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Month: October 2017

Trick or titrate!

Your students may have outgrown getting dressed up to go trick-or-treating, but they haven’t yet outgrown the desire to collect and consume as much candy as possible. So what can you do to help them avoid a belly ache and put that candy to good use? Perform an experiment!

Coincidentally, National Chemistry Week is coming up just before Halloween, and the theme this year is “Chemistry Colors our World”. We’ve put together a fun activity that incorporates core chemistry concepts, Halloween candy, and colour to fit the festive season. Instead of tasting the rainbow, your students will get to titrate the rainbow as they determine the amount of citric acid in Skittles™.

Citric acid (H3C6H5O7) is one of the first ingredients in Skittles which means your students can perform a titration with a base (in this case NaOH) to find the amount of citric acid per Skittle using the following balanced equation:

3 NaOH (aq) + H3C6H5O7 (aq) → 3H2O (l) + Na3C6H5O(aq)

Now, it’s likely that your students will volunteer to do a taste test to determine the candy’s acidity level. Remember, the first rule of the lab is that we don’t eat things in the lab. But that doesn’t mean we can’t create a nice Skittles solution!

Our recipe calls for 10 yellow Skittles added to enough water to make a 50mL solution— stirred, not shaken (until the candy dissolves). This process should take about 10 minutes.

With most titrations, chemists know that it’s all ‘bout that base— and now it’s time to prep it. Provide your students with a stock solution of 0.2 M NaOH and have them prepare a 0.020 M NaOH solution for the experiment. It’s always good practice for them to prep their own solutions using the appropriate glassware!

Once the base is ready, they should rinse and fill a Drop Dispenser with the titrant. Then they need to prepare the Skittles sample in a beaker by adding a 10 mL aliquot of the Skittles solution, some water, a few drops of phenolphthalein, and a pH Probe.

A drop dispenser with titrant along with the Skittles solution

The Skittles sample is still yellow from the dyes in the candy. Choose lighter colored Skittles for this step so the color change of the phenolthphalein indicator at equivalence will be obvious. Thymolphthalein will also work for some of the darker colored Skittles, and the pH data is like the pot of gold at the end of the rainbow and will show your students what their eyes might miss!

Now its time to titrate the treat. Start data collection and open the valve on the Drop Dispenser so that 1-2 drops of 0.02 M NaOH are added every second.

The Skittle solution starting to change color as the pH changes

Notice the subtle streaks of pink in the solution as the reaction proceeds and the pH changes.

Pink Skittle solution after titration

Students should record the volume when the pink is permanent and continue to titrate a few more milliliters of NaOH. Now they’ve created a great new Skittles color! More importantly, the data looks beautiful too.

Looking at the graph, the students can observe that the sharp change in the pH occurred when the indicator took on the permanent pink color.

NOTE: Even though citric acid is triprotic, there is only one noticeable equivalence point on the graph after all three of the ionizable hydrogens have reacted. This is because of overlapping buffer regions of the acids and their conjugates.

After the titration of the treat, it’s time for the tricky part— data analysis. Using the volume of NaOH at equivalence, the concentration of NaOH, and the balanced equation, the students can calculate the amount of citric acid in the 10 mL sample that was reacted. From here they can calculate the amount of citric acid in their original 10 Skittles sample and finally in each Skittle.

Now that they have the basic technique down, students can design their own experiment by coming up with a question that they could answer with a titration. For example, they could compare between different colours of Skittles, between Skittles and other candies containing citric acid, and between Skittles and lemon juice.

I hope your students enjoy this titration activity and get spooktacular results. Happy Halloween!

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Glow in the Dark Science!

Fall is in full swing and Halloween is approaching. It’s the time of year for glowing ghosts, ghouls, and… science experiments!

Things that appear to glow are luminescent. Luminescent materials are literally “cool” because they give off light without needing or producing heat. Luminescence can be broken down into the following main categories: fluorescence, phosphorescence, and chemiluminescence.

