The Syringe Argument
Sam seals the tip of a syringe holding 60 mL of air and pushes: the plunger slides in to about 30 mL. Filled with water instead, it will not move at all.
Why can air be squashed into half the space when water cannot?
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Science · Grade 8 Science
Chapter 1: Atoms and the Particle Model
A model is only worth having if it explains something.
Sam seals the tip of a syringe holding 60 mL of air and pushes: the plunger slides in to about 30 mL. Filled with water instead, it will not move at all.
Why can air be squashed into half the space when water cannot?
Make your guess to turn the page.
Still to come: 10 pages · Words to Know · What We Learned · 4 questions · Explain It in Writing · Ask and Wonder
The idea this lesson is working on — Students often picture air filling the spaces between particles, because empty space feels impossible. The book turns that picture into a claim with a prediction: air pushed out of a sealed syringe would bubble from the tip and could not return. Ask: if there is air between particles, what is between the particles of that air? The idea to reach is that the gaps hold nothing, and that emptiness is what makes a gas compressible.
A model earns trust by predicting — The particle model explains diffusion, compression, pouring and conservation of mass with one idea, and it predicts things that could turn out false, such as gases diffusing faster when warmer. A smell crossing a room in a minute is carried mostly by air currents; diffusion alone across several meters takes hours. That is why the book uses dye in still water as its example of particles spreading on their own.
About the numbers — Air at room temperature has a density of about 1.2 g per liter, so 60 mL is about 0.072 g. Halving the volume of sealed air needs about twice atmospheric pressure, roughly a firm two-handed push on a 60 mL syringe. Water is so stiff that the same push shrinks 60 mL by only about 0.003 mL. The trial results are a realistic illustration, not a published study.