In my first year of teaching high school chemistry one of my more humbling moments came nearly three quarters of the way through the course. One of my top students had a moment of reflection, which he happened to share out loud to the entire class.
“You mean that’s ALL the chemical elements? No ‘dirt’ atoms or ‘wood’ atoms?”
Chemistry can be very difficult for both teachers and learners. But what makes studying chemistry so different than other subjects? Chemical education research refers to three conceptual levels that are key in understanding chemical behavior.
Recognizing these three concepts can be very helpful for both teachers and students.
The Macroscopic Level
Laboratory work is typical in most science courses. This gives students the opportunity to observe the characteristics and behavior of matter first hand. Students can use their five senses to note color, odor, temperature, and texture of various types of matter. These same types of observations can help them detect chemical or physical changes. We often use instruments to enhance our observations at the macroscopic level, such as thermometers to measure temperature or a spectrophotometer to measure color.
The Microscopic Or Particulate Level
Everything important in chemistry happens on the level of atoms and molecules, the fundamental particles of matter. Some educators avoid using the term ‘microscopic’ for fear students might think this level could be observed with regular light microscopes like the ones they use in biology. Atoms are visible under sophisticated microscopes such as scanning tunneling microscopes (STM), but the issue remains. Chemical reactions happen because atoms make bonds and break bonds. Atoms are in constant motion, in all states of matter.
The Symbolic Level
Nothing is more representative in the eyes of the general public regarding chemistry than the chemical equation. Show the average person a complicated chemical formula and their response is, “that’s a chemical”. Show a person a container of sugar and you would never get the same response. We use symbols to represent the elements, formulas to represent compounds, chemical equations to represent reactions. We also use many mathematical figures and variables such as δH or ΔT. The symbolic level is the most abstract representation of chemical concepts, and yet it is the most typical way of explaining what goes on in chemistry, especially at the higher levels of education.
Failure to Communicate
The fact of representing matter on three different levels isn’t in itself what makes chemistry and other sciences hard to understand. It is more due to the fact that we rather heedlessly fail to connect the levels in a way that leads students to get a thorough understanding of the physical world around them.
I once thought up a design for a coffee cup that we could add to our products for sale in an ACS catalog. My idea was to do a take off on an old bumper sticker, the one that said, “If you can read this, thank an English teacher.” My idea was to have the coffee cup read, “If you can read this, _________, thank a chemistry teacher.” Where the blank was a chemical formula.
I tried out a number of formulas, such as Fe3O4 or C6H12O6, but to my amazement, few people could understand what they were saying. It didn’t leap out they were saying ‘Rust’ and ‘Sugar’. There wasn’t a strong link between the macroscopic identity of the materials and the symbolic name representing them.
Teachers often unwittingly change from one representation of matter to another without bringing students along with them, or even acknowledging the change. Teachers might talk about water, or use the chemical formula H2O, and then later switch to a structural formula for H2O (structural formula here) or a ball and stick formula (show here), or a space-filling formula (show here), as if they were all saying the same thing (which of course, they are!) It is very important to acknowledge that we have different ways of portraying matter, rather than just assuming kids will ‘get it’.
Solutions at Hand
One of the best things we can do to help students have a clear understanding of how matter behaves is to create links among all the conceptual levels we use. We are fortunate that there are terrific resources available today that makes creating these links easier than before. The ACS Middle School Chemistry (MSC) site is loaded with pictures, models and animations that can helps students navigate among the conceptual levels of chemistry.
A good example of how the MSC can help is shown in Lesson 1.2—Molecules in Motion. The lesson takes on a few key concepts:
* Heating a liquid increases the speed of the molecules.
* An increase in the speed of the molecules competes with the attraction between molecules and causes molecules to move a little further apart.
* Cooling a liquid decreases the speed of the molecules.
* A decrease in the speed of the molecules allows the attractions between molecules to bring them a little closer together.
The lesson starts with an inquiry investigation based on the question, “Is the speed of water molecules different in hot and cold water? What can we do to find out?” Students are coached to consider what kind of evidence they would need to demonstrate how hot and cold liquids differ. They also consider how to make a fair test by controlling variables.
The activity has students add food coloring to samples of hot and cold water. The idea is that if the molecules are moving faster in the hot liquid, the food color should move faster too.The student worksheet helps the students organize their observations and asks them to explain what they observe.
A key part of the worksheet asks them to do a drawing of how they think the molecules are moving. The students use ‘motion lines’ to indicate movement and are also asked to consider the space between the molecules. This simple illustration helps give a clear view of student thinking and helps reveal any misconceptions they might have.
Better yet is an animation which shows how the movement of particles changes in going from cold to hot. The animation has an image of water at the macroscopic level and an inset frame of a magnified view of the water at the molecular level. The temperature is changed as students slide a bar at the bottom of the animation. A few moments with the animation gives students a great image of how their macroscopic observations can be explained by models of the particles in motion.
All the lessons in the Middle School Chemistry collection have this same strong link among the conceptual levels of matter.
You can’t give students a clearer view than that!