Fluorescent materials will absorb energy, then quickly re-emit the energy. As a result, they only appear to “fluoresce” when they are in the presence of some form of radiation such as ultraviolet light.

The PASCO Spectrometer allows you and your students to experiment with fluorescence. Fluorescein, as the name implies, is a chemical that will exhibit fluorescence. In this demonstration, a small sample of fluorescein is diluted in water, then added to a cuvette. When held under a blacklight (ultraviolet radiation source) the sample will glow. In the Spectrometry App under Fluorescence, we can set an excitation wavelength to 405 nm.

excitation 405 nm
Spectrum of the 405 nm light used for fluorescence excitation.

When the cuvette with fluorescein is added to the Spectrometer, you can observe the “glow” indicating fluorescence.

PASCO spectrometer and sample
Fluorescein “glowing” in the PASCO Spectrometer.

Now we can observe the spectrum of the emitted light when fluorescein is excited with 405 nm light.

Fluorescence
The spectrum of fluorescein

By overlaying the spectra, we can compare the wavelength of the light that went into the sample and the light that was fluoresced by the sample.

Comparison of spectra
Notice the shift to a higher wavelength from excitation to emission.

Phosphorescent materials glow in the dark. Similar to fluorescence, they get excited by white or ultraviolet lights. But these materials slowly re-emit the energy in the form of light, even when the lights are turned off. Glow-in-the-dark toys are a great example of phosphorescence.

Finally, chemiluminescence occurs when a chemical reaction produces light without producing heat. Glow sticks are a perfect Halloween example of this. When the chemicals are mixed, a ghostly glow is given off.

So, the next time you see a glowing jack-o-lantern or an eerie zombie, don’t just think scary… think science.

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Glow Stick Kinetics

Glow Stick Kinetics

Want to illuminate the concepts of kinetics? Try Glow Sticks! Glow sticks are a very popular accessory around Halloween because they are cheap, portable and they give off a ghostly glow. But do students understand the chemistry behind the plastic tube that gives them light?

The “glow” from a glow stick is the result of a process called chemiluminescence – a chemical reaction that provides the energy to emit light. A glow stick is housing for two separate chemical solutions. The outer plastic chamber contains a mixture of a phenyl oxalate ester and a fluorescent dye. The inner glass vial contains hydrogen peroxide. When the glow stick is bent, the glass vial breaks releasing the hydrogen peroxide and activating the reaction.

Students familiar with glow sticks will remember that the light fades over time. This fading provides a perfect backdrop for introducing students to kinetics.

Initial Investigation

To study the glow stick fading, you can use a Light Level Sensor. Create a reaction vessel using a Calorimeter Cup and some black electrical tape.

Calorimeter Cup, Glow Stick, Electrical Tabe and Light Level Sensor

Tape the Calorimeter Cup to ensure a dark baseline. Leave a hole in the top for the Light Level Sensor.

Light Level Sensor over Reaction Vessel

Create a graph of Light Level vs. Time using SPARKvue® software. Remove the reactants from the plastic casing to eliminate interference with the Light Level Sensor.

CAUTION: The reactants are non-toxic and non-flammable, but contact with skin or eyes may cause discomfort. In case of contact, rinse with water. Reactants can also permanently stain clothing or furniture. Use appropriate precautions.

Carefully cut a hole in the top of the glow stick and pour the phenyl oxalate ester and fluorescent dye mixture into a small beaker. Then break the glass vial and pour the hydrogen peroxide into the small beaker.

Glow Stick reaction

Cover the reaction vessel with the lid and the Light Level Sensor, and start data collection.

Room Temp Glow stick light level
The light intensity is decreased to half its original value after about 5 minutes.

Exploration

Now that it is established that the reaction can be studied with the Light Level Sensor, you can ask your students to explore factors that would affect the reaction. Some possible ideas for exploration could include:

  • Can you make the reaction brighter?
  • How does temperature affect the reaction?
  • Does the concentration, or amount of reactants affect the reaction?

For example you could have the students look at temperature. Create a hot water bath. Cut the top off of the glow stick and heat it up in the hot water bath.

