Calibration Curves for Salt (NaCl) Solutions

August 29, 2014

Calibration curves produced by different student groups to determine the relationship between density and concentration of salt (NaCl) solutions.

Calibration curves produced by different student groups to determine the relationship between density and concentration of salt (NaCl) solutions.

To start with chemistry class, we’re studying the properties of substances (like density) and how to measure and report concentrations. So, I mixed up four solutions of table salt (NaCl) dissolved in water of different concentrations, and put a drop of food coloring into each one to clearly distinguish them. The class as a whole had to determine the densities of the solutions, thus learning how to use the scales and graduated cylinders.

However, for the students interested in doing a little bit more, I asked them to figure out the actual concentrations of the solutions.

One group chose to evaporate the liquid and measure the resulting mass in the beakers. Others considered separating the salt electrochemically (I vetoed that one based on practicality.

Most groups ended up choosing to mix up their own sets of standard solutions, measure the densities of those, and then use that data to determine the densities of the unknown solutions. Their data is shown at the top of this post.

Finding the mass of solution in order to calculate its density.

Finding the mass of solution in order to calculate its density.

The variability in their results is interesting. Most look like the result of systematic differences in making their measurements (different scales, different amounts of care etc.), but they all end up with curves where the concentration increases positively with density.

I showed the graph above to the class so we could talk about different sources of error, and how scientists will often compile the data from several different studies to get a better averaged result.

Then, I combined all the data and added a linear trend line so they could see how to do it using Excel (many of these students are in pre-calculus right now so it ties in nicely):

Trend line from combined data.

Trend line from combined data.

What we have not talked about yet–I hope to tomorrow–is how the R-squared value, which gives the goodness of the fit of the trend line to the data, is more a measure of precision rather than accuracy. It does say something about how internally consistent the data are, but not necessarily if the result is accurate.

It’s also useful to point out that the group with the best R-squared value is the one with only two data points because two data points will necessarily give a perfectly straight line. However, the groups that made more solutions might not have as good of an R-squared value, but, because of the multiple measurements, probably have more reliable results.

As for which group got the most accurate result: I added in some data I found by googling–it came off a UCSD website with no citation so I’m going to need to find a better reference. Comparing our data to the reference we find that team AC (the red squares) best match:

The straight line shows my (currently) accepted values for the concentration/density relationship.

The straight line shows my (currently) accepted values for the concentration/density relationship.

Citing this post: Urbano, L., 2014. Calibration Curves for Salt (NaCl) Solutions, Retrieved May 29th, 2017, from Montessori Muddle: http://MontessoriMuddle.org/ .
Attribution (Curator's Code ): Via: Montessori Muddle; Hat tip: Montessori Muddle.

Glass Bending

March 14, 2014

Students bend glass tubing for their steam distillation apparatus.

Bending the glass tubing is fairly straightforward, but looks awfully sciency.

Our steam distillation apparatus for extracting lavender oil started off fairly simply — a steamer connected to a glass tubing running under the cold water tap to a collection flask — and evolved from there. One of the final tweaks we attempted, was to make a coil in the glass tubing so the steam would have a longer transit through the ice-water bath to enhance condensation.

We heated the glass using a small butane burner until it became pliable, then bent the tube into shape around a piece of wet wood. Using the wood was not as effective as we’d hoped because the glass tubing is fairly thin and cools down quickly when in contact with the water.

You also have to be very careful when bending the tubing to make sure you don’t pull on it. Pulling stretches the glass, making the walls thinner, making it more likely to break. My students discovered this the hard way.

Citing this post: Urbano, L., 2014. Glass Bending, Retrieved May 29th, 2017, from Montessori Muddle: http://MontessoriMuddle.org/ .
Attribution (Curator's Code ): Via: Montessori Muddle; Hat tip: Montessori Muddle.

Biochemical Art

November 23, 2013

Hydrophobic art.

The art of hydrophobia.

