Grade: Grade 12 Subject: Science Unit: Science Electives Lesson: 5 of 6 SAT: ProblemSolving+DataAnalysis ACT: Science

CER Writing

Learn

CER (Claim-Evidence-Reasoning) is a framework for scientific argumentation that helps you construct clear, logical, and evidence-based explanations. This structure is essential for lab reports, scientific papers, and standardized test responses.

The CER Framework

Claim

A claim is a statement that answers a question or makes an assertion about a phenomenon.

  • Must directly answer the research question
  • Should be specific and testable
  • Stated clearly in one to two sentences
  • Does NOT include evidence or reasoning

Evidence

Evidence consists of data or observations that support the claim.

  • Must be specific and quantitative when possible
  • Comes from experiments, observations, or reliable sources
  • Should be relevant to the claim
  • Multiple pieces of evidence strengthen arguments
  • Include units and precise measurements

Reasoning

Reasoning explains HOW and WHY the evidence supports the claim using scientific principles.

  • Connects evidence to the claim logically
  • References scientific concepts, laws, or theories
  • Explains the underlying mechanism
  • Shows deep understanding of the topic
  • May address limitations or alternative explanations

Common Mistakes to Avoid

  • Making claims that are too vague or broad
  • Using evidence that does not directly support the claim
  • Confusing evidence with reasoning
  • Omitting scientific principles from reasoning
  • Including opinions instead of evidence
  • Failing to explain the connection between evidence and claim

Strengthening Your CER

  • Use transition words: "According to the data...", "This supports the claim because...", "Based on the principle of..."
  • Be specific: Include exact numbers, percentages, and comparisons
  • Connect to theory: Reference relevant scientific laws or concepts
  • Address counterarguments: Acknowledge limitations or alternative interpretations

Examples

Example 1: Strong CER

Question: Does temperature affect enzyme activity?

Claim: Enzyme activity increases with temperature up to an optimal point, after which activity decreases due to denaturation.

Evidence: In our experiment, amylase activity (measured by starch breakdown time) decreased from 180 seconds at 10C to 45 seconds at 37C. However, at 60C, starch breakdown took over 300 seconds, and at 80C, no breakdown occurred even after 10 minutes.

Reasoning: This pattern occurs because enzymes are proteins with specific three-dimensional shapes that determine their function. At low temperatures, molecules move slowly, resulting in fewer enzyme-substrate collisions. As temperature increases, molecular motion increases, leading to more frequent collisions and faster reactions. However, at high temperatures (above approximately 40C for most enzymes), the increased kinetic energy causes the hydrogen bonds maintaining the enzyme's shape to break, resulting in denaturation. Once denatured, the enzyme's active site no longer fits the substrate, and catalytic activity is lost permanently.

Example 2: Weak vs. Strong CER

Weak version:

Claim: Plants need light. Evidence: The plants in the light grew more. Reasoning: Light is important for plants.

Strong version:

Claim: Light is necessary for photosynthesis and plant growth.

Evidence: Plants grown under 12 hours of light daily averaged 15.3 cm in height after two weeks, while plants in complete darkness averaged only 3.2 cm and appeared yellow-white in color.

Reasoning: This difference occurs because light energy is required for the light-dependent reactions of photosynthesis, which produce ATP and NADPH needed to convert carbon dioxide into glucose during the Calvin cycle. Without light, plants cannot produce sufficient glucose for cellular respiration and growth. The yellow-white color of dark-grown plants indicates a lack of chlorophyll production, which also requires light for synthesis.

Practice

Complete these practice problems to strengthen your CER writing skills.

1. Write a claim for this question: Does salt concentration affect the rate of osmosis in potato cells?

2. Identify the problem with this evidence: "The reaction was really fast and made a lot of bubbles."

3. What scientific principle would you include in reasoning about why metal expands when heated?

4. Rewrite this vague claim to be more specific: "Exercise is good for you."

5. What is the difference between evidence and reasoning? Provide an example of each.

6. Write appropriate evidence for this claim: "Increasing the surface area of a reactant increases the rate of reaction."

7. Identify what is missing from this CER: "Claim: Acid rain damages limestone buildings. Evidence: Buildings in areas with high air pollution show more erosion. Reasoning: The acid reacts with the limestone."

8. Write a complete CER for: Does mass affect the period of a pendulum?

9. Why is it important to include scientific principles in your reasoning section?

10. How would you address a counterargument or limitation in a CER response?

Check Your Understanding

Question 1: Which component of CER explains the scientific principles connecting evidence to the claim?

  • A) Claim
  • B) Evidence
  • C) Reasoning
  • D) Conclusion

Question 2: Which of the following is the best example of evidence?

  • A) The experiment worked well
  • B) Plants need water to survive
  • C) The temperature increased from 22C to 35C over 10 minutes
  • D) Heat causes molecules to move faster

Next Steps

  • Practice writing CER responses for various experimental scenarios
  • Review scientific principles relevant to your experiments
  • Continue to the Unit Checkpoint to assess your understanding