Australian Curriculum v9 / ACiQ Year 7 Science - Unit 7 - Particle model, pure substances and mixtures

Particle model, pure substances and mixtures

Use particle theory to explain physical properties and distinguish pure substances from mixtures.

Updated 2026-07-24 - 8 min read

Use particle theory to explain physical properties and distinguish pure substances from mixtures. This note connects the core scientific model to a worked example, an inquiry design and the limits of the evidence.

Core model

The particle model connects an invisible scale to observable properties. Describe arrangement, motion and attraction; then use those features to explain shape, flow, compression or diffusion.

Accessible diagram description: Three equal boxes represent states. Solid: dots touch in ordered rows with short vibration marks. Liquid: dots are close but irregular with short movement arrows. Gas: dots are widely spaced with long arrows. A caption states symbols and spacing are not to scale.

A model is useful because it highlights relationships that help explain or predict evidence. It is not a perfect copy of reality. Always state what the representation includes, what its arrows or symbols mean, and one relevant limitation.

Authored representation workshop

Particle arrangements in three states

Accessibility description: The solid has six particles in fixed neighbouring positions with short vibration marks. The liquid has six close, irregular particles with short movement arrows. The gas has six widely separated particles with longer movement arrows. Symbols and spacing are not to scale.

Represented parts

  • Solid: close ordered particles, vibration only
  • Liquid: close irregular particles moving past one another
  • Gas: widely separated particles moving rapidly

Represented relationships

  • Solid: close ordered particles, vibration only -> Liquid: close irregular particles moving past one another (greater freedom of movement)
  • Liquid: close irregular particles moving past one another -> Gas: widely separated particles moving rapidly (much greater separation and motion)

Learner action: Compare arrangement, movement and spacing across all three states. Use those represented features to explain why a trapped gas compresses more than a liquid, then state why the dots are not literal scale particles.

Food-colouring spread time

Accessibility description: Spread time generally falls as water temperature rises. The second 45-degree trial is unusually slow and should be checked against actual temperature and drop size.

| Water temperature (degrees Celsius) | Trial 1 spread time (s) | Trial 2 spread time (s) | Trial 3 spread time (s) | | --- | --- | --- | --- | | 15 | 188 | 181 | 185 | | 30 | 124 | 119 | 122 | | 45 | 78 | 151 | 75 |

*Same water volume, clear container, dye-drop size and no-stirring rule.*

Learner action: Design an indirect particle-motion investigation using a defined colour-spread endpoint. Specify safe teacher-approved temperatures, volumes, a no-stirring rule and repeats; analyse the supplied times, investigate the slow warm-water trial and explain why colour spread is evidence for, but not a direct view of, particle motion.

The representation and table are supplied evidence. Do not replace a represented value, relationship or anomaly with an expected result. If a visual version is created, preserve this text-equivalent information and define every symbol, arrow, heading and unit.

Worked example

The reasoning routine is:

  1. identify the observation, measurement or represented relationship
  2. select the relevant science idea
  3. connect the idea to the evidence in a complete sentence
  4. qualify the answer when the evidence or model has a boundary

Inquiry connection

Question: How does water temperature affect the time a drop of food colouring takes to spread through still water?

Reasoned hypothesis: If temperature increases, spreading time will decrease because particles move more rapidly.

  • Independent variable: water temperature
  • Dependent variable: time to reach a defined colour-spread endpoint
  • Relevant controls: water volume, container, dye drop size and no-stirring rule
  • Hazard: warm water and spills may cause burns or slips
  • Risk control: use teacher-approved warm water, stable containers and prompt spill clean-up
  • Reproducibility detail: state temperatures, volumes, dye-drop method, endpoint and repeat count

Possible data finding: mean spreading time decreased as temperature rose, with one slow warm-water trial.

Evidence-based interpretation: The trend supports the particle-motion explanation; the slow trial may reflect a smaller dye drop or cooler actual temperature.