A beautiful demonstration of the interaction between detergents and fats (in milk). The food coloring acts as a tracer.

Citing this post: Urbano, L., 2013. Biochemical Art, Retrieved May 29th, 2017, from Montessori Muddle: http://MontessoriMuddle.org/ .
Attribution (Curator's Code ): Via: Montessori Muddle; Hat tip: Montessori Muddle.

The Effect of Baking Soda on Cookies

November 8, 2013

Cookies about to come out of the oven.

Cookies about to come out of the oven.

My chemistry students did a little experiment to investigate the effect of varying baking soda amounts on cookies. They did four batches of cookies based off of the recipe on the back of a bag of chocolate chips. The batches used:

  1. No baking soda.
  2. The amount of baking soda recommended by the recipe.
  3. Double the baking soda.
  4. The recommended amount of baking soda plus about 30 ml (1 tablespoon) of orange juice.

The last batch used orange juice for its acidity. We hypothesized that more baking soda, and more acidity, would increase the size of the cookies, making them fluffier. The hypothesis was supported by our rather tasty evidence.

Collecting data on cookie fluffiness and taste. Because of the smell emanating from the kitchen, there were no shortage of test subjects.

Collecting data on cookie fluffiness and taste. Because of the smell emanating from the kitchen, there were no shortage of test subjects.

Although our focus was on the physical chemical reaction of the baking soda (sodium bicarbonate) and acid to produce the carbon dioxide bubbles that make the cookies rise, the making of the cookies also allowed us to talk a little about the food science behind the role of the flour. Specifically, we discussed the long chain gluten proteins that stretch out and trap the bubbles. We’ll talk a bit more about this next time when we make bread.

Citing this post: Urbano, L., 2013. The Effect of Baking Soda on Cookies, Retrieved May 29th, 2017, from Montessori Muddle: http://MontessoriMuddle.org/ .
Attribution (Curator's Code ): Via: Montessori Muddle; Hat tip: Montessori Muddle.

CSI: TFS

November 2, 2013

Identifying the culprits using blood testing.

Identifying the culprits using blood testing.

At the suggestion of Mr. Elder, I put together a Crime Scene Investigation (CSI) simulation for one of our afternoon interim activities. Sixteen students were challenged to solve a murder/mystery using simulated blood tests, fingerprinting, hair analyses, and chemical tests for drugs. And the assailants and the victims were members of the group.

Knife at the crime scene.

Knife at the crime scene.

I set up the crime scene with four different lines of evidence — fingerprints, hair, blood, and drugs — and forensic methods, so I could break my students up into four groups. The students were all told that they were competing to solve the mystery; to find out what happened and who did what to whom. Without any coaxing, the groups each claimed proprietary rights one type of evidence and set about trying to solve the mystery on their own. Since none of the lines of evidence could explain everything from the crime scene they ended up having to combine what they all found.

A blood soaked murder weapon (also with fingerprints and hair sample).

A blood soaked murder weapon (also with fingerprints and hair sample).

The Crime Scene

There were two weapons lying on the floor: a bloody knife and a bloody rolling pin with a hair stuck to it. On the table above the weapons were a few lines of white powder. There seemed to have been originally four lines, but one and one half of them had been used. There were fingerprints and a strand of hair next to the powder lines.

Also on the table, close to the powder, were a deck of cards (with fingerprints), a set of poker chips, a scale, and another stray hair.

Fortunately for our detectives, the fingerprints and hair had already been pulled and tagged.

The crime scene setup.

The crime scene setup.

Acquiring the Evidence

It took quite a bit of effort to acquire and plant the evidence. Some of it, like the blood, was simulated, but I had to get the hair and fingerprints from the students themselves. Since the individuals who chose this activity were a self-selected fraction of the middle and high-schoolers, I wandered around the building at lunchtime at the breaks between classes trying to find one or two students who were by themselves or were in a group with others who had not chosen the CSI activity.

