Scientists use models to explain what cannot be directly seen, and graphs to reveal patterns in data. Knowing how to draw, read and evaluate these representations is tested in every exam paper.
← Back to Sec 1 Secondary HubA scientific model is a simplified representation of something in the real world. Models help us explain and predict behaviour, especially for things too small to see (like atoms) or too complex to describe fully.
Represents matter as tiny particles in different arrangements and motion to explain states of matter
Labelled diagrams of cells, apparatus, circuits — a type of visual model
Comparing something unfamiliar to something familiar, e.g. a cell is like a factory
Mathematical models that show how two variables relate to each other
Grouping organisms by shared features — a conceptual model of diversity
| Strength | Limitation |
|---|---|
| Simplifies complex ideas so they are easier to understand | May leave out important details |
| Allows prediction of new outcomes | Can create misconceptions if taken too literally |
| Provides a shared language for scientists | May not work well outside the conditions it was designed for |
Exam question type: "Suggest one limitation of this model." — always say what the model leaves out or oversimplifies.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement | Regular, closely packed | Irregular, close but not fixed | Random, very spread out |
| Motion | Vibrate in fixed positions | Slide past each other | Move freely and rapidly in all directions |
| Forces between particles | Strong | Moderate | Very weak / negligible |
| Volume | Fixed | Fixed | Fills container |
| Shape | Fixed | Takes shape of container | Takes shape of container |
Always link the particle model to a macroscopic (what you can see) observation:
Example: "A gas can be compressed because the particles are far apart with large spaces between them, so the particles can be pushed closer together."
Example: "A solid cannot flow because the particles are fixed in position and cannot move past each other."
⚠ Common mistakes:
Mean = Sum of all values ÷ Number of values. Always check for anomalous results before calculating — exclude them and note why.
Example: Readings of 12, 14, 13, 34, 12. The value 34 is anomalous. Mean of remaining values = (12 + 14 + 13 + 12) ÷ 4 = 12.75
Percentage change = [(new value − original value) ÷ original value] × 100%
A positive result is an increase; a negative result is a decrease. This lets you compare changes on a fair scale regardless of starting values.
Use the trend shown by a graph to predict values inside the range (interpolation) or beyond the range (extrapolation). Always state that extrapolation is less reliable because the trend may not continue.
| Command word | What it means | How much to write |
|---|---|---|
| State | Give a fact with no explanation needed | One sentence |
| Identify | Name or select something from the information given | One word or phrase |
| Define | Give the precise meaning of a term | One sentence |
| Describe | Give the key features or say what you observe — no explanation | 2–3 sentences |
| Explain | Give reasons — link cause to effect; use "because" or "so that" | 2–4 sentences |
| Compare | Say how two things are similar AND different | 1 similarity + 1 difference minimum |
| Suggest | Give a possible reason — you may need to apply knowledge to a new situation | 1–2 sentences |
| Predict | Say what you expect to happen — justify it | 1–2 sentences with reason |
| Evaluate | Judge the quality or reliability of something; give evidence and a conclusion | 3–5 sentences |
⚠ Critical trap: If the question says "explain", a description alone gets zero marks for the explanation marks. You must say why something happens — link your answer using "because", "so", "this means that" or "therefore".
The particle model shows particles of a solid as small spheres packed in a regular grid. A student says this model is not completely accurate. Suggest one limitation of this representation.
The model shows particles as identical, rigid spheres, but in reality, particles come in different sizes and shapes (e.g. different atoms and molecules). The model also shows them as stationary, whereas in reality, even particles in a solid are always vibrating.
A student records the temperature of water as it is heated from 20°C to 100°C over 10 minutes. She takes a reading every 2 minutes. Describe the steps she should follow to plot a line graph of her results.
Table shows mass of salt dissolved in 100 cm³ water at different temperatures: 20°C → 35 g, 40°C → 38 g, 60°C → 43 g, 80°C → 50 g.