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Teach High School Science

Engaging Science Students Through Discrepant Events
Judy Jones

Many years ago I came across a wonderful book by Tik Liem (Invitations to Science Inquiry). He wrote several books that contain many clever science demonstrations and experiments. His book introduced me to the idea of using “discrepant events” to engage the thinking powers of my science students.

A discrepant event is one where the result is the opposite of what was expected; in other words, the result contradicts the expectation of the student. By having a “belief” challenged, the student is sort of “thrown off balance” and this intellectual disequilibrium motivates the student to try and find out what happened. Although the use of discrepant events to teach science is more often applied in elementary classes, I have also found it very useful in my high school science classes.

The first discrepant event I tried came from Tik Liem’s book and I used it in my chemistry classes. I took two baby food jars (the lids are necessary!). I half filled one jar with water and half filled the other jar with rubbing alcohol. Then I took a kitchen candle and cut ½” slices of the candle wax. I put a chunk of candle wax in each jar and screwed on the lids. (The lids keep the students from being able to smell the alcohol!) I placed these jars on my demonstration table in the front of the room and just waited to see what would happen. Slowly at first, students noticed that the wax was floating in one jar (water) and had sunk to the bottom of the other jar (alcohol). They asked me why. I said “why do you think?” And that led to a week or so of proposed hypotheses that we kept on the board while the student puzzled out the possibilities. Eventually, I opened the jars and they were able to use other senses. The ultimate outcome was a terrific discussion of density, polarity, scientific method, and other concepts. My students were definitely engaged!

Another such activity is the well-known “drops on a penny.” In our state biology curriculum, students are expected to understand the characteristics of water that are relevant to living things. I start the class by asking students to hypothesize how many drops of water they could fit on the surface of a penny. The answers usually vary from 4-12 or so. Then I give students pennies, a variety of droppers, a small beaker of water and set them loose to test their hypotheses. The results totally surprise them (unless they have done this before). I have had students who are using small tipped droppers get close to 100 drops on their penny before the water spills over. The water “drop” ends up looking like a huge muffin on top of the penny. We end up having a great discussion about the variables involved and the nature of water. We discuss size of dropper, the use of heads or tails on the penny, the newness of the penny, the angle of the dropper, etc. We also discuss the polarity of water and both its cohesive and adhesive properties based on the hydrogen bonding from one water molecule to the next. Again, the students are highly engaged!

I have also designed activities to address common misconceptions so that students are faced with evidence that contradicts what they “believe.” In my biology classes I use a clever piece of software called CATLAB (http://emescience.com/bio-software-catlab.html). Using this software, students can create a group of parent cats and then mate them to try and discover how certain coat color traits are inherited. I start with a very simple trait. Is “ALL-WHITE” in cats dominant over “NOT ALL-WHITE” or is it recessive? First, I ask my students to hypothesize and explain their hypotheses. Almost every student will say that the trait is recessive because it is rare or because they know that albinism in humans is usually recessive. This first reason is a very common misconception. When the students mate a variety of ALL-WHITE and NOT ALL-WHITE parent cats, they get data that is totally inconsistent with ALL-WHITE being recessive. I love watching them drop their hypotheses (with reluctance!) and create a new model to explain their data. They are actively learning; their brains are engaged with trying to figure out why their hypotheses are not supported by the data.

For more discrepancy event activities visit

Teaching through discrepant events involves the 5 E’s of the constructivist model:
Engage: In this phase, students are invited to connect what they already know or believe with the learning experience facing them.

Explore: In this phase, students are actually collecting data or making observations.

Explain: Students try to explain their data or observations. In this phase I become very active, because I want my students to clearly understand the concepts. (I know that in some “constructivist” models, the teacher takes a more hands-off approach, but I have learned that many times students really need sequenced questions and close monitoring to ensure that they learn accurately from the experience.)

Elaborate or Extend: In this phase, students can take what they have learned and apply it to other situations. For example, in the CATLAB activity, my students have learned to analyze genetic data and this knowledge can be applied to many problems involving genetics.

Evaluate: Students and teachers both assess what and how much was learned from the activity.

Over the years, I have gathered many more discrepant events to use as demonstrations or in actual lab activities. I encourage you to begin your collection of similar events to puzzle, engage, and motivate your students.

Please share your ideas with me via e-mail.


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