Flame Tests Lab
♦Have you ever seen a fireworks display? Where do all of the colors come from?
In this activity, you will investigate the colors of flame produced by solutions of metal salts.
♦A flame test is a procedure used to test qualitatively for the presence of certain metals in chemical compounds. When the compound to be studied is excited by heating it in a flame, the metal ions will begin to emit light. Based on the emission spectrum of the element, the compound will turn the flame a characteristic color. This technique of using certain chemical compounds to color flames is widely used in pyrotechnics to produce the range of colors seen in a firework display.
♦Certain metal ions will turn the flame very distinctive colors; these colors in turn can help identify the presence of a particular metal in a compound. However, some colors are produced by several different metals, making it hard to determine the exact ion or concentration of the ion in the compound. Some colors are very weak and are easily overpowered by stronger colors. For instance, the presence of a potassium ion in a compound will color a flame violet. But on the other hand, even trace amounts of sodium ions in a compound produce a very strong yellow flame, often times making the potassium ion very difficult to detect. To counteract the effects of any sodium impurities, one can view the flame through a piece of cobalt blue glass. The cobalt glass absorbs the yellow light given off by sodium while letting most other wavelengths of light pass through.
♦In this activity, wooden splints dipped in solutions of metal salts are heated using a Bunsen burner, producing different colored flames. By comparing the color given off by an unknown with the known metal salts, the identity of the metal salt can be determined.
What the flame test is about
The normal electron configuration of atoms or ions of an element is known as the “ground state.” In this most stable energy state, all electrons are in the lowest energy levels available. When atoms or ions in the “ground state” are heated to high temperatures, some electrons may absorb enough energy to allow them to “jump” to higher energy levels.
The element is then said to be in the “excited state.” This excited configuration is unstable, and the electrons “fall” back to their normal positions of lower energy (ground state). As the electrons return to their normal levels, the energy that was absorbed is emitted in the form of electromagnetic energy. Some of this energy may be in the form of visible light. The color of this light can be used as a means of identifying the elements involved. Such analysis is known as a flame test.
To do a flame test on a metallic element, the metal is first dissolved in a solution and the solution is then held in the hot, blue flame of a Bunsen burner. This test works well for metal ions, and was perfected by Robert Bunsen (1811 – 1899). Many metallic ions exhibit characteristic colors when vaporized in the burner flame.
- Bunsen Burner
- Flame test wire (you can use also wood splints soaked in the solutions overnight)
- Solutions of the following :
- Lithium chloride (Li+)
- sodium chloride(Na+)
- potassium chloride(K+)
- Calcium chloride (Ca2+)
- Strontium chloride (Sr2+)
- Barium chloride (Ba2+)
- Copper (II) chloride (Cu2+)
- Light the Bunsen burner and adjust it so that it has a hot blue flame.
- Using a clean flame test wire, dip it into one of the solutions and then hold it in the hottest part of the burner flame.
- Observe the color of the flame. Try not to move the wire in and out of the flame so it does not get glowing red, because if you do, you will see only reddish glow from the wire, not from the chemical.
- Carefully record your observations in the data table. Be accurate here – your description of the color must be accurate enough to distinguish this metal ion from the other ions tested.
- Clean the wire with distilled water and test it to see if you do not see a color when placing it on the flame.
- If you need to clean it further, use the hydrochloric acid solution and then distilled water again before using the wire with a different solution
- (not done in 2019) When you examine the sodium and potassium ions, first look at their color alone, then test them again, looking through a Cobalt glass plate.
- When you have tested all the known solutions and can distinguish the color of each metal ion, obtain unknown solutions and determine which metal ions are present by performing a flame test and comparing this data to your previous data.
Data Table 1: Copy and complete this table using GRAPH PAPER OR COMPUTER PAPER – USE RULER!
Not all the elements are listed. Verify that the ions you used are listed in YOUR TABLE.
TO SEE A PERIODIC TABLE WITH THE LINE SPECTRA BY ELEMENT, CLICK HERE
Data Table 2: Copy and complete this table using GRAPH PAPER OR COMPUTER PAPER – USE RULER! – SHOW THE CALCULATIONS IN YOUR REPORT!
(As an example, if you saw the sodium flame color as yellow, look for the wavelength that correspond to that colour and then show the values for frequency and the energy in the cells to the right)
Analysis: ANSWER THE FOLLOWING QUESTIONS ABOUT THE LAB:
- List the colors observed in this lab from the lowest to highest energy.
- List the colors observed in this lab from the lowest to highest frequency.
- List the colors observed in this lab from the longest to shortest wavelength.
- What is the relationship (formula) and proportionality between frequency, and wavelength?
- What is the relationship (formula) and proportionality between frequency and the energy associated to the light?
- What difference you notice by using the cobalt blue glass? Explain.
- How are electrons “excited” in this part of the experiment?
- Why do we see different colors in flames?
- What particles are found in the chemicals that may be responsible for the production of colored light?
- Why do different chemicals emit different colors of light?
- Colorful light emissions are applicable to everyday life. Where else have you observed colorful light emissions?
Items to be included in the lab report:
- Purpose of the lab
- Lab setup
- Data tables (2)
- Calculations for filling the data tables. For your calculations you will use
- V= 3 x 108 m/s
- h= 6.626 x10-34 Jxs
no calculations will result in an I (incomplete) Calculations must include, formula, or reasoning, units and answer with units.
- Analysis questions. If these are missing you will have an “I”