The crime scene setup really only requires evidence of two people, but to keep it a little more mysterious I used a little misdirection. I got five students to contribute fingerprints and hair, but told them all that they’d be the murderer. I also got one person who was not in the class to contribute as well so we’d have a set of completely mysterious evidence.

Fingerprints

I pulled fingerprints by having students rub their fingers on a black spot I’d created using a basic number 2 pencil. The student would get the black graphite on their fingers and then touch their fingertips to the sticky part of some clear tape. The fingerprints turned out quite clearly that way.

Since I did not have time to figure out how to transfer the fingerprints to the surfaces I wanted them on, I just stuck the pieces of clear tape where I wanted them in the crime scene, which also saved the detectives a bit of time and effort.

Once I told them how to get the fingerprints from their peers, the students did not need any other guidance about how to analyze the fingerprints. They took the imprinted sticky tape and stuck them to a sheet of white paper, where the black prints showed up quite nicely. Then they fingerprinted everyone in the classroom and compared, looking for whirls and swirls primarily, but also basing their conclusions on the size of the prints which they took to be indicative of gender.

Comparing fingerprints.

Comparing fingerprints.

Of the four sets of prints, they were able to accurately identify the two people who were holding the knife and the rolling pin. The misidentified the one set that was from a person not in the class, and could not find the match for the last set.

Interestingly, of the four students in the group, two did most of the work while the other two wondered off to join other groups.

Hair

Hair was easy enough to collect since the students were quite happy to donate one or two for the cause. One hair per student would have been sufficient, but I kept loosing them until I just decided I’d stick them onto a piece of clear sticky tape and leave the sticky tape with hair attached at the scene of the crime.

Examining hairs under the microscope.

Examining hairs under the microscope.

With only a little nudging, the group working on the hair realized that they could get out one of the compound microscopes to examine their specimens, and compare them to the students in the class.

One major indicator that helped with the hair identification was the length. Two of the hair samples were from girls with long hair, while one was from a fairly short haired boy. I did consider just leaving pieces of the hair as evidence, instead of whole strands, but it’s a good thing I did not since, for one reason or another, the hair group had a difficult time identifying the owners of their samples (lack of effort might have been one part of it). It did help a bit that the two major perpetrators of the crime were members of that group.

Drugs

My idea here was to simulate a drug (cocaine) deal gone bad because of a contaminated/cut product. I laid out three lines of corn starch to simulate the cocaine and one line powdered glucose in between the last two cocaine lines to represent the adulterated drug. I removed the last cocaine line and half of the glucose line to make it look like someone had been ingesting the lines and stopped part-way through.

The lines of powdered substance (cocaine) were severely disrupted by student's sampling, but you can still see the two full lines to the right and the half line that the spatula is touching.

The lines of powdered substance (cocaine) were severely disrupted by student’s sampling, but you can still see the two full lines to the right and the half line that the spatula is touching.

Since we’ve been testing for simple and complex carbohydrates in biology and chemistry classes I told the group testing the drugs that the test for cocaine was the same as the iodine test for starch: if you add a drop of potassium iodine to a starch solution then it turns black.

If the students had examined the drugs on the table closely enough they should have been able to see that the glucose line was different from the others; it was not as powdered (so the crystals were small but visible), and it did not clump as much as the corn starch. However, they did not, and I had to hint that they should perhaps test all the lines of powder instead of just the first sample they took.

When they discovered that one of the powder lines did not react with the potassium iodine, I told them that a common adulterant was sugar so they should perhaps test for that. One of the students remembered the Benedicts solution test, which they were able to easily conduct since I’d already had the hot water bath set up for them.

Testing for glucose.

Testing for glucose.

Looking through the United Nations Office on Drugs and Crime’s Recommended Methods for the Identification and Analysis of Cocaine in Seized Materials, it seems that a common test for cocaine (the Scott test) turns a solution blue when the drug is present, so the next time I try this I may have to find some tests that produce a similar color change.

