Claim-Evidence Writing
Learn
Scientists do more than just conduct experiments - they must also communicate their findings clearly. In this lesson, you will learn how to write scientific arguments using the Claim-Evidence-Reasoning (CER) framework.
The CER Framework
- Claim
- A statement that answers a question or makes an assertion. It is what you believe to be true based on information.
- Evidence
- Data, observations, or facts that support your claim. This must come from reliable sources like experiments, measurements, or documented observations.
- Reasoning
- An explanation of WHY your evidence supports your claim. This connects the evidence to scientific principles and shows your understanding.
Why Use CER?
- It organizes your thinking logically
- It requires you to back up your ideas with facts
- It helps others understand and evaluate your conclusions
- It mirrors how real scientists communicate
CER Writing Tips
- State your claim clearly and directly
- Use specific numbers and details in your evidence
- Explain the scientific principle behind your reasoning
- Make sure evidence actually connects to your claim
- Avoid opinions - stick to observable facts
Examples
Example 1: Sound and Materials
Question: Does sound travel faster through solids or gases?
Claim: Sound travels faster through solids than through gases.
Evidence: According to experimental data, sound travels through steel at 5,960 meters per second, while it travels through air at only 343 meters per second. Sound through steel is about 17 times faster than through air.
Reasoning: Sound is a vibration that passes from particle to particle. In solids, particles are packed closely together, so vibrations transfer quickly from one particle to the next. In gases, particles are spread far apart, so it takes longer for vibrations to pass between them. This is why sound travels faster in solids.
Example 2: Light and Space
Question: Can light travel through empty space (a vacuum)?
Claim: Light can travel through empty space without a medium.
Evidence: We can see the Sun and stars even though the space between Earth and these objects is a vacuum with no air. Sunlight takes about 8 minutes to travel 150 million kilometers through space to reach Earth.
Reasoning: Unlike sound waves, which are mechanical waves that need particles to vibrate, light is an electromagnetic wave. Electromagnetic waves are disturbances in electric and magnetic fields, which can exist and propagate without any material. This is why light from distant stars can reach us across the vacuum of space.
Example 3: Weak vs. Strong CER
Weak response: "Sound is louder when you're close because it's just stronger near the source."
Strong CER response:
- Claim: Sound becomes quieter as you move farther from the source.
- Evidence: When standing 1 meter from a speaker, the sound measured 80 decibels. At 10 meters away, the same sound measured only 60 decibels.
- Reasoning: Sound waves spread out in all directions as they travel from their source. As the waves spread, their energy is distributed over a larger area. This means less energy reaches any single point, resulting in a quieter sound.
Practice
For each question, identify the claim, evidence, and reasoning, or evaluate the quality of a CER response. Click to reveal the answers.
Question 1: Write a claim to answer this question: "Does increasing the amplitude of a sound wave make the sound louder or quieter?"
Answer: A good claim would be: "Increasing the amplitude of a sound wave makes the sound louder." This is a clear, direct statement that answers the question.
Question 2: What evidence would support this claim: "Light travels faster than sound"?
Answer: Good evidence includes: Light travels at about 300,000 km/s while sound travels at about 343 m/s in air. Real-world observations: we see lightning before we hear thunder; we see fireworks explode before we hear the boom; we see a distant person clap before hearing it.
Question 3: Is this a good claim? "Waves are really cool and interesting." Explain why or why not.
Answer: No, this is not a good scientific claim. It is an opinion, not a factual statement that can be proven with evidence. A claim should make a specific, testable assertion about how something works or what is true.
Question 4: A student writes: "Sound needs a medium because my teacher said so." What is wrong with this evidence?
Answer: This is weak evidence because it relies on authority rather than data or observations. Better evidence would be: "In an experiment, a ringing bell in a vacuum jar became silent when air was removed, but the bell kept ringing when air was present."
Question 5: Complete the reasoning: "Sound travels faster in water than in air because..."
Answer: "...water molecules are packed more closely together than air molecules. Sound is a vibration that passes from particle to particle, and when particles are closer together, the vibration transfers more quickly. Water is denser than air, so sound can travel faster through it."
Question 6: Identify the claim, evidence, and reasoning in this response: "Mirrors reflect light. When I shined a flashlight at a mirror at a 45-degree angle, the light bounced off at a 45-degree angle on the other side. This happens because light bounces off smooth surfaces at the same angle it hits them."
Answer:
Claim: "Mirrors reflect light."
Evidence: "When I shined a flashlight at a mirror at a 45-degree angle, the light bounced off at a 45-degree angle on the other side."
Reasoning: "This happens because light bounces off smooth surfaces at the same angle it hits them."
Question 7: A classmate says, "I think high-pitched sounds have short wavelengths because they sound higher." Is the reasoning complete? What is missing?
Answer: The reasoning is incomplete. It does not explain WHY high-pitched sounds have short wavelengths. Better reasoning would explain that pitch is determined by frequency, and high-frequency waves (many vibrations per second) have shorter wavelengths because the distance between wave peaks is compressed.
Question 8: Write evidence for this claim: "Sound cannot travel through a vacuum."
Answer: Good evidence: "In the classic bell jar experiment, when air is pumped out of a jar containing a ringing bell, the sound fades until it is completely silent even though the bell is still vibrating. When air is allowed back in, the sound returns." Or: "Astronauts cannot hear each other speak in space without radios."
Question 9: What makes reasoning different from evidence?
Answer: Evidence is the DATA or OBSERVATIONS - the facts and measurements that can be directly seen or recorded. Reasoning is the EXPLANATION of WHY those facts support the claim - it connects the evidence to scientific principles and concepts. Evidence shows WHAT happened; reasoning explains WHY it supports your claim.
Question 10: Write a complete CER response for this question: "Can you see an object that does not emit or reflect light?"
Answer:
Claim: You cannot see an object that does not emit or reflect light.
Evidence: In a completely dark room with no light sources, you cannot see anything, even though objects are still present. When a light is turned on, you can immediately see the objects because light reflects off them to your eyes.
Reasoning: Vision works because light enters our eyes and is detected by cells in the retina. Without light entering our eyes from an object (either light it produces or light it reflects), there is nothing for our eyes to detect, so we cannot see it.
Check Your Understanding
After completing this lesson, you should be able to:
- Explain the three parts of a CER response
- Write a clear, testable claim
- Identify strong vs. weak evidence
- Write reasoning that connects evidence to scientific principles
- Evaluate the quality of scientific arguments
Next Steps
- Review any concepts that felt challenging
- Move on to the next lesson when ready
- Return to practice problems periodically for review