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Thinking and Working Scientifically: Models and representations

Science Stage 6 Chapter Notes

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Thinking and Working Scientifically: Models and representations
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In this chapter

  1. 1Introduction to Scientific Models
  2. 2Types of Models and Representations
  3. 3Making and Testing Predictions
  4. 4Developing and Evaluating Models
  5. 5Understanding the Limitations of Models

1. Introduction to Scientific Models

In science, we often study things that are too big, too small, too complex, or too dangerous to observe directly. To overcome this, scientists use models. A scientific model is a simplified representation of a real-world object, system, or process. It is not an exact copy, but a tool designed to help us understand, explain, and make predictions. For example, a model of the solar system helps us understand the movement of planets, even though it can't capture every detail of the real system. Models are essential for communicating complex scientific ideas to others and for forming the basis of new investigations.

Key term

Scientific Model: A simplified representation of an idea, object, process, or system that is used to describe and explain phenomena that cannot be experienced directly.

Examiner insight

Examiners reward answers that clearly state a model is a simplification used to explain a complex phenomenon.

Fun fact

The first physical model of a DNA double helix was built by Watson and Crick in 1953 using simple lab equipment, metal scraps, and cardboard cutouts.

Worked example 13 marks

A student uses a globe to explain why Australia has summer when the UK has winter. Explain why the globe is a useful model for this purpose. [3 marks]

  1. 1

    Step 1: Identify the type of model and its key feature. The globe is a physical model of the Earth. Its key feature is that it shows the Earth's spherical shape and its tilt on its axis.

  2. 2

    Step 2: Link the model's feature to the phenomenon being explained. The tilt is the crucial factor that causes seasons. The model allows the student to demonstrate how, as the Earth orbits the Sun, the tilt causes the Southern Hemisphere (Australia) to receive more direct sunlight at one time of year, and the Northern Hemisphere (UK) to receive more direct sunlight six months later.

  3. 3

    Step 3: Conclude why it is useful. The model is useful because it simplifies the vast Earth-Sun system into a manageable, visual tool that clearly demonstrates the complex concept of seasons caused by axial tilt, which is difficult to observe directly.

Recap

  • A scientific model is a simplified representation of a complex reality.
  • Models are used to help understand, explain, and predict phenomena.
  • They are essential tools for communicating scientific concepts.
  • No model is a perfect copy of the real thing.
  • Models allow us to study things that are otherwise impossible to observe directly.

Quick check

  1. State two reasons why scientists use models.2 marks

2. Types of Models and Representations

Scientific models come in various forms, and we choose a type based on what we want to explain. The main types are:

  1. Physical Models: These are tangible, three-dimensional representations that you can touch and see. They are scaled up or down to be observable. Examples include a ball-and-stick model of a molecule, a model of a volcano, or a globe representing the Earth.
  2. Conceptual Models: These are ideas or diagrams used to explain how a system works or how its parts are related. They are not physical objects but representations of thought. Examples include food webs, diagrams of the water cycle, flowcharts, or the particle theory of matter.
  3. Mathematical Models: These use equations, formulas, and graphs to describe the relationships between different variables in a system. They are powerful for making precise, quantitative predictions. Examples include Newton's second law (F=ma), equations describing population growth, or computer simulations of climate change.

Key term

Representation: The description or portrayal of something in a particular way, such as a diagram, graph, or equation.

Common pitfall

Confusing a conceptual model (like a diagram) with a physical model (a 3D object). A drawing of a DNA helix is conceptual; a 3D plastic kit of it is physical.

Worked example 13 marks

A weather forecast on television shows a map with moving symbols for high pressure, low pressure, and rain.(a) What type of model is this? [1 mark](b) Explain two features that make it a useful model. [2 marks]

  1. 1

    Step 1 (a): Identify the core nature of the model. It is a visual representation of ideas and data. This is a conceptual model (or a hybrid, but conceptual is the best fit).

  2. 2

    Step 2(b) - Feature 1: Explain its simplifying feature. The model simplifies vast amounts of complex atmospheric data (temperature, pressure, humidity) into simple, easily understood symbols (like a sun or a raincloud).

  3. 3

    Step 3(b) - Feature 2: Explain its predictive feature. The movement of the symbols on the map allows it to represent a prediction of how the weather system will change over time, making it useful for planning.

Recap

  • Physical models are tangible, 3D objects like a model of a cell.
  • Conceptual models are diagrams or ideas, such as a food web or the particle model.
  • Mathematical models use equations and graphs to describe relationships, like F=ma.
  • The type of model used depends on the scientific question being asked.