Limitation: Colour spreading is an indirect model of particle motion and the endpoint is judged visually.

This investigation frame recurs across Year 7 Science. A sound response names a testable relationship, matches the hypothesis to the same variables, manages a realistic risk, specifies quantities and measurement rules, analyses the full data pattern, supports a claim with evidence and states what the evidence cannot establish.

Common errors and corrections

  • Error: Saying particles themselves expand. Correction: Return to the core model and identify the exact evidence or relationship before answering.
  • Error: Saying solid particles are motionless. Correction: Return to the core model and identify the exact evidence or relationship before answering.
  • Error: Treating clear or safe as meaning chemically pure. Correction: Return to the core model and identify the exact evidence or relationship before answering.
  • Error: Saying different particle types in a mixture must be chemically bonded. Correction: Return to the core model and identify the exact evidence or relationship before answering.

Practice

  1. Describe each state using arrangement and motion.
  2. Explain gas compression.
  3. Identify a pure substance and mixture from text diagrams.
  4. State one strength and one limitation of the model.

Transfer task

Apply the topic to the investigation below without relying on a teacher or AI to mark the reasoning.

Evidence status: activity only. Use the task-specific rubric below for self-review or teacher feedback; completion does not automatically award mastery.

Context: How does water temperature affect the time a drop of food colouring takes to spread through still water?

  1. Rewrite the question if it does not clearly name the relationship and measurable outcome.
  2. Write the matching reasoned hypothesis: include the expected direction and the science idea that justifies it.
  3. Identify the independent variable, dependent variable and at least three relevant controlled variables.
  4. Write a six-step reproducible method. Include equipment, quantities, units, an ordered measurement rule and at least three repeated trials for each condition.
  5. State the hazard, possible harm and a practical control. The control must address the stated hazard rather than being generic advice.
  6. Design a results table with headings and units. State which graph or other representation would best show the relationship and why.
  7. Use this possible finding: mean spreading time decreased as temperature rose, with one slow warm-water trial. Describe the overall pattern, identify any anomaly and state a reasonable check.
  8. Write a conclusion using claim, specific evidence and scientific reasoning. Finish with this limitation: Colour spreading is an indirect model of particle motion and the endpoint is judged visually.

Correction guide: Compare the question and hypothesis to confirm they use the same two variables. In the method, circle every quantity and unit and underline every controlled condition. In the data response, separate the broad pattern from any anomalous result. In the conclusion, highlight the evidence sentence and box the limitation. If one of these parts is absent, revise that part before checking the scientific vocabulary.

An excellent transfer response is precise without pretending the evidence is perfect. It makes the chain from question to method to data to claim visible, and it explains how the core model applies in the unfamiliar context.

Task-specific inquiry rubric

  • Question and hypothesis for water temperature and time to reach a defined colour-spread endpoint: Names the same measurable relationship in both and gives a scientific reason. Revise if needed: Align the changed factor, measured outcome and predicted direction.
  • Fair, safe and reproducible method: Controls water volume, container, dye drop size and no-stirring rule, manages "warm water and spills may cause burns or slips", and specifies quantities, units, measurement rules and repeats. Revise if needed: Replace vague directions and generic safety advice with operational detail.
  • Data and analysis plan: Provides labelled headings and units, an appropriate representation, a way to describe the whole pattern and a rule for checking anomalies. Revise if needed: Show how evidence will answer the question before collecting it.
  • Scientific interpretation and boundary: Links the expected evidence to particle model, pure substances and mixtures and anticipates this boundary: Colour spreading is an indirect model of particle motion and the endpoint is judged visually. Revise if needed: Explain the science link and state what the design cannot establish.

Self-check

For each response, check:

  • Did I use the correct scientific vocabulary?
  • Did I refer to the specific evidence, feature or data?
  • Did I explain the link rather than only naming it?
  • Did I avoid claiming more than the model or data support?
  • If an investigation is involved, did I address question, hypothesis, variables, safety, reproducibility, data, evidence and limitations?

Sources