Lesson 1 of 6 · Atoms, molecules, phases & plasma
Build matter from particles
9 minute lesson · 3 checks along the way
Your goal for this lessonDistinguish a change of physical state from a chemical or ionization change, and explain why pressure belongs in a phase prediction.
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Keep identity separate from arrangement
A spacecraft, a cloud, and the Sun all contain matter, but their particles are arranged and interacting in very different ways. An contains a small nucleus surrounded by electrons. The number of protons identifies its element. Atoms can join into : each water molecule contains two hydrogen atoms and one oxygen atom. A molecule is not simply another name for any microscopic particle.
In a solid, particles remain close together and resist sustained changes of shape. They are not motionless. In a liquid, close neighbors rearrange so the material flows while remaining relatively difficult to compress. In a dilute gas, widely separated particles explore the available volume. These descriptions concern collective behavior, rather than suggesting that an individual water molecule becomes a tiny ice cube when water freezes.
Check 1 of 3
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Change state without changing the ingredient list
Melting ice and vaporizing liquid water can leave the water molecules chemically intact. Breaking those molecules apart into different substances is a different process. A depends on and pressure together; water does not have one universal boiling temperature for every planet or chamber. Lowering the surrounding pressure can make liquid boil at a temperature below its familiar boiling point near Earth’s sea level.
A solid can also pass directly into vapor, a process called . Phase changes require an energy account, even when the measured temperature stays nearly constant during an idealized transition. Energy changes the microscopic arrangement and interactions. Conversely, freezing releases energy to the surroundings. A thermometer reading alone cannot reveal how much of a sample has changed phase or how much energy has crossed its boundary.
Work through a decision
Decide what changed in a fictional sample
Imagine a sealed experiment that begins with 100 intact water molecules in ice. After a carefully controlled transition, instruments identify the same 100 intact molecules in vapor, with no chemical reaction. The inventory still contains 200 hydrogen atoms and 100 oxygen atoms. Neither the molecule count nor the elemental identity changed. The arrangement, spacing, and energy changed; expanding into a larger volume did not manufacture new matter.
Now compare a second report: some molecules have split, and a third report: electrons have been removed from some atoms. Those describe and , respectively. Choose measurements that distinguish the claims, such as spectra sensitive to molecular bonds or charge states. A changed photograph of the container’s contents might establish a visible difference, but it cannot by itself settle the microscopic explanation.
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Meet plasma without losing the particles
contains mobile charged particles, including free electrons and ions, whose electric and magnetic interactions can produce collective behavior. A plasma may also contain neutral particles; it need not have every atom completely stripped. Ionizing an atom changes its electron inventory, not its proton count. The resulting ion remains the same element unless a separate nuclear process changes its nucleus.
Solar Dynamics Observatory records radiation from the Sun’s plasma. The orange archive image represents extreme ultraviolet observations using visible display colors; it is not a close photograph of individual atoms or a fire burning in oxygen. To infer temperature or ion populations, researchers combine the instrument’s wavelength response with emission physics and other observations. The image gives this particle model a real astronomical setting while preserving the distinction between a measurement and its interpretation.
Solar Dynamics Observatory’s Atmospheric Imaging Assembly observed the Sun on March 28, 2013. The PIA17669 image displays extreme-ultraviolet measurements in orange, a visible color assigned for viewing. The source identifies the dark region as a coronal hole. This is a view of emission from solar plasma, not a visible-light photograph of flames.
Look closely
Can the orange display color alone establish the temperature or chemical identity of every bright region?
See what to notice
No. The recorded wavelength range, the detector response, and a model of emission are needed. Brightness can depend on several plasma properties and material along the line of sight. The assigned orange color helps us see the observation; it is not a thermometer or a view of individual atoms.
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Put it together
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