Expt 023 -- Ammonia Ghosts

Have you ever seen an ammonia ghost? You'll get a good look at one when you try this activity. They are pretty timid, though, for ghosts; in fact, a few drops of vinegar is all that it takes to scare them away! Ammonia diffuses through a paper towel into a large bottle of water containing phenolphthalein. Convection currents swirl the pink ghosts gently.

Chemical Concepts

1. Some compounds are different colors in the acid and basic forms. These compounds are called indicators.
2. Reactions of indicators are reversible.
3. Ammonia solutions are less dense than water.

Safety

Wear goggles at all times.

Procedure

1. Fill the large "beaker" with tap water to within 3-4 cm of the top, then stir in the 10-12 mL of 1.0% phenolphthalein solution. Place the beaker in position for the demonstration and allow 4-5 minutes for the solution to come to rest.
2. Prepare some dilute vinegar solution by mixing 1 mL of "white" vinegar with 9 mL of tap water.
3. Place the lead weights (or pebbles) in the baby food jar, then add the 20-25 mL of household ammonia.
4. Place two or three layers of paper towel over the mouth of the baby food jar, and secure them in place with a rubber band (this may require two sets of hands!). Cut away the extra paper towel fringe.

   !!!Click here to See Movie. Click |> or <| to step the slides forward or back.

5. Wet the paper towel with a milliliter or two of the dilute vinegar. Then carefully lower the baby food jar into the beaker of water/phenolphthalein solution. (Crucible tongs may come in handy. Avoid disturbing the water, and try to position the jar upright in the center.)

   !!!Click here to See Movie. Click |> or <| to step the slides forward or back.

6. Wait and observe. Add dilute vinegar to disperse the ghosts, and watch formation of the ghosts again. The reaction is reversible for many cycles.

   !!!Click here to See Movie. This movie is accelerated to 5 times faster than the actual experiment.

   !!!Click here to See Movie. This movie is real time.

7. Watch surface of the towel where the gas is diffusing into the solution.

   !!!Click here to See Movie. This movie is accelerated to 5 times faster than the actual experiment.

8. Also, to "scare the ghost away," a few drops of vinegar (undiluted) may be added and gently stirred in. The ghost quickly disappears, only to reappear later on.

Questions

1. What evidence is there that ammonia gas can diffuse?
2. What evidence is there that ammonia gas is soluble in water?
3. What evidence is there that a solution of ammonia in water may be less dense than pure water?
4. Why does the ghost disappear when the vinegar is added?
5. How would the demonstration be affected if more vinegar had been added to the container?

Handout

Name ___________________________ Class ________

Teacher __________________________

BeckerDemos 023 Ammonia Ghosts

Watch the movies, and answer the questions.

Curriculum-

This activity ties in well with discussions of acid and base, indicators, diffusion of gases, and solubility of gases.

Activity-

Laboratory or Demonstration

Although it is written here as a demonstration, this activity can easily be made into a fun lab exercise.

Safety-

Wear goggles.

Time-

Teacher Preparation: 15 minutes

Class Time: 10 minutes as a demonstration (As a student laboratory allow about 20 minutes.)

Materials-

• 20-25 mL of household ammonia {about 3 M NH₃(aq)}
  • or
• 20-25 mL of 7.5 M NH₃ (Add 10 mL of conc. ammonia to 10 mL of water.) This more concentrated solution is faster than household ammonia.
• 10-12 mL of prepared 1% phenolphthalein solution (Dissolve 1.0 g of phenolphthalein in 50 mL of 95% ethanol. Add 50 mL of water slowly with rapid stirring. Use a magnetic stirrer if it is available.)
  • or
• 2-3 Exlax^® tablets dissolved in 25 mL of rubbing alcohol
• 1 2-L clear plastic bottle
• scissors
• 1 baby food jar or similarly shaped small, clear container
• a paper towel
• a rubber band
• 80 - 100 g of lead fishing weights or small pebbles.
• stirring rod
• a disposable plastic pipet or eye dropper
• white vinegar
• water

Disposal-

Discard solutions at the sink. Store lead weights or pebbles for reuse.

Lab Hints-

1. Determine (mathematically or by trial and error in a bucket of water) how many lead weights it takes to make the empty baby food jar sink in water.
2. Use scissors to cut the top portion off of the 2-L plastic soda bottle to create a tall plastic beaker. Strip the label. Cut the sides of the bottom piece of plastic away leaving a saucer of plastic for support.

   !!!Click here to See Movie.

3. The dilute vinegar placed on the paper towels acts to provide a time delay for the basic ammonia must first neutralize the acidic vinegar before it can make the solution basic. This time delay allows the water to settle again after the jar has been added. The thickness of the paper towel also plays a role. If the ghost is taking too long to make its appearance, try using fewer layers of paper towel or a more dilute vinegar solution.
4. If local tap water is so basic that the phenolphthalein turns pink, add vinegar dropwise until the pink color disappears.

Observations-

Household ammonia is actually a solution of ammonia gas in water. The pungent odor one experiences when opening a bottle of ammonia is due to some of the ammonia gas coming out of solution. The same happens inside the baby food jar. The ammonia gas leaves the solution, diffuses up to the top of the jar, and there it re-dissolves into the water of the wet paper towel. As it does so, it turns the solution in and around the paper towel pink (ammonia acts as a base in water and the phenolphthalein indicator changes to pink in a basic environment). Ammonia not only produces a basic solution, it also does something that few other substances do; as it dissolves, the density of the solution decreases (Density of 17 M Ammonia -- 0.90 g/mL). And although this decrease is slight, it is enough to cause the pink solution to rise slowly upward through the beaker. The wispy, ephemeral appearance of this rising column of ammonia solution resembles (if you use your imagination) a pink ghost emerging from the jar. Furthermore, the shapes and movements generated seem to be different each time the demonstration is performed.

Answers-

Q1. What evidence is there that ammonia gas can diffuse?

A1. Since the ammonia solution at the bottom of the baby food jar was not in direct contact with the paper towel at the top, the only way it could have made it to the paper towel was to diffuse through the air in the jar.

Q2. What evidence is there that ammonia gas is soluble in water?

A2. The phenolphthalein solution around the paper towel starts to turn pink, indicating a base has been dissolved in it. Ammonia is the only base present.

Q3. What evidence is there that a solution of ammonia in water may be less dense than pure water?

A3. Like smoke, the pink portion of the solution starts to rise through the rest of the solution, implying that it is less dense.

Q4. Why does the ghost disappear when the vinegar is added?

A4. The ghost disappears, because the acidic vinegar neutralizes the basic ammonia, causing the indicator to change back to colorless.

Q5. How would the demonstration be affected if more vinegar had been added to the container?

A5. The more vinegar added, the more ammonia required to turn the solution pink (basic) again. Thus, it takes longer for the ghost to reappear.

Key Words 1-

gas, diffusion, acid, base, indicator, pH