Exploring Matter: Chemistry Demonstrations
In this issue of CoolStuff, we'll once again welcome guest author Patty Carlson. At New Trier High School, Patty is known for her ability to make chemistry come alive. Her flair for great demonstrations and labs certainly comes through in the upcoming collection of activities illustrating properties of matter. I know you'll enjoy sharing these marvels with your students! I have known and worked with Patty Carlson for over a decade and feel privileged to teach and grow with her. Patty is an energetic, creative, and caring professional who has earned the respect of her students and the admiration of her colleagues. She relates well to students, knows her subject inside and out, and is enthusiastic about sharing her knowledge with others. Patty is intrigued by the simplest things, which, I believe, explains her success. She seems more fascinated with the natural world around her each day and she shares this growing sense of wonder with her students.
Exploring Matter Activities
Chemistry is all about studying matter and how it changes. Fortunately many characteristics of matter are macroscopic, that is, we can directly observe them without the aid of any lens other than those in our own eyes. We can watch as matter is mixed or reacted and ultimately may be able to infer something about its deeper, more abstract structure (electron configurations that determine bonding, or the pairing/ unpairing of electrons that result in the magnetic properties of certain elements, the molecular shape of a molecule that influences its polarity, or the “packing” of atoms that determines the density of a substance.)
In the following activities students are encouraged to poke, prod, pour and play (wow…..that’s a lot of alliteration…) with matter and watch as it responds. I’ve found over the years that using familiar, household items as much as possible reduces the intimidation factor that some students feel in physical science classes, especially in the beginning of the year when many of the concepts explored below are introduced. The Reddi-Whip will go fast as will any extra soda you might have leftover to wash it down. Make sure you have lots of Styrofoam cups too because even my high school students want to see that acetone/cup demo over and over and over and over…..
1. Like Dissolves Like
Ever have a nasty stain on your shirt that won't come out in the wash, no matter how many times you try, and yet that same stained shirt comes back from the dry cleaner looking like new? If you have, you've experienced the chemical phenomenon of "like dissolves like". That is, substances tend to dissolve in things that are similar to them. By 'similar' in this case we mean in terms of their polarity. Some stains dissolve better in a polar substance like water and some stains require a more non-polar substance to dissolve them away.
Let's consider two solvents that are pretty different in their polarities in order to explore this topic. Water, which we said is a polar solvent, dissolves almost anything that is polar, such as salt and many other ionic compounds. Water can't dissolve everything, though. Try removing fingernail polish with water and you'll see what I mean. Acetone, a solvent with some non-polar properties, is commonly used to do that job. Acetone is an effective solvent for all sorts of non-polar substances.
Place two large glass beakers side-by-side. Pour water into the first beaker until it’s about half full. Place a Styrofoam cup in the water beaker. Nothing will happen. Styrofoam is non-polar, water is polar and, since “like dissolves like,” they will not dissolve in each other.
Now pour some acetone into the other beaker and place another Styrofoam cup into that beaker. You’ll see the cup slowly break down until it is just a glob of goo. Acetone can get in between the components of the polymer of plastic and allow the air in the cup to escape (don’t worry, they don’t use CFC’s in Styrofoam anymore so there is no harm to the environment when doing this demo).
The goo you retrieve from the beaker is actually polystyrene plastic (#6 in recycling code) and is the same plastic used to make plastic table ware, etc. You can shape it any way you wish while it is wet and it will harden over time as all the acetone completely evaporates away. In order to completely dissolve the plastic, you’d need a stronger and more non-polar solvent.
Place starch packing peanuts (the environmentally friendly packing option commonly used today) in a beaker of acetone. Since the starch packing peanuts are polar, they will not dissolve in acetone. Put the starch packing peanuts in a beaker of water, mix around a bit and you’ll see they dissolve readily
Old-fashioned Styrofoam packing peanuts are fun to play with too. You’ll need a large beaker filled about half full with acetone. Have someone ready with a large wooden spoon and start loading the Styrofoam packing peanuts into the beaker as your helper stirs like crazy. You’ll be amazed at how many peanuts will fit into the beaker.
