paper into the flask) and pour it into a 150 mL beaker.

Add 10 mL of the filtrate to 10 mL of the dirty water provided in a 100 mL beaker. Add solid sodium hydrogen carbonate slowly, with stirring; until the liquid is basic to litmus. Stir well and pour some of the slurry into a centrifuge tube and label it tube "1."

The aluminum hydroxide gel will settle out over time with its trapped "dirt," but we can speed up the process by using a centrifuge. Using untreated dirty water, fill a second centrifuge test tube (tube "2") to the same level as tube "1." Put the tubes in the centrifuge on opposite sides and centrifuge them for two minutes.

Decant the liquid from the centrifuge tubes into spectrophotometer tubes labeled “1” and “2”. Fill a third spectrophotometer test tube (tube "3") with untreated dirty water to the same height as the other two tubes. Compare the absorbance at 500 nm of each solution using a spectrophotometer. Record the results on the lab report form.

Put the waste from this part of the experiment into the waste beaker provided.

USE OF ACTIVATED CHARCOAL

Take two Erlenmeyer flasks of the same size and add 50 mL of deionized water to each flask. Add five drops of methylene blue dye solution to each flask.

Add about 0.5 g of activated charcoal to one of the flasks. Swirl the flask to mix the water and charcoal well.

Filter the charcoal from the water using the suction filtration method used earlier and compare the appearance of the treated water with the untreated water that still has methylene blue in it. Record the results on the lab report form.

The liquid waste from this part of the experiment can be rinsed down the drain.

Answer 1

1)

Al +      2 Na+         +      2 OH-      +    6 H2O →    2 Na+    +   2 Al(OH)4-          +       3 H2 (g).

When Al is treated with NaOH, H₂ is released. This reaction mixture is heated gently. Fizzing is due to release of hydrogen. So reaction mixture is heated to remove hydrogen gas from reaction mixture till fizzing stops. Stopping of fizzing indicates that there is no more hydrogen left and our reaction is completed.

2)

Centrifuge is done to reduce time of setting of suspended particles. Here we centrifuge the slurry, so that aluminium hydroxide gel settle out easliy and quickly.

3)

Would it be safe to drink above clarified water, depends upon the impurity, chemicals present in initially given dirty water and you skills and observartion.

If you were given a muddy water without any chemical dissolved in it then water obtained from above process is suitable for drinking.