1. Chapter at a Glance
By Grade 8 you have learnt that science begins with curiosity and grows with evidence. This year you will go further — from simply asking questions to investigating them like real scientists: framing a clear question, making a testable prediction (hypothesis), planning a fair test, controlling variables, collecting data and drawing conclusions. This opening chapter introduces these skills and previews the big ideas of Grade 8: the microscopic world, forces, particles of matter, light, ecosystems and our changing planet.
- Steps of a scientific investigation.
- Hypothesis, prediction, variables (independent, dependent, controlled) and fair tests.
- Recording data in tables and graphs; repeating trials; drawing conclusions.
- A preview of Grade 8 themes and how they connect.
- Scientific thinking in everyday life and the role of science in tackling climate change.
2. What Is a Scientific Investigation?
A scientific investigation is a planned way of finding the answer to a question using evidence. Scientists do not accept an idea just because someone important said it; they test it.
- Observe and ask a focused question — “Does the amount of sunlight affect how fast a plant grows?”
- Form a hypothesis — a possible explanation that can be tested: “Plants grow faster with more sunlight.”
- Make a prediction — “If I keep one plant in sunlight and one in shade, the sunlit plant will grow taller in two weeks.”
- Plan a fair test — decide what to change, what to measure and what to keep the same.
- Collect and record data — measure carefully, with units, and repeat the trials.
- Analyse and conclude — look for patterns; decide whether the hypothesis is supported.
- Communicate — share results so others can check and repeat them.
3. Variables and Fair Tests
A variable is any factor that can change in an experiment.
| Type of variable | Meaning | Example (plant growth and sunlight) |
|---|---|---|
| Independent variable | The one factor we deliberately change | Amount of sunlight |
| Dependent variable | What we measure to see the effect | Height of the plant |
| Controlled variables | Everything kept the same | Type of plant, pot size, soil, water, temperature |
A test is fair only when one independent variable is changed at a time and all others are controlled. A control set-up (for example, a plant given normal conditions) is used for comparison.
Hypothesis: salt water boils at a higher temperature than pure water. Independent variable: amount of salt (0, 1, 2 spoons). Dependent variable: boiling temperature. Controlled: volume of water, same vessel, same burner. Repeat each trial three times and take the average. If boiling temperature rises with salt, the hypothesis is supported.
4. Recording and Analysing Data
- Use tables with headings and units.
- Repeat trials and take an average — this reduces the effect of chance errors.
- Plot graphs to see patterns (bar graphs for categories, line graphs for changes over time).
- Be honest: record unexpected results too; they may lead to new discoveries.
| Day | Plant in sunlight (cm) | Plant in shade (cm) |
|---|---|---|
| 0 | 5.0 | 5.0 |
| 5 | 7.2 | 6.0 |
| 10 | 9.8 | 6.9 |
| 15 | 12.1 | 7.5 |
Sample data from a model classroom experiment, shown to practise reading a table.
5. A Preview of Grade 8 — Science All Around
| Theme | Investigative question | Chapter |
|---|---|---|
| Microscopic world | What lives in a drop of pond water? Are all microbes harmful? | 2, 3 |
| Electricity | Does a current-carrying wire behave like a magnet? Why does it get hot? | 4 |
| Forces and pressure | Why is it harder to push a box on a rough floor? Why do winds blow? | 5, 6 |
| Particles of matter | What is matter made of? How are mixtures different from compounds? | 7, 8, 9 |
| Light | Why does a spoon show two different images on its two sides? | 10 |
| Sky and time | Why does the Moon change shape? How were calendars made? | 11 |
| Ecosystems and Earth | How do living things depend on each other? What makes Earth special? | 12, 13 |
These topics are linked. Microbes recycle nutrients in ecosystems; particles explain dissolving and pressure; human activities that change the atmosphere affect climate and life. Scientific understanding helps us find solutions to problems such as climate change, pollution and disease.
6. Thinking Scientifically in Everyday Life
- Checking whether a costly “herbal” claim on an advertisement has evidence behind it.
- Finding which detergent removes a stain best by testing equal pieces of cloth.
- Comparing which water bottle keeps water cooler, using a thermometer.
- Questioning forwarded messages and rumours instead of believing them blindly.
