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Pure Substances, Mixtures, and Conservation of Mass
Start Date:Due Date:
Purpose: Determine if something is a pure substance or a mixture. Determine if mass is conserved in a chemical reaction.
Apparatus:
Mass Balance, 4 Test Tubes, Salt, Coffee Filter, Felt Tip Marker, Beaker, Plastic Cup, Elmer’s Glue, Borax, Food Coloring, Stir Stick, 2 Zip Lock Bags, Plastic Pipette, Baking Soda, Vinegar
Procedures:
- The first thing we will do is use a mass balance. This is a review, but accurate use of the balance is necessary for the rest of the lab.
To be sure you know how to use the balance, select any small object and use the balance to determine its mass. Mass the object to the nearest tenth of a gram. Ask your teacher for help if you are not sure how to use the mass balance accurately.
Please list the name of the object you are massing: ______
Please list the mass of the object: ______
Now trade objects with somebody and find the mass of their object!
Name of somebody else’s object ______
Mass of somebody else’s object ______
Compare your results with the person you traded with.
Answer Question 1: Did your measurements match the measurement of the other person? If not, why were they different?
- Now you will examine a mixture.
Fill a test tube ¼ full of water. Add a small amount of salt (about half a teaspoon full) to the test tube with water.
Now, place your thumb over the opening of the test tube and shake it. (Note that you should never use your thumb to cover a test tube unless you are certain that the contents are harmless!)
Answer Question 2: Observe what happens when the salt falls into the water and write what you see below.
Answer Question 3: After you shook up the water and salt, did the water change? Is it still water?
Answer Question 4: Did the salt change? Is it still salt?
- Now you will determine whether ink is a pure substance of a mixture.
Cut a coffee filter so a strip will hand down from the middle. Your teacher will demonstrate how to do this for the entire class.
Once the filter is cut, touch the tip of a felt tipped pen or marker to the middle of the strip that is hanging down for about 10 seconds to get a sample of ink.
Lay the filter over the top of a beaker with a little water in it and let the strip touch the water. The strip should reach the water, but the ink should be above the water level.
As the strip soaks up the water it will carry different dyes (colored molecules) at different rates, thus separating them, if the ink is a mixture.
Use this test to determine if ink is a mixture or a pure substance. This test will take about 10 or 15 minutes, so go on to the rest of the lab and check this later. When you do check back, tell me whether you think ink is a pure substance or a mixture below. Explain your answer too.
Answer question 5: Is ink a mixture? Defend your answer with evidence you gained in the lab.
- Mixtures are not like pure substances. The procedure below should show you some differences
A pure substance will be a solid, a liquid, or a gas. But mixtures do not always appear to be just a solid, a liquid, or a gas. Mixtures can appear to have properties somewhere in between the states of matter.
When you get to this part of the lab, ask your teacher to fill your small cup 1/3 full with glue. Glue is a mixture! Have your partner stir the glue vigorously while your teacher slowly adds about a tablespoon of the borax solution to your glue. As the partner stirs, the mixture should start to become like “Silly-Putty.”
More borax solution may need to be added until the entire mix becomes like “Silly-Putty.” Once the entire mix starts to become like putty, take it out and knead the substance like a baker would knead dough.
Answer question 6: Is the substance a solid or a liquid? Explain your answer.
Answer question 7: Examine the strange state of the substance and tell me if it is a pure substance or a mixture
Put the mixture into a zip lock bag and take it home if you want! DO NOT PLAY WITH THIS IN OTHER CLASSES!
- Now you will examine whether or not mass is conserved in a chemical reaction.
The Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction. However, it is not always clear where mass goes during some chemical changes. This lab procedure should help you realize that mass is conserved and that often the mass gets changed into something we can not see!
For example, consider a log burning in a camp fire. What happens to all the mass of the wood after it burns?
Answer question 8: Does the ash weigh the same as the log did?
Answer question 9: If not, why does the weight change and where does the weight go?
In the following procedure, make sure you measure the mass as accurately as possible!
Find the mass of the zip lock bag by itself and the mass of the plastic pipette by itself. Record the masses below.
Mass of plastic bag______
Mass of plastic pipette______
Place a couple of tablespoons full of baking soda into a “zip-lock” bag and measure the mass of the bag and baking soda together. Make sure you get as much air out of the bag as you can. Record your measurement below.
Mass of plastic bag and baking soda ______
Use a plastic pipette to suck up some vinegar into it. Mass the pipette and the vinegar together and record the mass below.
Mass of pipette and vinegar ______
Now, place the pipette with the vinegar into the “zip-lock” bag with the baking soda and quickly zip the baggy up. Try to get as much air out of the baggy as you can before you zip it, but make sure you keep the pipette out of the baking soda and DO NOT squeeze out any vinegar yet.
Measure the mass of the baggy with the baking soda, the pipette, and the vinegar all together.
Mass of everything together______
After you measure the mass of everything and you are certain the baggy is zipped, aim the pipette at the baking soda and squeeze out the vinegar.
You should see a reaction taking place. The baggy will expand as it fills up with a gas from the reaction. After the reaction stops, measure the mass of the baggy with everything still in it. Do not open the bag until you measure the mass after the reaction stops.
Mass of baggy after the reaction ______
Answer question 10: What was the mass of the baking soda you put in the baggy?
Answer question 11: What was the mass of the vinegar you sucked into the pipette?
Answer question 12: What is the mass of the baking soda and vinegar added together?
Answer question 13: How does the mass of the baggy after the reaction compare to the mass of the baggy with everything in it before the reaction?
Answer question 14: How does the mass of the baking soda before the reaction compare to the mass of the baking soda after the reaction?
Answer question 15: How does the mass of the vinegar before the reaction compare to the mass of the vinegar after the reaction?
Answer question 16: Use the Law of Conservation of Mass to explain what happened during your experiment.
Calculations:
Show all calculations on a separate sheet of paper.
Conclusions:
Discuss:
- The difference between pure substances and mixtures.
- How you can determine if something is a pure substance or a mixture.
- Sources of error and ways to correct them.