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Saturday, March 29, 2014

Chemistry Lab: Percent Composition


Easy!  
Fun!  
And students get to see a hands-on application of the problems we solve in the classroom.

For the extremely busy teacher, it doesn't get much better than this for a great chemistry lab.  When the materials list consists of.....


Baking soda, 2N sulfuric acid, a spot plate, a Beral pipet ( or any dropper), a balance, and a test tube

........ it is a good day for the chemistry teacher!!

The percentage composition of a compound is the percentage by mass of each of the elements in the compound.  The purpose of this experiment is to determine the percent composition of carbon in a sample of sodium bicarbonate, NaHCO3.  
When sodium bicarbonate reacts with sulfuric acid, carbon will be released in the form of carbon dioxide.  According to the Law of Definite Proportions, the mass percentage of carbon in the bicarbonate will be constant, no matter how much sodium bicarbonate is used in the experiment.

Students simply weigh out an amount of baking soda. Drops of 2N sulfuric acid are added to the baking soda, one drop at a time.  The reaction releases carbon dioxide. Students continue adding drops of acid until there is no further reaction.  
The mass of the apparatus is taken both before and after the completion of the reaction to determine the amount of carbon dioxide that has been released. From the mass of carbon dioxide released, students can mathematically determine the amount of carbon in the released sample, and therefore the percent composition of carbon in sodium hydrogen carbonate.

The student will determine the percent composition of carbon in sodium hydrogen carbonate, both experimentally and theoretically.  The student will then determine his/her percent error.




What are the "pros" to this lab?
  • It provides hands-on practice in determining the percent composition.
  • It reinforces the calculations that are being taught in the classroom.
  • Simple materials list.
  • Easy set-up and easy clean-up.
  • Allows students to compare a "true value" to their experimental data.
  • Student loves watching the bubbling in the test tube!!  :)
  • No real safety issues since a fairly dilute solution of sulfuric acid is being used.  (Students MUST wear safety goggles!)
What are the "cons" to doing this lab?  None that I can think of!

You can find this lab in my TpT store by clicking this link:  The Percent Composition of Carbon in Baking Soda.

Have fun teaching!

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!



Saturday, March 1, 2014

Chemistry Lab: How Big Is A Mole?

Two facts we all know about teaching chemistry:

  • The concept of a "mole" is absolutely crucial to the teaching of our class.
  • Many of our students struggle with this concept every single year.
According to the textbook:  A mole is the quantity of a substance that has a mass in grams equal to its molecular mass and contains Avogadro's number of particles.   I think that many students memorize the definition of a mole, but fail to grasp the concept of a mole.  I want my students to be able to visualize a mole and to have a grasp on the size of a mole of substance.

I just added a new lab to my yearly chemistry curriculum.  It is not really an "experiment" but more of a review and reinforcement of the mole concept. The idea of the activity is very simple:  Students are given ordinary household substances and are asked to determine how many moles are in a teaspoon of the substance.

Purpose: 
  1. To determine the number of molecules and/or atoms in small amounts of everyday substances.
  2. To determine how many moles of chalk it takes to write your name on the board.

Materials:               

  • Balance           
  • Weighing dishes        
  • Plastic spoons
  • Water              
  • Salt (NaCl)                 
  • Sugar (C12H22O11)
  • Chalk              
  • Chalkboard    
I had the students find the mass of a teaspoon of a substance.  From this mass, I had students determine the number of moles of substance, the numbers of molecules of substance, and the number of a particular atom within the substance.  The bottom line is that this activity provides repetitive practice in mole conversions.

I was a little concerned that my "sophisticated" high schoolers would find this activity too elementary.  Wrong!!  They embraced the activity whole-heartedly and came away with a better understanding of the "size" of a mole.

The activity concludes with a student-designed experiment.  I asked my students to design an experiment to determine the number of moles of chalk required to write their name on the chalkboard.  Students had to write the steps of their procedure, construct a data table, and convert their data to moles and molecules.  SO SIMPLE, but my students had a great time with this.  They enjoyed comparing the "size" of their name to that of their classmates.

Click above picture to view this product in my TpT store.