Lab 2 - Sep Era Ting Mixtures

Lab 2

Name_____________________________

How Can the Percent Composition of a Salt, Sand and Iron Mixture be Determined? Pre-Lab Assignment This written pre-lab is worth 15% (3 points) of your lab report grade and must be turned in to your lab instructor before lab begins. 1. 2. 3.

Read the entire lab handout. Make a list of all chemicals used in this lab (may be names or chemical formulas). Write a brief summary of the experiment that will be performed today. Hints: Do not just list each step of the procedure, but practice your summarizing or outlining skills.

Experimental Question How can the percent composition of a salt, sand and iron mixture be determined? Learning goals • Make detailed qualitative observations in your data record. • Make metric measurements with proper units. • Use different types of glassware and understand their functions. • Use a Bunsen burner properly. • Investigate the principle behind filtration. • Use different methods for separating different kinds of mixtures.

Background: Scientific Method Scientific inquiry (also called the scientific method) is a process that scientists use to explore the natural world and propose explanations based on the evidence they find. The main purpose of the CH 104 lab is to give you experience doing scientific inquiry on concepts covered in the CH 104 course. You will be practicing the scientific method through a series of steps each week. Your Mastering Chemistry tutorial on the scientific method has a good description of the scientific method: The scientific method is a procedure used to search for explanations of nature. The scientific method consists of making observations, formulating hypotheses, designing and carrying out experiments, and repeating this cycle. Observations can be either quantitative or qualitative. Quantitative observations are measurements consisting of both numbers and units, such as the observation that ice melts at 0ºC. In contrast, qualitative observations are observations that do not rely on numbers or units, such as the observation that water is clear. A hypothesis is a tentative explanation of the observations. The hypothesis is not necessarily correct, but it puts the scientist's understanding of the observations into a form that can be tested through experimentation. Experiments are then performed to test the validity of the hypothesis. Experiments are observations preferably made under conditions in which the variable of interest is clearly distinguishable from any others. If the experiment shows that the hypothesis is incorrect, the hypothesis can be modified, and further experiments can be carried out to test the modified hypothesis. This cycle is repeated, continually refining the hypothesis. If a large set of observations follow a reproducible pattern, this pattern can be summarized in a law—a verbal or mathematical generalization of a phenomenon. For example, over the years people observed that every morning the sun rises in the east, and every night the sun sets in the west. These observations can be described in a law stating, "The sun always rises in the east and sets in the west." After a great deal of refinement, a hypothesis can lead to a theory. A theory is an explanation of why something happens. For example, Newton's theory of gravitation explains why objects tend to fall toward the Earth (as well as explaining the interactions between the Earth and the other planets, etc). However, theories can still be further refined or even replaced. Einstein's theory of general relativity was able to better explain certain astronomical observations related to gravity, and therefore it replaced Newton's theory of gravitation (although Newton's theory still holds true under most everyday conditions). Similarly, the geocentric theory (that the Earth is the center of the universe) was replaced by the heliocentric theory (that the Earth revolves around the sun) based on further observations and testing of predictions. Note that a scientific theory is not the same as the popular definition of a theory—namely, a "guess" or "speculation." Instead, a theory is an explanation that can hold up against repeated experimentation. It may not be perfect, but it is the best explanation possible based on available evidence.

Lab 2: How Can the Percent Composition of a Salt, Sand and Iron Mixture be Determined

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The Experiments portion of the scientific method given above can be broken down even further: Experimental question: should be answered by the experiments. Procedural plan: takes into account many factors such as changing only one variable at a time while controlling all other variables, performing multiple trials, considering limitations of time and materials, running a control, etc. Data collection: includes using appropriate glassware and instruments to make accurate quantitative measurements with correct significant figures, making qualitative observations, and organizing your data into a data table. All data collected in the lab must be recorded directly onto your paper at the time of the observation. There will be no measurements on scratch paper or re-copying direct data later. Don’t worry – you’ll have the chance to organize your data into a more readable format in the data tables after data collection has been completed. Data Analysis: may include mathematical calculations, graphing, or otherwise manipulating your raw data (measurements) to answer the experimental question. Also includes examination of the level of experimental uncertainty. You may often find your qualitative observations important in drawing conclusions. Conclusion: final result of the data analysis which answers the experimental question. Hypothesis may be confirmed or denied. The results of this cycle of scientific inquiry must be shared with the scientific community and the general population. Therefore communication is also a key component of the scientific method. Scientists’ work must be available to others to analyze, critique, reproduce, and build upon.

Procedure Please work with a partner; record his/her full name. My lab partner is: ________________________________________ Write down all qualitative and quantitative observations and measurements directly on this sheet. Note that the equipment needed for each experiment is located either in your lab drawer, on the cart in the center of the lab or in a labeled cabinet or drawer. Critical Thinking Question 1: What measurements are needed to calculate the percent of iron in the mixture?

Obtain and make qualitative observations on the mixture as described below. • Shake the bottle containing the mixture of salt, sand and iron to mix it well. Obtain about 10 mL of the mixture in a small beaker. Make observations on the mixture.

