Experiment 12
What is a flame?
For this experiment, you will answer the question, "What is a Flame?" You will need to propose at least five hypotheses to be tested, using the scientific method. These hypotheses must be approved by your instructor during the first day of this lab. You will work with your usual lab partner. If you need any specialized equipment or chemicals not used for making gases, you need to request these from the stockroom by filling out a request form prior to the lab period you will need the items, not the same day you need them.
After you have performed your experiments as a group, and answered your hypotheses, you must report your results in an individual formal written lab report (format for this written lab report is available online). You must also turn in your lab notebook, which must also be well-written, legible and easy to follow your experimental approach. Your lab notebook and the formal written lab report will both be graded. The written lab report is worth a total of 50 points (your lab notebook scores for the entire semester are worth 100 points), so your written lab report is a big part of your lab grade. Most students will score 40-45 points on their lab report, based on how well the paper is written, how your experiments were used to determine what a flame is, and how original and creative you were in doing this experiment.
Each student writes his own report, it is not a group activity, even though you do all your experiments together.
Experimental Protocols
Day 1: Formulation of Hypotheses to be Tested
Listed below are some of the areas that you might use to help answer the question, "What is a Flame?" Do not limit yourself to the ideas listed below. Try to be original, as original thought and experiments will be rewarded with a higher grade. The hypotheses shown below are merely a few starting points to use in your analysis. For example, most of us know that oxygen is required for combustion. What you need to do in this experiment is to show that oxygen is required for combustion (we have been taught this all our lives). You will need to design experiments that either utilize oxygen, or implicate the use of oxygen in the combustion process. It is not enough to say, "oxygen is required for combustion." You must prove this hypothesis.
Possible areas to address include, but are not limited to the following:
Be both quantitative and qualitative in your analysis. Be descriptive, and record all your data. Be original and creative. You may not always be able to absolutely prove a hypothesis, but you can, in many cases, indicate that your results are "consistent" with the hypothesis you are trying to prove. For example, you may characterize a black residue produced by a candle flame and propose this material is carbon. You may not definitively prove it is carbon, but you can do experiments which show that your solid is "consistent" with being carbon.
You need to put together at least five hypotheses to be tested. These can be anything related to a flame. However, you must include each of the following in your analysis:
All of your experiments with laboratory prepared gases must be performed during the second day of lab.
Day 2: Preparation and Analysis of Gases
You can prepare gases to examine during your experiment. This section describes the chemical methods required to prepare different gases.
Individual gases are prepared using the chemical procedures shown below, which describe the protocol and setup conditions for the collection of gases. This section describes how to produce and collect gases, but does not describe how these gases can be used in the experiment. How to use these gases is up to you as to how they help you determine what a flame is.
The reactants for each of the individual reactions are placed in a 250-mL Erlenmeyer flask. A thistle-tube assembly (a two-hole stopper with a long tube with a funnel at the top and another tube with rubber tubing attached) is placed into the opening of the flask. Solid reactants are added to the flask prior to attachment of the thistle-tube assembly. Liquids are added through the thistle tube. The bottom of the thistle tube (inside the flask), must be covered by liquid in the flask (either water or added liquid reagents). The outlet tubing is used for the collection of gases. To collect the gases, you will use a downward-displacement procedure. The collection bottles are filled with water. Using a pneumatic trough, add enough water to cover the shelf. Cover the water-filled bottle with a glass plate, invert the bottle and place the opening of the bottle under water, resting on the submerged shelf (be sure to remove the glass plate from the mouth of the bottle). When you are ready to start collecting your gases, place the mouth of the inverted bottle over the bubbling tubing, and collect your gas, as the water is forced out of the bottle (downward displacement of the water). After the bottle is full of gas, place a glass plate over the opening (before you remove the bottle from under the water to seal the gas in the bottle) and place the bottle on the bench. For O2 and CO2, store the bottles in the usual position (since the O2 and CO2 are more dense than air, they will stay in the bottom of the bottle). For the H2, however, store the gas with the bottle inverted (bottom-side up) because the H2 is less dense than air, and will stay at the top of the inverted bottle. If you stored the H2 bottle in the usual position, the glass plate might leak, and allow the H2 gas to escape. Whenever you use the H2 gas for experimental purposes, keep the bottle inverted. The O2 and CO2 bottles are used as normal, not inverted.
