Construct and critique simple particle diagrams
Check arrangement, spacing and motion together instead of choosing a state from one symbol.
Section 2.1.2 · Particle model
Core and Extended; Physics 0625, 2026–2028, section 2.1.2. Describe particle arrangement, separation and motion; relate temperature to motion and absolute zero; explain gas collisions and Brownian motion. Forces, force per unit area and precise atom/molecule distinctions are developed in the labelled Supplement lessons. Written practice is self-checked.
Before you begin
- Describe the solid, liquid and gas particle models.
- Read motion arrows and state what they represent.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which motion belongs to the simple liquid model?
What you will learn
- Represent all three states with appropriate particle arrangements and motions.
- Identify contradictory labels and limitations in a schematic.
Use a three-part checklist
For each diagram check arrangement, separation and motion. A simple solid has close particles retaining relative positions and vibrating locally. A liquid has close particles in an irregular arrangement moving past one another. A gas has widely separated particles in random travel through accessible space. A correct label requires a consistent combination, not just a particular colour or circle count.
Short local two-way arrows can indicate solid vibration, while representative travel arrows show movement in liquid and gas examples. Unless a scale is supplied, arrow length is not a numerical speed measurement. A gas diagram with every arrow parallel represents a directed component or a misleading simplification, rather than the required random motion by itself.
State what the drawing cannot prove
The symbol count can be kept equal when comparing a fixed represented sample, but twelve circles do not mean that a real sample contains only twelve atoms. The size of a circle is not a calibrated atom diameter. Empty-looking regions outside a liquid drawing may contain unrepresented gas, and a regular solid example does not establish that every solid has an identical lattice.
A diagram is an explanatory model, not independent observational proof. Use it to express a proposed account and check consistency with the stated properties. Measurements, observations and their limitations provide evidence for evaluating a model. In particular, a pre-drawn random path should not be described as a new recorded Brownian-motion experiment.
Pause and explain
Does an unscaled arrow twice as long establish twice the particle speed?
Worked example
A drawing is labelled liquid but shows widely separated particles travelling throughout a box. Diagnose and correct it.
Show the worked solution
- Wide separation and box-wide travel describe a gas model.
- For liquid, draw close irregular particles within the occupied liquid region.
- Show movement past neighbours rather than changing the symbol sizes.
Answer The existing drawing represents gas; a liquid needs close mobile particles in its occupied region.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Give one suitable drawing description for each state, including arrangement, separation and motion.
Give me a hint
Use the full checklist for each of the three states.
Compare my reasoning
- Solid: close particles retaining relative positions with local vibration.
- Liquid: close irregular particles moving past one another.
- Gas: widely separated particles moving randomly throughout available space.
Three consistent diagrams with state-appropriate positions and motions.
Look for these in your work
- Represent all three features.
- Keep the models distinct.
A liquid diagram has close particles but labels them 'no motion'. A gas diagram has large gaps but all arrows point right. Identify what each needs corrected.
Give me a hint
Correct the motion description without ignoring the valid spacing clue.
Compare my reasoning
- The liquid needs particles moving past each other.
- The gas needs random particle motion, unless rightward arrows are explicitly only a bulk component.
- Both may use unscaled representative arrows rather than a fabricated speed scale.
Add liquid relative motion and a random gas-motion account.
Look for these in your work
- Explain both corrections.
- Preserve useful arrangement evidence.
A twelve-circle diagram is claimed to prove a real sample has twelve atoms, a precise density and recorded random motion. Evaluate the claims.
Give me a hint
Ask which quantities or observations the schematic actually supplies.
Compare my reasoning
- The circles are representative symbols, not a complete atom count.
- No calibrated volume or particle mass gives a measured density.
- A drawn path is not itself an observation recorded from an experiment.
None of those measurement or observation claims follows from the schematic alone.
Look for these in your work
- Identify missing calibration.
- Distinguish model from evidence.
Common mistakes
- Colour and count alone do not identify a state.
- A schematic is not experimental proof.
- Unscaled arrows do not measure speed.
What can you explain now?
What three features should a particle diagram communicate?
Compare with the explanation
Arrangement, separation and motion.
Together these distinguish the simple state models.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Construct and critique the three state models and explain why symbols and unscaled arrows cannot establish measured quantities.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.