Definition
What is a scientific hypothesis?
A hypothesis is a testable explanation or prediction about the expected relationship between variables. It gives the experiment a clear direction and can be evaluated using evidence.
A hypothesis should not be a random guess. It should be based on observations, background research, scientific principles, or previous evidence.
Hypothesis structure
Use the if-then-because format
Hypothesis formulaIf [independent variable changes], then [dependent variable will change], because [scientific reason].
This structure clearly separates the changed factor, the measured response, and the scientific explanation.
Writing process
Six steps for writing a strong hypothesis
Review the scientific question
Identify the exact relationship the investigation is designed to test.
Identify the independent variable
Determine what factor will be deliberately changed, selected, or compared.
Identify the dependent variable
Determine what response or outcome will be measured.
Predict the expected relationship
State how the dependent variable is expected to respond when the independent variable changes.
Add scientific reasoning
Explain the scientific principle, mechanism, or prior evidence supporting the prediction.
Check testability
Confirm that the prediction can be evaluated using measurements, observations, and a practical experiment.
Worked example
Scientific question to hypothesis
Scientific question
How does water temperature affect sugar dissolving time?
Independent variable: water temperature. Dependent variable: time required to dissolve.
Hypothesis
If water temperature increases, sugar will dissolve in less time.
This is expected because faster-moving water particles interact more frequently with the sugar.
Quality criteria
Characteristics of a strong hypothesis
- It directly answers the scientific question.
- It identifies the expected relationship between variables.
- It can be tested using measurable evidence.
- It includes scientific reasoning.
- It is specific and clearly worded.
- It can be supported or not supported by results.
Examples
Examples of scientific hypotheses
- If daily light exposure increases, then plant height will increase because more light supports photosynthesis.
- If ramp height increases, then a model car will travel farther because it begins with greater gravitational potential energy.
- If reactant concentration increases, then reaction rate will increase because particles collide more frequently.
- If exercise duration increases, then heart rate will increase because working muscles require more oxygen.
Common mistakes
Hypothesis problems to avoid
- Writing a statement that cannot be tested.
- Leaving out the independent or dependent variable.
- Giving a prediction without scientific reasoning.
- Using vague terms such as better, worse, or more effective without defining a measurement.
- Writing the hypothesis after seeing the final results.
- Claiming that the hypothesis must be proven.
Evidence
Supported does not mean proven
Experimental evidence may support, partially support, or not support a hypothesis. One investigation rarely proves a general scientific explanation.
An unsupported hypothesis is not automatically a failed experiment. It may reveal an incorrect assumption, an uncontrolled variable, a limitation, or a new scientific question.
Related resources
Build the complete investigation
Start with a focused Scientific Question, review the full Steps of the Scientific Method, and organize the final investigation with the Lab Report Template.
Questions and answers
Hypothesis FAQ
What is a scientific hypothesis?
A scientific hypothesis is a testable prediction about the expected relationship between variables, supported by scientific reasoning.
Does a hypothesis have to be correct?
No. A hypothesis is evaluated using evidence. An unsupported hypothesis can still lead to useful conclusions and improved investigations.
Should a hypothesis use an if-then-because format?
The if-then-because format is useful because it identifies the independent variable, dependent variable, and scientific reasoning, but other clear formats may also be acceptable.
What is the difference between a hypothesis and a prediction?
A prediction states what is expected to happen, while a strong hypothesis also explains why that outcome is expected.