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Thursday, March 13, 2014

Flame Tests: A Favorite Chemistry Lab


This experiment is a classic, must-do experiment in your chemistry class!

This flame test lab is always a favorite of mine, and a much loved lab by all of my students.  The best time to use this lab is when teaching the following concepts:  atomic structure, electron configurations, energy levels, ground state and excited state.  
Background:  A flame test is used to detect the presence of certain metal ions.  The test involves heating a sample of the element and observing the resulting color of the flame.  When atoms of elements are heated to high temperatures, some electrons may absorb enough energy to allow them to move to higher energy levels.  The element is then said to be in the "excited state".  This excited atom is unstable, and the electrons quickly return back to their positions of lower energy or their ground state.  As the electrons return to their ground state, the energy that was absorbed is given off in the form of visible light.  The color of this light can be used to identify the elements involved.  In a flame test, the element will give off a characteristic color that serves as a simple method of identification of that element.   

Purpose:

  • To observe the characteristic colors produced by metallic ions when heated in a flame.
  • To identify an unknown metallic ion by means of its flame test.
  • To identify the components of a mixture using cobalt glass.
Materials Required:  Bunsen burner,  Lab apron,  Wood Splints,  Safety goggles,  Unknown solutions,  Test Tubes,  Cobalt glass squares,  Test Tube Racks,  Nitrate solutions of sodium, potassium, lithium, calcium, strontium, barium, and copper.

Tips and Suggestions:
  • I like to set up different lab stations and have the students rotate through the stations.  I set up stations for the 7 different metal ions that I will have the students test.  Students are required to rotate through these 7 stations first.
  • After the students have identified the colors of the above 7 metal ions, I assign an unknown for them to identify.  
  • Finally, I have the students use cobalt glass to identify the components of a mixture.
  • I have used metal inoculating loops as a means of heating the metal ion sample, but I prefer to use wood splints.  They are cheap and disposable.  Be sure to soak them in the metal nitrate solutions prior to the start of the lab. 
I set up individual lab stations for each metal ion and have students rotate through the stations.
This is a quick and easy way to set up the lab.

In a flame test, the element will give off a characteristic color.  It is difficult to catch on camera, but the below photos show the characteristic colors of barium (lime green) and lithium (crimson).



After students have observed all colors, I assign them an unknown element to identify.  

Using cobalt glass, students determine the identity of elements in a mixture.
When viewing the flame test with the naked eye, the student will see the yellow color of sodium.
When viewing the flame test while looking through the cobalt glass the student will see the violet color of potassium.
To me this lab serves a greater purpose than just learning to identify metal ions from their flame tests.  This lab gets my students so excited about chemistry.... and that excitement makes all the difference in the world when trying to teach chemistry to high school students.

This lab is in my TpT store and can be viewed at this link.

Have fun teaching!



3 comments:

  1. I think that you are incredible for doing all of this. You have reminded me of my love of teaching science!! Thanks!
    A Former Science Teacher

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  2. I teach 11th grade Chem, and have also gotten away from using the loops. We've started using q-tips and they work well too. I've also found that having them wet the q-tip and place it in the dry salt, then hold in the flame gives a very vibrant color, but makes quite a mess over the course of the day. I'll have to give the wood splints a try. :)

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    Replies
    1. Thank you for your comments. I am so glad to hear from a teacher who is still doing this lab. I received several comments from pinterest saying that this lab was not suitable for high school students and should only be done as a teacher demonstration. I am so disappointed that teachers might be depriving students of such a classic activity. I have not tried the qtips but have tried dipping directly into the dry salt... Way too messy!! If you try the wood splints be sure to soak them overnight first!! Thanks for visiting my blog.

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