Wednesday, August 15, 2012

The Law of Conservation of Matter Lab

Teacher Notes:
Chm.2.2.3 Analyze the law of conservation of matter and how it applies to various types of chemical equations (synthesis, decomposition, single replacement, double replacement, and combustion).              

This is a double replacement reaction.  Within the reaction is a decompostion reaction.  The specific kind of double replacement is a neutralization reaction.  The NaHCO3 is the base and the CH3COOH is the acid.

The first reaction is  NaHCO3   +       CH3COOH       H2CO3    +     CH3COONa     

The products are called sodium acetate and carbonic acid.

The second reaction      H2CO3       CO2   +   H2O      which is a decompostion reaction

The Overall equation reaction is:

NaHCO3   +       CH3COOH          CH3COONa  +   CO2   +   H2O

Students will find the molar mass on each side of the yield sign to be the same.

Helping Students Explain Experimental Errors
Some chemistry students will state that the Law of Conservation of Matter is not supported in this lab.  When teaching with a lab, the teacher must be careful in not telling or even hinting the expected data the student should collect during a lab.  The student collects the data that happens during a lab, whether it supports or does not support a theory or law.  It should be the teachers job to get the students to ask questions on why the data supported or did not support the theory or law.  That is part of the data analysis and is very important in teaching about experimental errors. Many students do not understand experimental error.  Many will state human error.  There may be human error but humans can make many kinds of errors during an experiment.  Where and how do humans make these errors? Are there other incidents that cause errors that are not human?  Did the lab equipment raise concerns for the errors?  These statements must be very specific for the student to understand why or why not laws or theories are supported or not supported.

Teaching Students About Being a Scientist
Before this lab, it is important that the students know the parts of a scientific investigation and can eventually formulate this themselves.  It could be taught during this lab.  Many educators believe that students should devise all this themselves but many other educators believe that this is done gradually.  In the first lab, I told the Purpose (question), hypothesis, materials, procedure, data table and conclusion in the first lab and gradually left off more information each time for the students to complete themselves.  For example,  students in the beginning of chemistry find it difficult to make a hypothesis about what they think will happen.  Discussions about the definintion of hypothesis and that alone helps them become better at making a hypothesis. In the first lab, I modeled the scientific investigation so that they knew exactly what was expected.  In the last few labs, the students are given a purpose.  In groups, or with a partner, the students develop their own hypothesis, list the materials needed, write the procedure, develop the data table, complete the experiment, do the data analysis, and write their conclusion which should discuss the experimental errors and what changes would be done if the lab were done again.  In the semester project, students complete the parts of the scientific investigation by themselves, including the purpose (or question).


In many labs, I added conclusion questions that they might see on a test.  I feel that it made a connection with what was discussed in class. Also, it is a help when studying and reviewing for a test.  Yes, I asked the students to write the questions in the lab book so that they would know what the question was.

Helping Students To Think 
During a lab, I rarely give additional instructions (beyond the written lab instructions) or give  answers to an experiment, but will answer any questions that will help them.  Yes, it causes frustration, but they learn that I am teaching them how to think.  Sometimes I will answer a question with a question, to again, help them with a thinking process.  I think it is important for students to use a marbled lab book.  Every teacher can form their own opinion about this technique but I found that students take more ownership of their lab work.  It also is a good way to organized their lab work.

Helping Students Learn How to Make a Data Table
To teach students to make a data table, read the lab.  In the first lab, I teach them how to pick out what must be part of the data table.  In this lab you will notice that the data to record are the following:
* mass of the sodium bicarbonate (this is only to make sure that a reasonable amount is used)  If you
   don't want them to mass it, show them how much to use)
* mass of the set-up before the reaction
* mass of the set-up after the reaction
* observation of the reaction (smell, temperature, sight, sounds)

Helping Students with Data Analysis
Data analysis includes graphs and/or equations and math related to the experiment. It is important to teach them how to organize this.  Without help, some will include data analysis in the procedure or the data table. (Some simple labs will not have data analysis but all need to talk about the conclusion. This lab has no data analysis unless you change the procedure)

Lab safety is of upmost important in chemistry.  That topic should be covered before any lab experimenting is done. Lab techniques can be taught and demonstrated as a lab calls for it.

Now for the lab.  This lab is usually the first chapter or unit in chemisty or physical science.

The Law of Conservation of Matter Lab            

Safety: Safety googles, apron (if using HCl), closed toed shoes (if using HCl)

Purpose: (or Title of Lab)  To demonstrate that the mass of the reactants is equal to the mass of the product after a chemical reaction has occurred.

Hypothesis:  ?  (The student may need help in the first attempt to hypothesize.)

Materials:  Balance, 250mL beaker, ballon, spatual (or spoon), test tube, 100 mL beaker, disposibleregular stem pipette, acetic acid or vinegar (CH3COOH),  baking soda or sodium bicarbonate (NaHCO3),  safety goggles,

1.  Tare the balance.
2.  Obtain 100mL beaker that is 1/2 full of acetic acid.  (Dilute HCl can also be used but care must be
      used when handling it.  It can bleach clothing.  If using HCl, using an apron is advised.)
3.  Fill the test tube 1/2 full of acid.  Place the test tube in the 250 mL beaker.
4.  Obtain a balloon and sodium bicarbonate, NaHCO3.
5.  Place a scoop (3-4 grams) of  NaHCOin the balloon.
6.   Stretch the opening of the balloon over the open end of the test tube.  Be careful not to let any of the sodium bicarbonate spill into the test tube.
7.  Place the test tube/balloon in the beaker.  Make sure the balance is tared and determine the total mass of the balloon, test tube and beaker.  Record the data in the data table.  This would be the "before the reaction" data.
8.  Empty the contents of the balloon into the test tube.  Record the observations.
9.  After the reaction is complete do not remove the balloon yet.  Record the mass of the test tub, balloon and beaker.  The balloon can be removed after the set-up has been massed and recorded.

Data Table:

Data Analysis: (Students need to include the part of the lab but state that there is no data analysis)


Questions:  Write the question and answer in complete questions.
1.  How do you know that a chemical reaction occurred?
2.  What filled the balloon?
3.  What would have happened if the balloon was not attached to the test tube?
4.  What does the Law of Conservation of Matter state?
5.  Did this experiment support the Law of Conservation of Matter?  Why or why not?

Remember that there has to be a paragraph about the experimental errors and what changes would be made if the lab were done again.

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