What’s in the bubbles of boiling water?

I asked my students this year “what is inside the bubbles of boiling water?” Without hesitation, all 24 students in my advanced chemistry class answered “hydrogen and oxygen gas.” These students were sure of their answer because they knew that water is made of hydrogen and oxygen.

A typical chemistry class is taught from the top down; you start with the most complicated model of the atom and go from there. Do you really need the electron-cloud model of the atom to understand the gas laws? What about thermodynamics? What about bonding? Yes, I teach bonding without the electron-cloud OR Bohr model! The boiling water misconception is why.

My chemistry class starts with the Democritus model: everything is made of particles. This does not mean everything is made of atoms. Students may describe air particles, water particles, desk particles and even students particles. We use this model to explore mass, volume, and density. Then we observe diffusion and infer that particles must be moving and particle speed corresponds to particle temperature. This inference allows us to explain the gas laws and thermodynamics.

Let’s stop here for a second. If my students had no prior knowledge and I asked them “what’s inside the bubbles of boiling water?” at this point in our curriculum, they certainly would not answer “hydrogen and oxygen gas.” They would think more simply and answer “water gas particles.” Misconception averted.

Continuing on, after finishing our thermodynamics unit, the students observe water being broken into two separate gases in the Hoffman apparatus and must infer there is something smaller than a water particle. Those smaller particles can somehow “hook” together to form new substances. This is the Dalton model. We use the Dalton model to explore mixtures and pure substances as well as the Laws of Definite and Multiple Proportions.

Next, the students observe electrostatic attraction and must infer that particles have a mobile negative charge. This is the Thompson plum pudding model. This model gets my students through bonding. We talk about bonding in terms of attractions and physical properties instead of anthropomorphizing the octet rule. The Thompson model takes us through moles, chemical reactions and stoichiometry.

It is not until the end of the year that we observe graphs of ionization energies to infer that electrons exist in different energy “levels” which leads us to the Bohr model. My students then leave my classroom with a deep conceptual understanding of chemistry and how the world works at the particle level as opposed to a disjointed collection of equations and rules.

Chemistry was not discovered by reading textbooks so why should we teach it that way?

I cannot take credit for any of this. This concept was developed by the wonderful people at Arizona State University and is part of the Modeling Instruction curriculum. Find a workshop near you this summer at modelinginstruction.org.


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