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Class 10 · Science and environment

Why did the plants die? Alternative explanations

The class plants are turning yellow. Someone says, "It's the water." How do you know it is not something else?

What you will learn: List several explanations for one observation and design tests to tell them apart.

Small test at the end: scroll down to the end of this page for five short questions that check what you have learnt. Tap an answer, then press Check answer. Go to the questions ↓

Jump to: Check yourself · For grown-ups · For teachers

Safety first

No sprays or chemicals. Wash hands after.

Keep it safe:

  • No pesticides or chemicals: look and record only.
  • Wash hands after handling soil or plants.

You will need

  • Paper and pencil
  • A hand lensOptional, for looking under leaves.
  • Pot plantsOptional; the table works without real plants.

Time: About 40 minutes.

The story

The plants on the classroom shelf are turning yellow. "It's the water," says Tanmay, certain. Dhruv opens his notebook. "Maybe. But it could be light, pests or fertiliser." How can they find out which explanation fits the evidence?

The idea

One observation can have several explanations. List them before choosing one.

Each explanation predicts something different. If overwatering is the cause, the soil should stay wet for days. If light is the cause, plants by the window should be fine.

Design a test that tells the explanations apart: change one thing (such as moving one plant to the light) and keep the rest the same.

Even when a test supports one explanation, say that another cause may also be involved. Strong conclusions need more than one test.

A table of four explanations for yellow, dying leaves. Overwatering predicts soil that stays wet for days; test by feeling the soil daily. Too little light predicts that plants by the window are fine; test by moving one to the light. Pests predict insects under the leaves; test by looking with a lens. Too much fertiliser predicts a white crust on the soil; test by checking the soil top.Yellow, dying leaves: four explanationsExplanationIt predicts...TestOverwateringSoil stays wet for daysFeel soil dailyToo little lightPlants by window are fineMove one to lightPestsInsects under leavesLook with a lensToo much fertiliserWhite crust on soilCheck soil top
A table of four explanations for yellow, dying leaves. Overwatering predicts soil that stays wet for days; test by feeling the soil daily. Too little light predicts that plants by the window are fine; test by moving one to the light. Pests predict insects under the leaves; test by looking with a lens. Too much fertiliser predicts a white crust on the soil; test by checking the soil top.

Worked example

Class 10B investigates

  1. Observation: plants on the dark side of the room are yellowing. All are watered the same.
  2. Explanations: overwatering, too little light, pests, too much fertiliser.
  3. Checks: soil dries in 2 days (not overwatering); no insects; no crust. Plants by the window are fine.
  4. Test: move one yellowing plant to the window for two weeks, watering it the same. It recovers.

The evidence supports too little light. They note that one test is not proof, and keep checking.

Try it

  1. Write the observation.

    What exactly did you see, and where?

  2. List explanations.

    At least three.

  3. Write predictions.

    If this explanation is right, what else should be true?

  4. Check and test.

    Check each prediction. Design one fair test.

  5. Conclude carefully.

    Which explanation fits best? What could still be involved?

What you make (paper or offline)

Your explanations table

A table of explanations with predictions and tests.Yellow, dying leaves: four explanationsExplanationIt predicts...TestOverwateringSoil stays wet for daysFeel soil dailyToo little lightPlants by window are fineMove one to lightPestsInsects under leavesLook with a lensToo much fertiliserWhite crust on soilCheck soil top
A table of explanations with predictions and tests.

At least three explanations.

A prediction and a test for each.

A careful conclusion that says what is still uncertain.

Make it on paper. Keep it with you. Nothing to upload.

Check yourself

Five questions about what this page teaches. Choose an answer and press Check answer, or open "See the answer and why". Nothing is saved or sent: your choices stay on this screen and disappear when you leave or reload the page.

  1. Question 1 of 5Why list several explanations before deciding?

    See the answer and why
    • A. So you do not have to test anything Not this one. Listing explanations is what makes testing possible.
    • B. To make the report longer Not this one. Length is not the point.
    • C. Because the first idea is the right one Not this one. The first idea is often wrong. That is why you list others.
    • D. One observation can have several causes Correct. Listing them stops you jumping to the first idea.
  2. Question 2 of 5If overwatering is the cause, what would you predict?

    See the answer and why
    • A. The soil stays wet for days Correct. Too much water keeps the soil wet.
    • B. Insects under the leaves Not this one. That fits pests.
    • C. Plants by the window are fine while the others die Not this one. That fits too little light.
    • D. The soil is dry and cracked Not this one. That fits too little water, not too much.
  3. Question 3 of 5Plants on the dark side died; plants by the window are fine; all were watered the same. Which explanation fits best?

    See the answer and why
    • A. Too much fertiliser Not this one. Nothing suggests the dark-side plants got more fertiliser.
    • B. Too little light Correct. Light is the main difference between the two groups.
    • C. Overwatering Not this one. All got the same water, but only the dark-side ones died.
    • D. The plants were the wrong colour Not this one. Leaf colour is the result, not the cause.
  4. Question 4 of 5Which test best separates "too little light" from "overwatering"?

    See the answer and why
    • A. Water all the plants more Not this one. That changes water, and could make overwatering worse.
    • B. Move one plant to the window and keep its watering the same Correct. Only the light changes, so any recovery points to light.
    • C. Paint the pots a brighter colour Not this one. Pot colour does not change light reaching the leaves.
    • D. Look at the plants once and decide Not this one. Looking once does not test either explanation.
  5. Question 5 of 5A test supports one explanation. What should you still say?

    See the answer and why
    • A. Nothing else needs to be checked Not this one. Further checks make the conclusion stronger.
    • B. Another cause may also be involved Correct. One test supports an explanation but does not rule out everything.
    • C. All other explanations are impossible Not this one. One test cannot rule out everything.
    • D. This test proves it forever Not this one. Science stays open to new evidence.

Think back

Which explanation did you believe at first? Did the evidence change your mind?

Why is it hard to let go of a first idea?

Investigate further

  • Apply this to a bicycle that keeps getting flat tyres.
  • Read about a historical mystery with several explanations.

For grown-upsTap a heading to open it. The safety notes are at the top of the page.

Before you start
  • Optional: a plant that is not doing well at home.
Questions that help
  • "What else could cause that?"
  • "If that were true, what would we expect to see?"
What's really going on

Generating and testing rival explanations is how detectives, doctors and scientists avoid jumping to conclusions.

No pesticides are needed; looking and recording is enough.

Other ways to do it
  • Use "Why is the phone battery draining fast?" with made-up observations.

For teachersLearning aims, classroom ideas and what to look for.

Learning aims
  • Generate alternative explanations and predictions.
  • Design a discriminating test and state uncertainty.
Subject links
  • Science: plant needs and scientific method.
  • Language: reasoning and hedging.

These links are general. They have not been checked against a specific board syllabus.

In the classroom
  • Present a mystery; groups compete to list explanations and the best test to tell them apart.
What to look for
  • Different prediction for each explanation.
  • Conclusion that admits uncertainty.
ProLearners Assist