An interesting and often surprising property of a substance is its density, or the ratio of a certain mass of that substance to its volume. As long as you keep the temperature the same, the density of a particular substance never changes. You may have felt how heavy a chunk of lead is compared to a chunk of aluminum of the same size or perhaps you’ve held a jar of metal mercury and been amazed by how heavy even a small amount of this element is. These are differences in density. Since chunks of lead and jars of mercury are a little hard to come by, let’s explore this with some pretty ordinary stuff: Coke and Diet Coke.
Get a large, glass beaker (or aquarium) filled with water, a can of Coke and a can of Diet Coke. Place both cans in the water. The Coke will sink; the Diet Coke will float. Ask students to hypothesize about why this is so. (Caption: The difference between the two densities is real, but subtle. Make sure to do this in a large volume container (1000-2000 ml) in order to make the difference as obvious as possible. The density if Coke is slightly above 1.0 g/ ml and the density of Diet Coke is just about 1.0 g/ ml. The density of water (at room temp) is 1.0 g/ml. We assume the aluminum cans are identical in density.)
Challenge your students to design an experiment that will allow them to determine exactly what the densities of the two sodas are. This can be done easily using small graduated cylinders (10 ml) and an electronic balance. For example, they can pour 2 ml of coke into the graduated cylinder, place the cylinder on a balance and record the mass. (Of course, they should correct for the mass of the graduated cylinder.) This would be their first “data point”. They can repeat this technique with 4 ml, 6 ml and 8 ml of the Coke and corresponding masses for those volumes. This entire process is repeated with Diet Coke. Make sure students don’t get them mixed up. They may taste different, but they look identical in the lab.
Once they have gathered their data, can find the density by one of two methods: graphing the data and finding the slope of the mass vs. volume line (most accurate), or simply finding the average density from the data points. When graphing, students should include 0,0 as a data point, since zero volume of soda has a mass of zero.
The students will probably guess that the only real difference between these sodas is the sugar content. Coke contains approximately 39 grams of sugars (high fructose corn syrup and/or sucrose, which is regular old sugar) . Diet Coke contains Nutrasweet (aspartame) and since Nutrasweet is SO much sweeter than sugar, only about 100 milligrams per can are required to get it to match Coke’s level of sweetness. That’s a pretty big difference and the reason for the difference in densities of the two sodas.
If you want to make it more interesting, try the new low-carb Coke, C2, and see where its density falls with respect to the other two. It contains a combination of artificial sweeteners (aspartame, acesulfame potassium, and sucralose, which is Splenda) in addition to high fructose corn syrup and/or sugar. You can also try different brands. Tab contains saccharin and Diet Rite uses a combination of artificial sweeteners, giving them a slightly different density.
III. Density Columns
Here’s a Demo and/or Activity that uses the concept of “like dissolves like” and density! You’ll need: Dark Karo syrup, Water (with food coloring too help students identify which layer it is in the column), Vegetable oil, Rubbing alcohol (isopropyl alcohol), and Large glass cylinder (or any long tube will do. It doesn’t have to be graduated).
To do this as a demo, take the glass cylinder and pour in the dark Karo syrup (the most dense in this list). Then carefully pour in the colored water. You’ll note that they mix a little bit (there will be a “blur” between the two layers), but they are still distinctly layered. (The sugary syrup has some polar properties and the water will dissolve it at the point of contact.) Then pour in the vegetable oil. Because oil and water don’t mix (oil is non-polar, water is polar) they will also form distinct layers. For the last layer, add the rubbing alcohol. This can get messy and the column will need time to settle itself down. The alcohol will dissolve in water (alcohol has a polar region), but the oil will form a barrier between the water and alcohol. When you pour the alcohol into the column, it will come into contact with the oil and go from clear to murky. Again, there will be a blurring of the “line” between the two layers due to partial solubility (rubbing alcohol has non-polar parts too and oil is non-polar so a little mixing will occur).
To do this as a lab activity, give students smaller columns and the same 4 liquids. Let them pour the liquids in any order they wish. Based on their observations, they should be able to figure out which liquids are more dense than which others. Finally, they will be able to rank the liquids according to their relative densities.