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Samples of the mixture are set up under dissecting microscopes in the lab. Examine the mixture under the microscope and record the appearance of the particles you see. Distinguish between the salt crystals, the grains of sand and the iron filings in drawings and your description

Lab 2: How Can the Percent Composition of a Salt, Sand and Iron Mixture be Determined

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Record a hypothesis for this experiment (an answer to the experimental question):

Critical Thinking Question 2: Do you expect teams to get the same or different percent compositions of the mixture? Explain briefly.

Take your beaker containing the salt, sand & iron mixture along with the small flask to the top loading electronic balance and weigh about 5 grams of the mixture according to the instructions below. Mass measurements using a top-loading balance: • When using a balance to make mass measurements, be sure all containers and substances are at room temperature. • Place a piece of weighing paper on the balance pan. Close the top of the balance. Wait until the reading holds steady and push the “tare” button. The balance should now read “0.000” or “0.00” grams, depending on the precision of the balance. • Hold the mouth of the beaker over the weighing paper, then tap on the side of the beaker so that small amounts of the mixture pour onto the paper. Keep adding the mixture until you have a mass between 4.5 and 5.5 grams. • Record the actual mass of your sample in grams, including all digits, even zeros at the end. For example, if the balance shows a readout of 5.200, you need to record “5.200 g” on your paper, NOT “5.2 g”. • Carefully pour the sample from the paper into the flask. • Dispose of the paper in the trash can. mmixture = _____________ g Separate the iron from the mixture: • Spread the mixture into a very thin layer over a full sized piece of paper. • Weigh and record the mass of a second piece of weighing paper and set it aside. • Wrap a small square of clear plastic over the magnet. Remove the iron filings by passing the magnet closely over the surface of the entire mixture. Repeat several times to make sure you’ve collected all the iron. • Holding the magnet over the weighing paper, carefully remove the plastic and allow all the iron to fall onto the paper. Weigh and determine the net mass of the iron filings. miron = _____________ g • Describe the appearance of the mixture after removing the magnetic component.

At your bench, use the graduated cylinder to measure 25.0 mL of water as precisely as you can (see instructions below). Note that 25.0 is where the liquid is right on the line - use a dropper for the last few drops. Volume measurements using a graduated cylinder: • To read the volume: Place you cylinder on a flat surface. Put your eye level with the meniscus → meniscus (meniscus: the curve of the liquid surface).Read the volume at the bottom of the meniscus. • Pour the water into the flask containing the mixture. • Describe the appearance before, during, and after swirling the mixture continuously for 1 minute:

Lab 2: How Can the Percent Composition of a Salt, Sand and Iron Mixture be Determined

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Critical Thinking Question 3: What happened to the salt? Where is the salt now?

Filter the mixture as follows: • Place the funnel in a ring clamp and set the clamp so that the end of the funnel feeds into the mouth of a small beaker. • Use a PENCIL (not pen) to label a piece of filter paper with your initials. Weigh the filter paper. mfilter paper = _____________ g • Fold a piece of filter paper into quarters and place into the funnel, forming a cone. Wet the paper with a squirt from a wash bottle to hold the paper in place. • Transfer the liquid into the funnel by pouring the liquid down a stirring rod (see instructor demo). Rinse with a small amount of water from the wash bottle. • The liquid (called the filtrate) should come through the filter with a clear appearance. If necessary repeat the filtration step with new filter paper. • Use the 50 mL graduated cylinder to measure the total volume of the filtrate. Record the volume (estimate between the lines for the last digit). Vfiltrate =_____________mL • Describe the appearance of the liquid filtrate and the solid remaining on the filter paper.

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Place a new beaker under the funnel. Transfer the remaining solid onto the filter paper in the funnel. Once all of the solid has been transferred to the filter paper, rinse with about 5 mL of ethanol. Place the filter paper in the oven to completely dry the residue. The ethanol filtrate may be disposed of down the sink. Note: while the solid is drying, proceed with the rest of the experiment. Weigh the filter paper plus the dry residue. Describe the appearance of the residue. m filter paper + residue = _____________g

Critical Thinking Question 4: Where is the sand now? Where is the salt now?

Evaporate the filtrate as follows: • Weigh an evaporating dish and record the mass. m evaporating dish = _____________g • Pour the filtrate into the evaporating dish. • Adjust the clamp so it rests 3-5 inches above the Bunsen burner. • Connect the burner to the gas outlet, and have the instructor inspect your set-up before you light the burner.

Instructor initials: ______ • • •

Wear safety goggles. Watch your hair and clothing, and have proper footwear. Clear the work area around the burner. Light a match and hold it over the end of the burner. Open the bench valve completely. Adjust the flame color to blue by turning the barrel, and adjust the height of the flame using the knob at the base of the burner so that it just touches the bottom of the gauze on the ring clamp.

Lab 2: How Can the Percent Composition of a Salt, Sand and Iron Mixture be Determined • • •

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Allow the liquid to come to a gentle boil in the evaporating dish and keep heating until the water completely evaporates. Be careful of splattering. Turn off the burner by closing the bench valve. Record descriptions before, during and after heating.