Oxygen
You will produce oxygen (O2) by reacting hydrogen peroxide (H2O2) in the presence of MnO2, which is the catalyst (a catalyst is not consumed but simply speeds up the rate of reaction). The oxygen you collect must be stored in covered bottles until ready to be used (O2 is more dense than air, and will stay in the bottom of the bottle). The equation below shows this reaction.
Use a pea-sized amount of MnO2 (not very much), and put it into the reaction flask. Add the thistle tube assembly, and add about 25 mL DI water (if the bottom of the thistle tube is still above the water level, add a little more water to cover the bottom of the tube). Acquire about 50 mL of 9% H2O2 (hydrogen peroxide) and start the reaction by adding 5-10 mL into the funnel of the thistle tube. You should observe a gas forming, and bubbling into the pneumatic trough. Start collecting the oxygen gas, filling the first bottom. Continue collecting O2, until you have the number of bottles you desire. Continue to add the hydrogen peroxide solution, to maintain an active gas-producing reaction. When you are through, empty the reaction mixture into the appropriate waste container.
Hydrogen
You will produce hydrogen (H2) by reacting zinc metal with concentrated H2SO4.
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
Place about 10 g of zinc in a 250-mL Erlenmeyer flask, and add about 2 mL of 0.1 M CuSO4(aq) as catalyst (do not add more CuSO4). Add enough water (less than 50 mL) to cover the bottom of the thistle tube which has been inserted into the flask. Initiate the reaction by adding 4-5 mL of 18 M H2SO4 (concentrated sulfuric acid) through the thistle tube. Add more acid, in 4-5 mL increments, as needed, to maintain a steady production of gas (H2) that can be collected into the bottles. Continue to collect the number of bottles that you need for your experiment. Discard the contents of the reaction mixture into the appropriate waste container.
The first bottle collected will contain some oxygen (as part of the air which initially filled the flask) until totally displaced by the generated hydrogen. To avoid collection of oxygen, allow the gas to flow into the pneumonic trough for a minute so that you can collect only hydrogen.
Carbon dioxide
You will produce carbon dioxide (CO2) by reacting 6 M HCl with calcium carbonate (marble chips; CaCO3).
Reaction with calcium carbonate: CaCO3(s) + 2 HCl(aq) → CO2(g) + H2O(l) + CaCl2(aq)
Start with about at least 5 g of CaCO3(s) in the Erlenmeyer flask. Insert the thistle tube assembly, and add enough water to cover the bottom of the thistle tube. To initiate the reaction, pour the 6 M HCl into the thistle tube, and you should observe gas forming, and bubbling into the pneumatic trough. Collect the number of bottles of CO2 gas needed for your experiment. Discard of your reaction mixture in the appropriate waste container.
Day 3: Finalization of Experiment
Today you will finish your experiment. All experiments can be performed today, but experiments with prepared gases must have all been performed during the second day of lab.
The following table can be used in conjunction with the eperiments you may have designed. For example, if you do an experiment designed to see which gas or gases in the air are used during combustion, you can compare the amount of gas used during the reaction, to the percent of a particular gas in the atmosphere.
| Component1 | Content2 |
| Nitrogen Oxygen Argon Carbon dioxide Neon Helium Krypton Xenon |
78.084 20.9476 0.934 0.0318 0.00182 0.00052 0.00011 0.000009 |
| 1Principal gases in the atmosphere. 2Gas content is in mole percent |
|