Blood/DNA testing

Simulating the blood testing was one of the trickier parts of the procedure for my part since I had to keep things very organized when students started being sent to me to be blood tested.

The blood was actually a few drops of food coloring diluted into 10 ml of water. I used three drops of red in each case to try to at least get it to a somewhat blood-like color, but then in mixed in one or two other colors to get five unique blood types.

The number of drops of food coloring mixed with 10 ml of water to get the 5 blood types.

  • Type 1: 3 red + 1 blue
  • Type 2: 3 red + 1 green
  • Type 3: 3 red + 1 yellow
  • Type 4: 3 red + 1 green + 1 yellow
  • Type 5: 3 red + 1 blue + 1 yellow

To match everything up with the crime scene, I assigned Suspect A to have Blood Type 2, and Suspect B to have Blood Type 4. So a sample of Blood Type 4 went on the knife, and a sample of Type 2 went on the rolling pin.

As a result, when the blood type testing group wanted to blood test everyone in the classroom, I had them send the students to me one at a time and I handed each student a small cup with a random sample of one of the Blood Types, except for the two students whose blood were on in the crime scene. With 16 students, we ended up with three or four students with each blood type.

Blood type testing using chromatography.

Blood type testing using chromatography. The little containers of food coloring can be seen to the upper left.

This blood sample -- from the rolling pin -- is beginning to separate into its constituent colors (red, yellow and blue).

This blood sample — from the rolling pin — is beginning to separate into its constituent colors (red, yellow and blue).

The students took their blood samples back to the testers who I’d shown a simple chromatography method. They’d cut out thin (< 1cm wide) strips of coffee filter, put a drop of the blood sample on the middle of the strip, and then taped it down to a sheet of clear overhead transparency film. Although any clear glass or plastic would have worked, the transparency film was nice because you could tape five coffee filter strips to one sheet and then loosely roll the sheet up and put one end into a partially filled beaker of water (see Figure above). Capillary action sucked the water up the strips and smeared out the blood samples so you could see its constituent colors. The method worked pretty well, and the students were able to compare the blood at the crime scene to their test results to identify the small group of people who shared the suspect blood types. It was a lot of work, and it would have taken much longer if the group doing it were not amazingly organized and worked extremely well together.

This method is more akin to blood type testing than DNA testing, which I’d have liked to simulate better, however I did not have the time to work on my chromatography method.

In Conclusion

It took a little coaxing to get them to the right conclusion in the end, but I and the student had a lot of fun solving the mystery.

Citing this post: Urbano, L., 2013. CSI: TFS, Retrieved May 29th, 2017, from Montessori Muddle: http://MontessoriMuddle.org/ .
Attribution (Curator's Code ): Via: Montessori Muddle; Hat tip: Montessori Muddle.

Testing for Sugars and Starch

October 23, 2013

Testing for sugar using Benedict's solution.

Figure 1. Testing for sugar using Benedict’s solution. The test tube on the left indicates the presence of simple sugar, while the three on the right show none.

The standard biology-class test for simple sugars is to mix equal parts of Benedict’s solution and your sample solution, then heat it up in a hot water bath (80-100 ºC) for about five minutes. If there are simple sugars the mixture will turn from blueish green to reddish orange.

A ring shaped (cyclic) glucose molecule. Image via Wikipedia.

Figure 2. A ring shaped (cyclic) glucose molecule. Image via Wikipedia.

Simple sugars are those basic building blocks (monomers), which are chained together to form the more complex sugars and starches. The simplest are the monosaccharides (mono=one and saccharide=sugar) like glucose and fructose. Glucose is a chain or ring (see Figure 2) of six carbon molecules with the chemical formula C6H12O6. If you link two glucose molecules together, you get a disaccharide (di-two), which is called maltose.

Ms. Mertz had students tape two glucose molecules together to form maltose.

Figure 3. Ms. Mertz had students tape two glucose molecules together to form maltose.

