In doing chemistry demonstrations, entertainment should only be a side-benefit. If students are not encouraged to form hypotheses to figure out what happened, eventually they will get bored with the demonstrations, especially when better ones can easily be found on Youtube. (Albeit, there are arguments to be made for how a live one is always more exciting than a digital version)
Here is an example of a little “discovery” exercise based on a demonstration.In a 250 ml Erlenmeyer flask, water was added along with two drops of phenolphthalein. In a second flask that would end up at the bottom, we added about 5 ml of ammonia. We covered the water-filled flask with a wet filter paper and inverted it. We then placed it over the ammonia flask.
After a few seconds, in the water-filled flask, we noticed a thin fuchsia-coloured funnel forming in the center of the liquid. It elongated progressively until it reached the top of the liquid, which gradually turned a deep shade of fuchsia. The base of the upper flask had a similar colour change, and a chaotic swirling pattern emerging from the funnel eventually coloured the entire upper flask. The lower flask remained clear.
Why was there a colour change?
Why did we get a “funnel” shape?
Why a swirling pattern?
To help ourselves figure out what happened the students and I suggested additional experiments
|Add litmus to the smelly liquid||It turns blue|
|Add NaOH to phenolphthalein||The solution turns deep pink.|
|Put wet filter paper on top of ammonia liquid and nothing else.||We can still smell the gas.|
|Measure the temperature above the ammonia.||The temperature drops.|
|Repeat the experiment without ammonia liquid. Instead use a drop of NaOH solution on the filter paper.||We get similar observations—with
the thin pink funnel,
but everything seems to happen more slowly.
Answer You might be thinking,” Enough already with this ammonia demo!” But the beauty of this so-called “discovery” ( or constructionist) activity is that by showing others what was on our minds, we kept learning from each other. By that I certainly include myself because if it had not been for your hypotheses and suggested experiments, I probably would not have given as complete an explanation as what many of you offered on the lab test. So here is a summary of all your best thoughts and reflections.
1. How did it all start?
Ammonia is an aqueous solution of NH3 gas and water. The gas can free itself from water’s bonds and come out into the air. How do we know? We can smell it. That’s what the ammonia did in the lower flask.
2. What’s the connection between diffusion and translations?
When the ammonia first begins to pop out of the water, it tends to be momentarily concentrated near the surface of the water. Here the molecules are more likely to bump into their own kind than into air molecules. But the translations and collisions take them a little beyond the crowd, and as they start to bump into air molecules, they slowly get surrounded by more air molecules. That’s what we mean when we talk about diffusion being a movement from a high concentration (original crowded ammonia area at the surface of the water) to a lower concentration (where there was mostly air).
3. What evidence did a temperature drop provide?
A student suggested that we measure the temperature of the air above the ammonia in the lower flask. It would drop slightly because evaporation has a cooling effect. Ammonia steals heat from the air and uses the energy to free itself from water’s grasp.
4. What happens with the filter paper?
The NH3 gets back into the water because the filter paper is wet. NH3, when dissolved in water, actually reacts with H2O to make NH4+ (aq) and OH–(aq) ions. That’s consistent with the litmus test result: red litmus turns blue if dipped into an ammonia solution.
5. Why did a pink colour appear in the upper flask?
Phenolphthalein is a pH indicator. When it bonds to OH–(aq) ion, its molecular structure is such that it absorbs certain parts of white light and reflects fuchsia (deep pink). The evidence for this idea comes from the fact that the indicator turns pink in the presence of NaOH base or ammonia base.
6. Why did the pink colour rise?
Phenolphthalein is less dense than water; that’s why the pink colour rose to the top. Many students thought the NH3 was rising through the water, but others realized that it is the phenolphthalein that turned pink when the NH3 and water produced OH–. Moreover, when the same experiment was repeated without ammonia but with a drop of NaOH on filter paper instead, a rising pink projection still occurred. OH- was what they had in common. This proved that a gas was not needed for the pink color to move so it was not its cause.
With the NaOH version, everything was slower and less intense because the only NaOH present was in the drop. But NH3 keeps hitting the filter paper and keeps providing more OH–(aq).
7. Why did we observe a chaotic pattern?
It was the result of Brownian motion, which is caused by the random rotations of water molecules as they collide into rising phenolphthalein molecules. It is similar to what you see if you place a drop of food coloring into a flask of water. It will send coloured offshoots in every direction, and just like the water and phenolphthalein, it will seem to slowly stir on its own. The flask will be entirely mixed and uniformly coloured.