Four Chemistry Misconceptions and the Demos That Expose Them
Students can state a law correctly and still not believe it. The demonstrations below are ordinary; what makes them work is a single change in how they are run. Ask students to predict the result and commit to it before anything happens. The prediction is what surfaces the misconception. Run the same demo without it and most of the room will watch, agree, and carry the wrong model into the next unit intact.
1. Mass is conserved — except when it visibly is not
Students learn the law of conservation of matter early and can usually recite it. Many nonetheless believe that in some changes, mass genuinely disappears. The recitation and the belief coexist quite comfortably.
The demo. Run the same reaction twice, once in a closed container and once open to the room, weighing before and after each time. Ask for a predicted final mass in both cases, in writing, before you start.
What it exposes. The closed system holds and confirms what they said. The open system appears to lose mass, and the discussion that follows is the lesson: nothing vanished, a gas left. Students who predicted a loss in the closed case have just shown you exactly what they think a reaction does, which no amount of restating the law would have revealed.
The same idea, cheaper
You do not need apparatus for the smaller version. Crumple a sheet of paper and ask whether the mass changed; weigh it. Burn it and ask again; weigh what remains. Then push outward with a question that has no equipment at all: a tree grows for a century, dies, and rots away to almost nothing. Where did its mass go? That question reliably separates students who have absorbed conservation from students who can recite it.
2. Air does not weigh anything
Closely related, and worth separating out, because it is the belief that makes the open-system result feel correct to students. If gas has no mass, then mass really did disappear.
The demo. Take a sealed container of air — one of the pump-style bottle stoppers used to keep drinks carbonated works well — and weigh it. Ask students to predict whether the reading will change when you pump more air in. Then pump, and weigh again.
What it exposes. A visible increase, from adding something students were confident was nothing. It is a short demo and it makes gases behave like matter for the rest of the year, which pays off again in the gas laws unit — one of the later stops in the first-year sequence, and the point where physical chemistry starts in earnest.
3. There is air between the nucleus and the electrons
Teach the solar-system model of the atom and then, before moving on, ask a question the model itself never addresses: what is in the space between the nucleus and the electrons?
A meaningful number of students will answer air. Others will say dust, or nothing-in-a-vague-sense rather than nothing-in-a-vacuum sense. It is a revealing answer because it is not careless — it follows logically from a diagram that draws the atom as objects in a room. The model produced it.
What to do with it. This one is a discussion rather than a demonstration. It is a good moment to be honest that the diagram is a simplification, and that air is itself made of atoms, so an atom cannot contain any. It sets up the later shift to electron configuration and orbitals as a genuine correction rather than an arbitrary complication.
4. Dissolving is a chemical change
Stirring sugar into water makes it disappear, and disappearance reads as a reaction. The distinction between a physical change and a chemical change is one students can define and then apply incorrectly within the same lesson. It is general chemistry's most reliable trap.
The demo. Two beakers. In one, dissolve salt or sugar in water. In the other, run something unmistakably chemical — a colour change, a gas, a precipitate. Ask students to predict, for each, whether the original substance could be recovered. Then recover the dissolved solid by evaporation.
What it exposes. Recoverability is a criterion students can actually apply, and it survives contact with harder cases better than "a new substance is formed" does. It also gives solution, solubility and mixture a concrete anchor before the numbers arrive in the molarity unit.
Why predict-first works
A demonstration shown without a prediction is a performance. Students watch it, see the result, and file it as a thing that happened — and a wrong model survives contact with a result it was never asked to forecast. Requiring a committed prediction, ideally written down, changes the demo into a test the student has taken and can now mark themselves. It costs about ninety seconds.
It also gives you diagnostic data you cannot otherwise get. Reading twenty-eight predictions tells you which misconception is actually present in this class, which is rarely the one you assumed.
Questions
Why do students still think mass disappears after learning conservation of mass?
Because reciting a law and holding a mental model are different things. Students can state that matter is conserved while still picturing a burning or dissolving substance as ceasing to exist, particularly when they also believe gases have no mass. Asking them to predict a final mass in an open and a closed system brings the gap into the open, where restating the law does not.
What is the best way to run a chemistry demonstration?
Have students commit to a prediction, in writing, before you begin. Without a prediction a demonstration is something students watch rather than something that tests their model, and an incorrect model survives it untouched. The prediction also tells you which misconception this particular class actually holds.
How do you show students that air has mass?
Weigh a sealed container of air, ask students to predict whether adding more air will change the reading, then pump air in and weigh it again. The increase is small but real, and it addresses the belief that underpins the idea that mass vanishes in open-system reactions.
Is dissolving a physical or a chemical change?
Physical. No new substance is formed and the dissolved solid can be recovered, usually by evaporating the solvent. Students commonly classify it as chemical because the solid appears to vanish, so recovering it in front of them is more convincing than repeating the definition.
Why do students say there is air inside an atom?
Because the solar-system diagram draws the atom as objects separated by space, and everyday experience says the space between objects contains air. The answer follows logically from the model they were given, which makes it a good opportunity to say plainly that the diagram is a simplification and that air is itself made of atoms.