Solubility of Ammonia in Water

Description

Flasks filled with dry ammonia gas are submerged under water and opened. Indicators may be used. This experiment may be scaled down and performed by students in two ways. First, they may fill bulbs of plastic transfer pipets with ammonia and use these for study. Second, they may stack two identical vessels. A few drops of concentrated ammonia are placed in one. A few drops of phenolphthalein are placed in the second. The second vessel is then filled with water, covered with a piece of filter paper, inverted, and stacked over the first vessel. Ammonia from the lower vessel rapidly passes through the gas phase into the upper vessel producing spectacular plumes of red-pink color.

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• An implosion is possible if the flask containing the ammonia gas has a partial vacuum. The ammonia gas is toxic.
• Sodium hydroxide is highly caustic.

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Observers wear eye protection.

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Both traditional and microscale procedures are included. Use the one suggested by your instructor.

Microscale

1. Pick up two Beral pipets, one long and one short; a 100-mL beaker; and a 250-mL beaker. (Notice that the longer pipet has a slightly narrowed end so that it will fit into the end of the smaller pipet - try this out. The longer pipet will be your gas generator and the smaller will be the collecting tube.)
2. Fill the 100-mL beaker with cold water and set it aside. Fill the 250-mL beaker with water and heat it until bubbles are visible on the inside of the beaker. The water should be hot but not boiling.
3. A small beaker containing concentrated NH₃ should be placed under the hood. At the hood, squeeze the longer pipet to push out the air and place the tip into the ammonia. Half fill the bulb with concentrated NH₃.
4. At the laboratory workplace, set the shorter pipet onto the longer pipet so that the elongated tip of the longer pipet extends into the bulb of the shorter pipet. Place the bulb of the longer pipet in the hot water bath to warm the NH₃ solution. (As the solution is warmed, NH₃ is released and vigorous bubbling is observed.) Pump the smaller pipet once to fill it with the gas. Since ammonia is lighter than air, it rises and displaces the air. Remove the longer pipet from the hot water bath and detach the smaller pipet.
5. Observe the odor of the gas collected in the small pipet by wafting a small amount of it from the tip of the pipet towards the nose. (Describe the characteristic odor of ammonia.)
6. Place the tip of the short pipet into the cold water in the small beaker. Squeeze gently to push out one bubble of ammonia which will allow a small amount of water to enter the tube.
7. Repeat the procedure using an acid/base indicator in the cold water.

Traditional

1. There are several ways to fill a round bottom flask with ammonia gas. The best way is to use a cylinder of ammonia gas as the source. Few high school laboratories store gas cylinders, however.
2. To generate ammonia chemically, work in a hood. Use a spoon or spatula to mix equal volumes of solid ammonium chloride and solid sodium hydroxide on a piece of glazed paper.
3. Clamp the test tube to a support stand.
4. Use a piece of glazed paper to slide the solid mixture into a large test tube.
5. Clamp a dry 500-mL round bottom flask using a support stand and a clamp in the inverted position near the test tube.
6. Fit a 30-cm length of flexible plastic tubing to a one hole stopper for the test tube. Adjust the test tube to a 30 degree angle. Stopper the test tube. Place the other end of the tubing in the mouth of the round bottom flask.
7. Heat the solid mixture with a Bunsen flame. Gas is evolved. After gas has evolved for 1-2 minutes, stop heating. Remove the plastic tube. Stopper the round bottom flask. If moisture is present in the flask, there is a possibility that there will be a partial vacuum in the flask. In this case, handle the flask cautiously.
8. A third way to fill a flask with ammonia gas is to start with concentrated ammonium hydroxide. Attach a clamp to a 500-mL round bottom flask. Place 25 mL of concentrated aqueous ammonia (ammonium hydroxide) in the flask. Hold the flask so that the neck points downward. Heat the flask to boiling using a flame.
9. Once the liquid has bubbled vigorously for 20 seconds, invert the flask discarding the liquid into a beaker, and stopper the flask.
10. Once a flask is filled with ammonia gas, the solubility of ammonia in water can be demonstrated. Fill a large, clear, colorless tank 2/3rds full with water that has been colored with food coloring. Immerse a stoppered flask filled with ammonia gas under the water, neck down. Remove the stopper, and note evidence for reaction.
11. It sometimes helps to perform a control experiment and open a flask filled with air under the surface of the water to show no effect.
12. The procedure may be repeated using a pH indicator such as phenolphthalein in place of the food coloring. In this event, the base properties of ammonia may be discussed.
13. An equally dramatic though less spectacular demonstration uses two identically-sized vessels such as two 25-mL Erlenmeyer flasks.
14. Add a few drops of phenolphthalein solution to a vessel nearly filled with water.
15. Fill completely with water.
16. Place a piece of filter paper over the top of the vessel.
17. Place about 1 mL of concentrated ammonium hydroxide into a similar vessel.
18. Invert the water-filled vessel. Place it on top of the vessel containing the concentrated ammonium hydroxide. Note any evidence for reaction.
19. Ordinary laboratory flasks may also be used.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 059 Solubility of Ammonia in Water

What will happen if the flask is filled:

1. with oxygen?
2. with carbon dioxide?
3. with hydrogen chloride?

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Purpose

To illustrate the great solubility of ammonia gas in water.

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• ammonia gas (cylinder; concentrated ammonium hydroxide, Bunsen burner, 250-mL beaker; 10 g ammonium chloride, 10 g sodium hydroxide, large test tube with one-hole stopper and flexible plastic tubing, plastic spoon, glazed paper, Bunsen burner, 2 support stands with clamp)
• 500-mL round bottom flask
• one-hole stopper to fit equipped with wooden dowel
• large plastic tank (trough, aquarium) 2/3rds filled with water
• food coloring
• phenolphthalein
• 1 mL concentrated ammonium hydroxide
• 2 25-mL Erlenmeyer flask (or similar vessel)
• 1 circle of filter paper (any size)
• distilled water in 125-mL plastic squeeze bottle

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• It is difficult to fill a flask with dry ammonia. Three procedures are described. The gas cylinder procedure is preferred.

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Teacher preparation: 30-40 minutes

Presentation: 5-15 minutes

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• Fill the round flask with gaseous ammonia. Obtain the ammonia from a tank, or generate it by a chemical reaction. Use a round bottom flask to minimize implosion hazard.
• If the flask is filled by heating concentrated ammonium hydroxide and draining the liquid, a partial vacuum may be present. Shield such a flask. Work in a hood.
• Use extreme caution when handling and heating sodium hydroxide.
• Provide adequate ventilation. Have observers wear eye protection. Wear heavy gloves.

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The materials used in this experiment may be disposed of safely at the sink. Use caution when adding tap water to the tube containing the reaction mass left after heating the solid mixture; unreacted material may release heat and gases. Neutralize and discard.

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Presentation Question:

• What will happen if the flask is filled: with oxygen?; with carbon dioxide?; with hydrogen chloride?
  • Only the hydrogen chloride gives a similar result to ammonia. Carbon dioxide, though soluble, dissolves slowly. Oxygen is only slightly soluble in water.

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• ammonia
• gas
• gas solubility
• phenolphthalein
• implosion

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