Quick check

  1. Is a diagram showing the carbon cycle a physical, conceptual, or mathematical model?1 mark

3. Making and Testing Predictions

A key purpose of a scientific model is not just to explain what we already know, but to make predictions about things we haven't yet observed. These predictions are called hypotheses. A scientist can use a model to say, 'If this model is correct, then we should observe X when we do experiment Y.' For example, the particle model of matter predicts that if you heat a gas in a sealed container, the pressure will increase. Scientists can then carry out an experiment to test this. If the evidence from the experiment matches the prediction, our confidence in the model grows. If the evidence contradicts the prediction, the model is shown to be wrong or incomplete, and it must be revised or replaced. This cycle of prediction and testing is fundamental to how science progresses.

Key term

Hypothesis: A testable prediction, based on a model or theory, about the relationship between variables.

Examiner insight

Marks are often awarded for clearly explaining the logical link between a model's core principles and the specific prediction being made.

Worked example 14 marks

Using the particle model, predict what will happen to the rate of diffusion if the temperature of a liquid is increased. Explain your prediction. [4 marks]

  1. 1

    Step 1: State the core ideas of the model. The particle model states that liquids are made of particles that are in constant, random motion and have energy.

  2. 2

    Step 2: Link the change (temperature) to the model. According to the model, increasing the temperature increases the kinetic energy of the particles.

  3. 3

    Step 3: Describe the effect on the particles' behaviour. Increased kinetic energy means the particles will move faster and more randomly.

  4. 4

    Step 4: Formulate the prediction. Diffusion is the mixing of particles due to their random motion. If the particles are moving faster, they will mix more quickly. Therefore, the model predicts that the rate of diffusion will increase as temperature increases.

Recap

  • Good models can be used to make testable predictions, known as hypotheses.
  • Scientists test these predictions by gathering new evidence through observation or experiment.
  • If evidence supports the prediction, the model is strengthened.
  • If evidence contradicts the prediction, the model must be revised or discarded.
  • This cycle of prediction and testing drives scientific progress.

Quick check

  1. What is the scientific term for a testable prediction made by a model?1 mark

4. Developing and Evaluating Models

Scientific models are not static; they evolve. The development of a model is an iterative process. It often begins with a simple model to explain initial observations. As new evidence is gathered (often by testing the model's predictions), the model is evaluated. Does it fit the new data? If not, it must be adapted, refined, or sometimes completely replaced by a better model. A classic example is the model of the atom, which progressed from Dalton's simple sphere (c. 1803), to Thomson's 'plum pudding' model (c. 1897), to Rutherford's nuclear model (c. 1911), each change driven by new experimental evidence. When evaluating a model, scientists ask:

  1. Does it explain all the available evidence?
  2. Does it make accurate predictions?
  3. Is it as simple as possible?
  4. Is it consistent with other scientific ideas?

Key term

Evidence: Scientific data or observations that are used to support or refute a hypothesis or model.

Common pitfall

Thinking that a scientific model, once established, is permanent and never changes. Science is a process of refining understanding, and models are a key part of that.

Fun fact

The geocentric model of the solar system, which placed Earth at the centre, was the dominant scientific model for over 1500 years before being replaced by the heliocentric model.

Worked example 14 marks

For centuries, the geocentric model (Earth at the centre of the universe) was accepted. This was replaced by the heliocentric model (Sun at the centre of the solar system). Explain, using the concept of new evidence, why this change occurred. [4 marks]

  1. 1

    Step 1: Describe the initial model. The geocentric model explained the simple observation that the Sun and stars appear to move across the sky.

  2. 2

    Step 2: Introduce the new evidence. With the invention of the telescope, astronomers like Galileo made new, more detailed observations. For example, he observed that Jupiter had moons orbiting it, proving that not everything orbited the Earth.

  3. 3

    Step 3: Explain the conflict. This new evidence contradicted the predictions of the geocentric model. The model could not easily explain Jupiter's moons or the observed phases of Venus.

  4. 4

    Step 4: Conclude with the model change. The heliocentric model provided a simpler and more accurate explanation for all the available evidence, including the new telescopic observations. Therefore, the scientific community eventually replaced the old model with the new one.

Recap

  • Models are developed and refined over time in an iterative cycle.
  • New evidence is the primary driver for changing scientific models.
  • A model is evaluated based on its ability to explain evidence and make accurate predictions.
  • The model of the atom is a key example of how models evolve.
  • A good model is often the simplest one that explains all the available data.