Critical Thinking Question 5: Once the water has evaporated, is there anything remaining in the evaporating dish? If so, what could it be?

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Allow the evaporating dish to cool enough so you cannot feel heat rising from it (usually about ten minutes). Use crucible tongs to grasp the evaporating dish while balancing it on the wire gauze and take it to the weighing room. Do not squeeze the tongs to hard. If your sample spills see instructor.

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Tare the balance to zero and weigh the dish with the residue.

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m evaporating dish + residue = ______________ g

Clean-up • Rinse out the evaporating dish in the sink. • When the ring clamp and gauze have cooled sufficiently, disassemble and return all equipment to the appropriate areas. • Leftover dry mixture may be returned to the community area. Sand may be discarded into the trash. Rinse the flask, funnel and other glassware with water before returning them to your drawer. • Wipe your bench with a wet paper towel. • Return your key to the key box. Congratulations – you have just completed the first part of your scientific inquiry investigating different separation techniques.

Lab 2: How Can the Percent Composition of a Salt, Sand and Iron Mixture be Determined

Data Analysis (complete during lab) SHOW ALL WORK. Use proper units and significant figures throughout. 1.

Record raw data in Data Table: Transfer all the previous measurements to the Data Table below. Do NOT make direct measurements here! This is the FINAL, NEATLY prepared version of your data after completing data collection. A Data Table is used to organize the observations and measurements made during the lab. Be sure to include units. Mass of mixture Mass of iron filings Volume of Filtrate Mass of filter paper Mass of filter paper + sand residue Mass of empty evaporating dish Mass of evaporating dish + salt residue

part x100% whole Note: when doing any calculations, make units cancel. In this case, each mass must be in the same units (e.g. grams).

2.

Calculate the percent of iron in the original sample.

3.

Calculate the mass of the salt residue.

4.

Calculate the percent of salt in the original sample.

5.

Calculate the mass of the sand residue.

6.

Calculate the percent of sand in the original mixture.

Percent =

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Lab 2: How Can the Percent Composition of a Salt, Sand and Iron Mixture be Determined

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Results Table. Used to organize and display the end results of your calculations in one place. Mass (g)

Calculated Percent of Mixture

Iron filings Salt Sand Total (add columns)

7.

Is your total 100%, greater than 100%, or lower than 100%? Why might that be?

8.

Share your data by entering your percentages in the class spreadsheet. Answer the following questions, then your instructor will lead you in a class discussion of the class data. a.

Did all samples come from the same mixture?

b.

Did all teams get the same percentages? i. What is the average percent for each component? ii. How far from the average are the highest and lowest reported percentages for a given component?

c.

Why might different teams have different percentages?

Get Instructor’s Initials and you can leave the lab.

Instructor ______

Lab 2: How Can the Percent Composition of a Salt, Sand and Iron Mixture be Determined

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Questions 1.

Give an example of qualitative data from this lab. Give an example of quantitative data. In each cases, be specific, using an example from your collected data. You may quote data from previous pages. Qualitative: Quantitative:

2.

Refer to Data Analysis #7 and #8. How confident are you of your results (the percent composition of each component of the mixture)? Explain using your experimental notes and/or the class data. Include any sources of experimental uncertainty (experimental reasons your numbers may not be accurate – this does NOT include mathematical errors).

3.

You started with a mixture of salt, sand and iron. a. Iron was the first component removed from the mixture. What unique physical property of iron allows the separation?

b.

After filtration and evaporation the mixture is separated into two components. What is the physical property of salt that allows this method to work?

c.

Is sand a pure substance? Explain using your microscope observations (macroscopic observations in lab).

d.

Is salt (NaCl) a pure substance? Explain.

4.

The salt could be seen in the microscope as distinct from the sand. After dissolving in water, the salt reappears after boiling off the water. On the basis of these observations classify the dissolving of salt as either a physical change or a chemical change and explain your reasons.

5.

Filter paper has tiny fibers that criss-cross each other leaving tiny holes between the fibers that liquids can go through called pores. Filter paper can be purchased with different pore sizes depending on what you want to filter. Dry salt cannot pass through the filter paper, yet in the experiment you just performed the salt/water mixture passed through the filter paper. Formulate a hypothesis explaining on a particulate level (on the level of atoms, molecules and ions) why it is possible for the water and the dissolved salt to do so, while the sand stays behind.

Lab 2: How Can the Percent Composition of a Salt, Sand and Iron Mixture be Determined

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6.

Use specific examples from this lab to explain how it follows (or does not follow) the steps of the scientific method.

7.

Explain how any one aspect of this lab relates to your daily life. [Hint – look at the learning goals, techniques used in lab, chemistry concepts explored. For example, how do you separate mixtures in your daily life? Do you use any of the techniques listed here such as evaporation, filtration, or magnetic properties? Do you use any similar glassware or equipment? If so, for what purpose?]

Conclusion Answer the Experimental Question using grammatically correct English sentences. How can the percent composition of a salt, sand and iron mixture be determined?

Turn in the Entire Packet at the beginning of the next lab period