- Rubbing alcohol 0.87 g/ml
- Vegetable Oil 0.91 g/ml
- Water 1.00 g/ml
- Dark Karo Syrup 1.37 g/ml
To add a little complexity to this activity, ask the students to infer the approximate densities of the following solids: Ball bearing, Plastic bead, Cork, Rubber stopper
They can do this by dropping the objects, one-by-one, into the column and see if they float or sink in a particular layer. If they know the numerical value for the densities of each of the 4 liquids, they can approximate a value for the density of each of the solids. Students should observe the following sequence, in order from least to most dense: Cork – Rubbing Alcohol – Vegetable Oil – Plastic Bead – Water – Rubber Stopper – Karo Syrup – Ball Bearing
IV. Classification of Matter
Matter is anything that has mass and takes up space (has volume). We can separate the matter that we know about into two huge categories; mixtures and pure substances. Well, what are mixtures? Mixtures are physical combinations of at least two pure substances. Most of us are much more familiar with mixtures than pure substances and they are indeed much more common in our everyday experiences. For more on mixtures, check this out: http://www.factmonster.com/ce6/sci/A0833482.html.
Mixtures can be further categorized into homogeneous and heterogeneous mixtures. Homogeneous mixtures are mixtures with the same composition throughout. Let’s say you stir some powdered Kool-aid mix into a pitcher of water. Once the powder is dissolved, doesn’t that Kool-aid look and taste the same from the first sip to the last? Compare that to some orange juice with pulp in it. Let’s say your brother never shakes up the carton when he pours himself a glass of juice. By the time you get it, there is a huge blob of pulp at the bottom of the carton. Now your glass is a combination of juice and big globs of pulp. That, my friend, is a heterogeneous mixture and a gross one at that. Heterogeneous mixtures are not uniform in composition at all. Now, what are pure substances? These are either individual elements right from the Periodic Table or compounds (chemical combinations of those elements). The element Iron, for example, is a pure substance. Let’s say we let that iron sit around outside for while and we notice it starts to rust. It has undergone a chemical reaction and combined with oxygen in the atmosphere to create iron oxide, which is a compound, and, by itself, also a pure substance. These elements aren’t “mixed’ together like the mixtures we talked about before, they are BONDED together in a chemical way that won’t allow you to un-bond them very easily. Getting confused? Let’s take a look at some examples and maybe things will clear up.....
This is a station-based activity (or “smorgasbord” as my buddy Chris Chiaverina calls them) so you’ll need lots of lab space and a place for kids to walk around in small groups. Collect items like the following (and/or add your own!) and place them around the lab benches. Ask the students to
Identify the category of matter:
- Is it a pure substance? If so then is it an element or is it a compound?
- Is it a mixture? If so, then is it a heterogeneous mixture or homogeneous mixture?
- (Optional) Have the students write down the criteria they use for their categorization schemes
Devise a separation strategy for any mixtures found. In other words, if you think you’ve spotted a mixture, how would you separate it into different components, and (if possible) all the way to the pure substances that comprise the mixture? (Remember, pure substances cannot be separated by physical means. They must be separated chemically, or, in the case of elements, by splitting atoms! That’s beyond the scope of the activity for the day.)
- Aluminum foil: (Pure substance, element)
- Lucky Charms: (Heterogeneous mixture.) Separate physically. Visually identify the cereal from the sugary charms and manually sort into two piles. Separation beyond this is too difficult.
- Orange juice with pulp: (Heterogeneous mixture.) Separate by gravity filtration of the pulp.
- Salt water: (Homogeneous mixture.) Separate by boiling away or evaporating the water and leaving the salt crystals behind.
- Salt, sand and water: (Heterogeneous mixture.) Separate by filtering out the sand, boiling off water and leaving salt crystals behind.
- Reddi Whip dessert topping: (Homogeneous mixture…really a colloid, but that may be too fine a point here.) Separate gas from solid portion by heating it. Gas will bubble out since less soluble at higher temps, leaving solid portion behind. Separation beyond this is too difficult.
- Oil and vinegar salad dressing:(Heterogeneous mixture.) Separate by difference in density. (A separatory funnel is a good tool for this.)
