Chemistry “Chemical and Physical Changes”
I use this lab early in a chemistry course before students have written a standard chemical equation. I will have discussed with my students and demonstrated to my students how to tell the difference between a chemical change and a physical change. I use the 5 evidence method to determine a chemical change.
1. an unexpected color change
2. an unexpected odor change
3. a change in temperature (make sure the students understand that cooling also indicates a temperature change
4. release of a gas
5. a precipitate formed
I do not have students measure out exact quantities of materials. This is not a quantitative lab. I give the procedure in an exact format. I will use 3 different chemical reactions for this lab. This will be my students 1st attempt at a correct lab writeup. It allows discussion to take place as to what a purpose or hypothesis looks like and where measurements are recorded. I tell them that the purpose of this activity is two fold.
1. To identify any evidences for chemical changes
2. To write out chemical equations as they see the reactions take place
I like to use the following reactions because they involve cheap materials.
1. I have the students measure out ¼ test tube of copper(II) sulfate and have them dissolve it in distilled water. In a small beaker, I have the students dissolve 3 pieces of sodium or potassium hydroxide into a set amount of distilled water. I have the students pour the contents of the test tube into the contents of the beaker. A precipitate forms. If the students have paid attention, they will have measured the temperature of both of the solutions before they pour them together so they can determine if a change in temperature has taken place. This seems to be a common mistake. The resulting material can be poured down the sink.
2. I have a [1.0] solution of hydrochloric acid already made. The students measure enough HCl into a test tube to fill it 1/3 to ½ full. They then record the temperature. I have them get 2 pieces of mossy zinc metal and put them into the test tube. They will see a rapid evolution of gas and a temperature increase. Caution, when students are discarding the material, have them put the unused zinc in a different container. Zinc will clog your sinks.
3. I have students use the same [1.0] HCl for this part of the experiment. I have the students measure out 1 “scoop” of baking soda into a small beaker. They then measure 30 ml of the HCl into a graduated cylinder. After taking the temperature of the acid, they then pour the acid into the beaker with the baking soda. A rapid evolution of gas is given off and a different kind of temperature change will be noticed. The resulting material can be poured down the sink.
Discussion items
1. When I have the students attempt to write out a chemical equation, I courage them to write it out as they see it. For example, in the 1st procedure, many students will write out the following
Blue liquid + clear liquid = whitish gunky solid + dark blue liquid
Other students will research each chemical by using their book or doing a google search at home. They may write out the equation as such..
CuSO4 + KOH = whitish gunky solid + dark blue liquid
A common mistake that students make. They put the water into the equation. This lends itself to a discussion that only chemical changes go into an equation. The dissolving of a solid into a liquid is a physical change and not a chemical change.
Reasons for doing this lab.
1. This allows a discussion on safety procedure. The students can see the value of safety glasses and the value of cover gowns and the value of proper attire like closed shoes and no bare legs. It allows me to talk about proper disposal of products and how to enter and leave the laboratory setting.
2. It allows me to evaluate and correct a proper lab evaluation. In student conclusions, I want a listing for chemical change evidence for each procedure and their attempt at writing chemical equations.
3. It allows the students evidence of chemical reactions.
4. It allows me a beginning to show the writing of a chemical equation from an observed chemical reaction.
5. It reinforces the differences between chemical changes and physical changes
Materials that will be needed.
Chemicals. Test tubes, beakers (can be either plastic or glass), tongs for test tubes or test tube holders (you can also use a beaker to put the test tube into.), digital thermometers work best, but alcohol will work, container for used zinc, graduated cylinder.