Quick check

  1. State one reason why a scientist might decide to change an existing scientific model.1 mark

5. Understanding the Limitations of Models

No model is perfect. By definition, a model is a simplification of reality, which means it has limitations. It deliberately leaves out certain details to make the concept easier to understand and work with. For example, when we use the particle model, we draw particles as simple spheres, but we know real atoms and molecules are not solid spheres and have complex internal structures. When using a model, it is just as important to understand its limitations as it is to understand what it explains. Acknowledging a model's limitations is not saying the model is 'wrong' – it is about defining the boundaries of its usefulness. The key is to choose the right model for the job and to be aware of the aspects of reality it does not capture.

Key term

Limitation (of a model): An aspect of a real-world phenomenon that a scientific model does not accurately represent or account for.

Examiner insight

Examiners look for specific, well-explained limitations, not just vague statements like 'it's not realistic'. For example, instead of 'the model is too simple', say 'the model simplifies atoms into spheres, ignoring their internal structure'.

Fun fact

Subway maps are a famous example of a useful but limited conceptual model. They simplify the complex, winding routes of train lines into straight lines and fixed angles to make the map easy to read, but they completely distort the actual geography and distances.

Worked example 14 marks

A ball-and-stick kit is a physical model used to represent molecules. State two limitations of using this model to represent a water molecule (H₂O). [4 marks]

  1. 1

    Step 1: Identify a first limitation related to the components. The model uses coloured balls to represent atoms. A limitation is that it doesn't show the internal structure of the atoms (protons, neutrons, electrons). Also, atoms are not solid, hard spheres.

  2. 2

    Step 2: Explain this limitation. This is a limitation because it gives a misleading impression of what an atom is actually like, which is mostly empty space with a tiny nucleus and electron clouds.

  3. 3

    Step 3: Identify a second limitation related to the representation. The model uses sticks to represent covalent bonds. A limitation is that it doesn't show that the 'bonds' are actually shared pairs of electrons, which are regions of electron density, not rigid sticks.

  4. 4

    Step 4: Explain this limitation. This is a limitation because it implies the molecule is rigid and static, whereas in reality the bonds can vibrate and bend, and the electrons are constantly in motion.

Recap

  • All scientific models are simplifications and therefore have limitations.
  • Limitations are the aspects of reality that a model does not accurately represent.
  • Understanding a model's limitations is crucial for using it correctly.
  • Stating a model has limitations is not the same as saying it is 'wrong' or 'useless'.
  • The particle model, for instance, ignores the forces between particles and their internal structure.

Quick check

  1. State one limitation of using a diagram of a food web.1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define the term 'scientific model' and give one example of a physical model.2 marks
  2. For each of the following, state whether it is a physical, conceptual, or mathematical model: (a) A graph showing the change in a car's velocity over time. (b) A plastic skeleton in a science lab. (c) A diagram showing the process of photosynthesis.3 marks
  3. A student creates a diagram of a food web for a local pond. (a) What type of model is this? (b) Explain two ways this model is useful for a scientist studying the ecosystem.3 marks
  4. The particle model of matter represents substances as collections of particles. Describe how this model explains why gases are easy to compress but liquids are not.4 marks
  5. A scientist proposes a new mathematical model to predict the spread of a forest fire. (a) Describe what the scientist must do to test the model's predictions. (b) List two criteria the scientist would use to evaluate if the model is a good one.4 marks
  6. Explain two limitations of using a simple diagram of the solar system, with planets shown as dots on circular orbits, to represent the real solar system.4 marks
  7. The model of the atom has changed significantly over time, from Dalton's simple sphere to Rutherford's nuclear model. Explain why scientific models, like the model of the atom, change over time. In your answer, refer to the role of new evidence.5 marks
  8. Climate scientists use complex computer simulations to model the Earth's climate. (a) State one specific, testable prediction these models might make. (b) Explain why, despite being very complex, these models still have limitations. (c) Why are they still considered scientifically useful?6 marks
  9. A student says, 'The ball-and-stick model of a methane molecule is wrong because atoms aren't really coloured balls and bonds aren't sticks.' Evaluate this statement from a scientific perspective, explaining why the model is still useful.5 marks
  10. Compare and contrast a physical model, such as a model of the lungs, with a conceptual model, such as a flowchart of the circulatory system. You should mention one similarity and two differences.5 marks

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