- Chocolate Silk Jif: (Homogeneous mixture.)Separation strategy: good luck J Some homogeneous mixtures are so uniform, even at the microscopic level, they seem extremely difficult to separate by conventional means.
- Aspirin (make sure this is pure aspirin with nothing added, like buffers or anything else): (Pure Substance, compound – acetylsalicylic acid. All aspirin is this compound. People buy different aspirin products for different reasons. Buffered aspirin helps those prone to stomach upset, etc…)
- Juice Bar Candy Refreshee Spray Perfume: (Homogeneous mixture.) Separate by differences in boiling point using fractional distillation. (What’s that?
- Iron and Sulfur (literally iron filings and powdered sulfur): (Heterogeneous mixture) Separate by difference in magnetic properties.
Demo Idea: Combine the iron and sulfur mixture from # 11 in a test tube. Heat over a bunsen burner under the hood, you can show the students that a new substance is chemically formed. It is a pure substance and a compound, iron sulfide (FeS). It no longer has any magnetic properties at all and proves that is has been chemically changed from two elements to a single compound with completely different physical properties.
V. What's the Matter?
We are all probably aware of the basic states of matter: solids, liquids and gases. When we see a solid, we expect it to act like a solid, that is, have a definite volume and a distinct shape (at a given temperature). When we see liquids we expect them to behave like liquids. They should flow easily, no matter how hard or gently we stir them around.
Are there substances that don’t behave the way we think they should? Sure! They’re called non-Newtonian substances.
Things you’ll need:
- Electronic balance
- 150 ml beaker
- Glass stirring rod
- Disposable cup
- Hot plate
- 10 ml graduated cylinder
- Hot mitts
- De-ionized water
- Polyvinyl alcohol (PVA) the powder form
- Saturated Borax solution (add enough borax to water so that it turns cloudy. You can put it on a magnetic stirrer to keep the particles suspended)
- Mass out 2.00 grams of the PVA. Set aside.
- Pour 50 ml of deionized water into the beaker. Insert the thermometer into the beaker and place the beaker on the hot plate. Heat gradually to about 90 degrees. Do not let it boil rapidly or you will lose too much water and your slime will be stiff.
- SLOWLY sprinkle in the 2.00 grams of PVA and stir constantly with your glass stirring rod. You will know if you are going too fast if there is a glob of material at the bottom of your stirring rod.
- After you have completely stirred in all 2.00 grams of PVA, turn off the hot plate and keep your beaker on the hot plate so it doesn’t cool off.
- Get 5.0 ml of Borax solution.
- Take your disposable cup and simultaneously pour the PVA solution from the beaker (use hot Mitts!!) and the Borax solution together in the disposable cup (NOT the beaker) and stir.
- A gel-like substance (SLIME) should form immediately. If it doesn’t, keep stirring. Sometimes when the solutions get too hot it takes a while to get the slime to form.
- Add food coloring to make really gross slime.
Now, why is this a Non-Newtonion Substance? Because it behaves differently depending on how gently or strongly you stir or pull it. If you pull it slowly, it will stretch and ooze sort of like a liquid. If you pull it apart quickly, it will stiffen up and break cleanly in two as if it were a solid.
You’ve heard of quicksand, right? It is also a non-Newtonian substance. Maybe you’ve seen movies where someone is trapped in quicksand and cant’ get out. The harder they thrash around to get out, the worse it is for them. Can you explain why? Think about your slime. The harder you force it, the more rigid it becomes, so the person gets even more stuck in the quicksand. To save themselves, they should move very slowly to get out so they quicksand would behave more like a liquid and not resist the person as much.
Ketchup is another non-Newtonian substance, but it behaves in the opposite way. Glass bottles of ketchup used to be common, but they are probably only seen now in restaurants. Ever try to get the ketchup out of this kind of container? It flows better with more agitation! You may have had to shake the bottle pretty violently several times before it actually starts to flow out of the bottle.
Easy Slime Alternative Lab
- 1 cup of cornstarch
- 1/2 cup of water
- food coloring
- Put cornstarch in bowl
- Slowly and with stirring (hands are fine) add the water.
- Add food coloring as desired
- Test your cornstarch slime by hitting hard, then softly. Try to stir it quickly, then very slowly and gently. Note observations and have fun!