Ms. Mertz did this experiment with her biology class last week using apple juice, oatmeal, corn syrup, honey, and an unknown as samples.

Ms. Mertz pulls samples out of the water bath.

Figure 4. Ms. Mertz pulls samples out of the water bath.

The biology class also tested for starch. Starches are really long chains of sugar molecules called polysaccharides. The simple sugar, Benedict’s solution test does not pick them up because the solution only reacts at the ends of the molecule, and with the long chains of the starch there aren’t that many sites for reactions.

Sample solutions to be tested.

Figure 5. Sample solutions to be tested.

The test for starch is to add a few drops of potassium iodine solution to your sample. Starch turns the resulting solution a bluish black.

Testing for starch using iodine solution.

Figure 6. Testing for starch using iodine solution.

Citing this post: Urbano, L., 2013. Testing for Sugars and Starch, Retrieved May 29th, 2017, from Montessori Muddle: http://MontessoriMuddle.org/ .
Attribution (Curator's Code ): Via: Montessori Muddle; Hat tip: Montessori Muddle.

Chasing Water

October 15, 2013

Ms. Mertz’s biology class chased water droplets around a piece of wax paper to study the properties of water. It was pretty neat how she had them name the droplets and then use a toothpick to drag them around, join them up, and split them apart.

Dragging water droplets around a piece of wax paper using a wooden toothpick.

Dragging water droplets around a piece of wax paper using a wooden toothpick.

The water sticking together to form droplets is due to the hydrogen bonding between the water molecules: each water molecule has a slightly positively charged end and a slightly negatively charged end that causes molecules to stick together.

Hydrogen bonding among water molecules is due to the shape of the individual molecule. Since the molecule is "bent" one end has a slightly more negative charge and the other a slightly more positive. Image by User Qwerter at Czech wikipedia.

Hydrogen bonding among water molecules is due to the shape of the individual molecule. Since the molecule is “bent” one end has a slightly more negative charge and the other a slightly more positive. Image by User Qwerter at Czech wikipedia.

The ability to drag the water droplets around using a toothpick is because the cellulose fibers in the wood have their own slight charges that make them hydrophilic.

The students then tried dragging the water droplets around using a small piece of plastic straw, which was not supposed to be hydrophilic. However, it was a little hard to tell the difference between the straw and the wood. We’re not sure why, so we’ll have to revisit that part of the experiment again.

Ms. Mertz followed up with another nice little demonstration of the effect of soaps on water. She sprinkled some black pepper onto the surface of some water in a bowl, and then took a toothpick, dipped it into a bottle of liquid dish-washing soap, and then dipped the tip into the center of the bowl. The result was quite immediate, and quite dramatic.

A bowl of water with black pepper floating on top, just before dipping in the soap covered toothpick.

A bowl of water with black pepper floating on top, just before dipping in the soap covered toothpick.

After dipping in the soap covered toothpick.

After dipping in the soap covered toothpick.

The soap molecules are forced to form a thin layer on top of the water as their charged end is pulled down toward the water and their uncharged, hydrophobic end is pushed away.

Citing this post: Urbano, L., 2013. Chasing Water, Retrieved May 29th, 2017, from Montessori Muddle: http://MontessoriMuddle.org/ .
Attribution (Curator's Code ): Via: Montessori Muddle; Hat tip: Montessori Muddle.

Copper Plating

October 11, 2013

As an introduction to ionic compounds, my chemistry students hooked up a dime to an electrode in a copper chloride solution. It’s not exactly copper plating, but the color is quite interesting.

A copper plated dime.

A copper plated dime.

It was also interesting to see how the color of the copper chloride solution changed as well: from a dark to pale blueish green as the copper was extracted by the electrolysis.

Citing this post: Urbano, L., 2013. Copper Plating, Retrieved May 29th, 2017, from Montessori Muddle: http://MontessoriMuddle.org/ .
Attribution (Curator's Code ): Via: Montessori Muddle; Hat tip: Montessori Muddle.

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