7. Master Table: Key Terms
| Term | Meaning |
|---|---|
| Investigation | Planned search for an answer using evidence |
| Hypothesis | A testable possible explanation |
| Prediction | What we expect to happen if the hypothesis is true |
| Independent variable | Factor deliberately changed |
| Dependent variable | Factor measured |
| Controlled variable | Factor kept the same |
| Fair test | Only one variable changed at a time |
| Conclusion | Decision based on evidence about the hypothesis |
8. Master Summary Table
| Step | Key action | Common error |
|---|---|---|
| Question | Make it specific and testable | Too vague (“Why is nature beautiful?”) |
| Hypothesis | Testable “if…then” statement | Cannot be checked by experiment |
| Fair test | Change one variable | Changing two at once |
| Data | Units, repeats, averages | Single trial only |
| Conclusion | Based on evidence | Ignoring results that do not fit |
9. Mind Map
- Question, hypothesis
- Test, data, conclusion
- Independent, dependent
- Controlled; fair test
- Tables, graphs
- Repeat and average
- Microbes, forces, matter
- Light, sky, ecosystems
10. Tips and Tricks
- “I change, D depends” — Independent is changed; Dependent is measured.
- Write hypotheses as “If … then …” statements.
- Always add “repeat the experiment and take the average” in experiment-design answers.
- Mixing up hypothesis (explanation) and prediction (expected result).
- Calling a test fair when two factors were changed.
- Claiming a hypothesis is “proved” after one trial — say “supported by the evidence”.
11. Important One-liners
- A scientific investigation answers a question using evidence.
- A hypothesis must be testable.
- The independent variable is the one we change.
- The dependent variable is the one we measure.
- Controlled variables are kept the same.
- In a fair test only one variable is changed.
- Repeating trials makes results more reliable.
- Results must be shared so others can check them.
12. Exam-pattern Questions with Detailed Answers
In an experiment, the factor that is deliberately changed is called the:
- Dependent variable
- Controlled variable
- Independent variable
- Conclusion
Show answer
We change the independent variable and measure its effect on the dependent variable; controlled variables are kept the same.
Answer: (C) Independent variable
What is a hypothesis?
Show answer
A possible explanation for an observation that can be tested by experiment.
Answer: A testable explanation
Why should an experiment be repeated several times?
Show answer
To reduce the effect of chance errors and make the results more reliable; the average of several trials is more trustworthy.
Answer: For reliable, averaged results
A student tests whether temperature affects how fast sugar dissolves. Name the independent, dependent and two controlled variables.
Show answer
Independent: water temperature. Dependent: time taken for sugar to dissolve. Controlled: amount of sugar, volume of water, size of sugar particles, stirring.
Answer: Temperature; dissolving time; sugar amount, water volume
Write a hypothesis and prediction for: “Does the type of surface affect how far a toy car rolls?”
Show answer
Hypothesis: Rougher surfaces produce more friction, so the car rolls a shorter distance. Prediction: If the car is released from the same height on glass, wood and carpet, it will roll farthest on glass and least on carpet.
Answer: More friction → shorter distance
What is a control set-up? Why is it used?
Show answer
A set-up kept under normal conditions without the change being tested. It provides a standard for comparison so that any difference can be linked to the independent variable.
Answer: Standard for comparison
From the plant growth table above, by how much more did the sunlit plant grow than the shaded plant in 15 days?
Show answer
Sunlit growth = 12.1 − 5.0 = 7.1 cm; shaded growth = 7.5 − 5.0 = 2.5 cm. Difference = 4.6 cm.
Answer: 4.6 cm more
How can scientific thinking help when you receive a viral message claiming a “miracle cure”?
Show answer
Ask for evidence: Who tested it? Was it tested fairly on many people? Do reliable sources (doctors, official health agencies) support it? Without evidence, do not believe or forward it.
Answer: Demand evidence before accepting claims
Riya puts plant A in sunlight with daily water and plant B in shade with water every third day. B grows less. Can she conclude that sunlight caused the difference? Explain and correct the plan.
Show answer
No. Two variables (light and water) were changed, so the difference may be due to either. She should give both plants the same water, soil and pot, and change only the light.
Answer: Not a fair test — change only sunlight
An unexpected result appears in one of five trials. Should it be ignored? Explain.
Show answer
It should be recorded, not hidden. Check for a mistake in that trial (wrong measurement, changed condition). If an error is found, repeat it; if not, it may point to something new worth investigating. Honest recording is part of good science.
Answer: Record it, check for errors, repeat
Design an investigation to find which of three bottles (steel, plastic, clay) keeps water coolest.
Show answer
Fill each with the same volume of water from the same source at the same starting temperature; keep them in the same place. Measure temperature with a thermometer every hour for 4 hours; repeat on three days and average. Independent: material; dependent: temperature; controlled: volume, place, time, starting temperature.
Answer: Same conditions, vary only the bottle material
13. Quick Revision Summary
- Investigation: question → hypothesis → prediction → fair test → data → conclusion → communication.
- Independent (changed), dependent (measured), controlled (kept same) variables.
- Fair test: one change at a time; use a control; repeat and average.
- Record honestly in tables and graphs.
- Grade 8 explores microbes, health, electricity, forces, pressure, matter, light, sky, ecosystems and Earth.