Hot water bath

Once the hot water bath with the glow stick has reached a sufficient temperature (about 600C), pour some of the hot water into the Calorimeter Cup. Add a Fast Response Temperature Probe to the water in the Calorimeter Cup. Build a page with a graph of Light Level vs. Time and digits display of Temperature. Prepare the glow stick components as before and start data collection.

Glow Stick - high temperature reaction
The initial light level is much higher than the room temperature glow stick, and the reaction happens at a much faster rate.

Using the Light Level Sensor, a Calorimeter and a glow stick provides a unique way to to explore kinetics with materials that are both familiar and fun!

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Leaf Absorption Spectra

Leaf Absorption Spectra

Fall is the perfect time of year for you and your students to investigate plant pigments. If the curriculum sequence doesn’t fit, you can always squirrel away some data for later! PASCO’s Wireless Spectrometer (and its free Spectrometry software) makes it quick and painless to do data collection and get full spectrum scans. No more warm-up time, tedious wavelength adjustments, or students waiting in line!

Download Free Lab Activity

Analyzing the absorption spectra is a popular lab activity that uses simple materials and can easily be extended into a student inquiry. PASCO’s biology team developed this new experiment to complement existing manuals and is available free to download.

Sample data showing the absorption spectra from several tree species (Tree of Heaven, Big Leaf Maple, and Black Oak leaves).

Enzyme Data-palooza

Enzyme Data-palooza

Many biology courses at every level start the year studying enzymes since they are integral to so many biological processes. No matter what your preferred enzyme and substrate, sensors can help you get more data, and better data, faster. As you can see, we like to catalyze with catalase since it’s widely available (in nearly all living things) and easy to process for lab use. For example, using the Wireless Pressure Sensor with a simple yeast suspension and 3% H2O2, students can begin collecting data for under $100/station – and look at how much data can be produced!

Figure 1. Each setup uses a 15 mm test tube, #1 1-hole stopper, 3 mL of 3% H2O2, and 1 mL of yeast suspension in a water bath at a set temperature.
Figure 2. SPARKvue monitors bath temperature while recording pressure data for each reaction.

Table 1. Rate of Reaction for Yeast and Hydrogen Peroxide

Condition Trial 1
kPa/min)
Trial 2
(kPa/min)
Trial 3
(kPa/min)
Average
(kPa/min)
3°C 0.280 0.155 0.733 0.389
15°C 0.547 0.633 0.589 0.590
25°C 0.981 1.015 1.134 1.043
35°C 1.591 1.732 1.643 1.655
45°C 1.045 0.941 0.899 0.962
Figure 3. Graph of Reaction Rate vs Temperature.

With more data, it’s possible to produce meaningful descriptive statistics with your students. This supports students’ math literacy and is useful to identify runs/groups that are outliers from the class data. While these outliers are often procedural errors, they can provide a useful springboard to inquiry. Ask students to determine why the group’s results differ. Are their results reproducible? If time allows, they can explore additional variables that can add to their understanding of protein structure, enzyme reactions, and even evolution.

The data above was produced using the Wireless Pressure Sensor. The protocol is the same for the Oxygen Gas Sensor; download the lab for your preferred approach.

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Ocean Acidification

Ocean acidification is a byproduct of increasing atmospheric CO2 levels around the globe, which is also causing climate changes. This global phenomenon can be difficult conceptually for students, but it is easy to model using sensors.

Here’s a quick video using the Wireless CO2 and Wireless pH sensors:

Using a few simple materials, this quickly demonstrates to students what impact gaseous CO2 can have on the pH of water as it dissolves. Ask students to consider the limitations of this model: How does it differ from earth systems? How could the model be improved?

Looking to extend this into a student lab? We’ve got an inquiry lab where students can act as the CO2 sources and monitor pH with a sensor, while learning more about the chemistry behind ocean acidification.

Free Downloads:

Here are a handout and PowerPoint presentation that have been used when performing this activity as part of a PASCO workshop.

 

 


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