Introduction

Before scientists carry out an experiment, they usually have an idea about what they think will happen and why. This idea helps them plan their investigation and decide what evidence to collect.

Scientists express these ideas by writing hypotheses and making predictions.

Although these terms are often used together, they are not the same thing. A hypothesis explains why something might happen, while a prediction states what is expected to happen if the hypothesis is correct.

---

What Is a Hypothesis?

A hypothesis is a testable explanation for an observation or a scientific question.

A good hypothesis is:

• Based on scientific knowledge.
• Testable through investigation.
• Supported by logical reasoning.
• Able to be shown as supported or not supported by evidence.

A hypothesis is not simply a guess.

It is an educated explanation based on what is already known.

---

What Is a Prediction?

A prediction states what you think will happen during an investigation if the hypothesis is correct.

Predictions describe the expected outcome of an experiment.

Example:

Prediction:

Plants receiving more sunlight will grow taller than plants receiving less sunlight.

A prediction does not explain why the plants will grow taller—that is the role of the hypothesis.

---

Hypothesis vs Prediction

---

Writing a Good Hypothesis

Many hypotheses are written using an If... then... because... structure.

Example:

If bean plants receive more sunlight,

then they will grow taller,

because sunlight provides the energy needed for photosynthesis.

This format includes:

• the independent variable
• the expected outcome
• the scientific explanation

---

Making Logical Predictions

Predictions should be based on:

• scientific knowledge
• previous observations
• evidence from earlier investigations

Example:

Question:

How does water temperature affect the time taken for sugar to dissolve?

Prediction:

Sugar will dissolve faster in hot water than in cold water.

This prediction is based on what we know about particle motion.

---

Testing a Hypothesis

A hypothesis is tested by carrying out a fair experiment.

Scientists:

1. Plan the investigation.
2. Identify the variables.
3. Collect measurements.
4. Analyse the results.
5. Decide whether the evidence supports the hypothesis.

Importantly, scientists do not prove a hypothesis to be absolutely true. Instead, the evidence may support or not support it under the conditions tested.

---

Revising a Hypothesis

Sometimes the results do not match the prediction.

This does not mean the experiment has failed.

Instead, scientists:

• examine their methods,
• analyse the evidence,
• consider alternative explanations,
• revise their hypothesis if necessary.

Improving explanations based on new evidence is a normal and important part of science.

---

Example Investigation

Question

How does the amount of fertilizer affect the growth of tomato plants?

Hypothesis

If tomato plants receive more fertilizer, then they will grow taller because fertilizer supplies nutrients needed for growth.

Prediction

Plants given extra fertilizer will have a greater average height after four weeks.

Investigation

Plants are grown under identical conditions, except for the amount of fertilizer they receive.

Conclusion

The results are used to determine whether the evidence supports the hypothesis.

---

Why Are Hypotheses Important?

Hypotheses help scientists to:

• focus their investigations,
• identify relevant variables,
• design experiments,
• interpret results,
• develop scientific understanding.

Without hypotheses, investigations would lack direction.

---

Real-World Applications

Scientists use hypotheses and predictions in many areas, including:

• Medicine
• Agriculture
• Environmental science
• Engineering
• Astronomy
• Chemistry
• Biology
• Physics

Every major scientific discovery begins with questions that lead to hypotheses and predictions.

---

Worked Examples

Example 1

Which statement is a hypothesis?

A. Plants will grow 5 cm taller.

B. Plants grow taller in sunlight because they can carry out more photosynthesis.

Answer:

B

It explains why the plants are expected to grow taller.

---

Example 2

Which statement is a prediction?

Answer:

Plants placed in sunlight will grow taller than plants kept in darkness.

---

Example 3

Complete the hypothesis.

If the temperature of the water increases,

then ____________________,

because ____________________.

Possible Answer:

...the sugar will dissolve more quickly,

because the water particles move faster and collide with the sugar more often.

---

Example 4

What should a scientist do if the results do not support the hypothesis?

Answer:

Review the evidence, consider possible sources of error, and revise the hypothesis if necessary.

---

Example 5

Why must a hypothesis be testable?

Answer:

Because scientists need to collect evidence through observations or experiments to determine whether the evidence supports the hypothesis.

---

Did You Know?

In the 1800s, Louis Pasteur tested the hypothesis that microorganisms come from existing microorganisms rather than appearing spontaneously. His famous swan-neck flask experiment provided strong evidence against the idea of spontaneous generation and helped establish the foundations of modern microbiology.

---

Key Terms

---

Key Takeaways

• A hypothesis explains why something is expected to happen.
• A prediction states what is expected to happen if the hypothesis is correct.
• Good hypotheses are testable and based on scientific reasoning.
• Predictions should be logical and supported by existing evidence.
• Experiments provide evidence that may support or not support a hypothesis.
• Scientists revise hypotheses when new evidence becomes available, helping scientific knowledge become more accurate over time.