What Is a Hypothesis?
Taught in US schools
Key Takeaways
- A hypothesis is a testable prediction about what will happen in an experiment.
- A good hypothesis is written as an if-then statement: 'If [cause], then [effect].'
- A hypothesis is not a guess - it must be based on prior knowledge or observation.
- A hypothesis can be disproven - that is not failure. It is how science works.
What Is a Hypothesis?
A hypothesis is a testable prediction about what will happen in a scientific experiment. It is not simply a guess - it is an educated prediction based on prior knowledge, observations, or research.
A hypothesis is the second step of the scientific method: after asking a question and doing background research, a scientist proposes a hypothesis before conducting an experiment.
What Makes a Good Hypothesis
A strong hypothesis has three characteristics:
- Testable - you can design an experiment to test it
- Falsifiable - it can be proven wrong (not all predictions are falsifiable)
- Based on evidence - it connects to what is already known, not invented from nothing
"I think the plant will grow" is not a good hypothesis - it's too vague and not based on any specific variable. "If a plant receives 8 hours of sunlight per day, then it will grow taller than a plant that receives 4 hours, because plants use sunlight for photosynthesis" is a strong hypothesis.
Writing a Hypothesis: The If-Then Format
The most common format for writing a hypothesis in elementary science is the if-then statement:
If [the independent variable changes in this way], then [the dependent variable will change in this way].
Examples:
- "If we add salt to water, then it will take longer to boil."
- "If the ramp is steeper, then the marble will roll farther."
- "If plants are watered with soda instead of water, then they will grow more slowly."
Adding "because" strengthens the hypothesis by connecting it to prior knowledge:
- "If plants are watered with soda, then they will grow more slowly, because plants need plain water, and the sugar and chemicals in soda could harm them."
What Happens When a Hypothesis Is Wrong?
A hypothesis that is disproven is not a failure - it is science working correctly.
When an experiment disproves a hypothesis, it tells us something important: our prediction was wrong, which means something else must be true. This leads to a new hypothesis, more testing, and deeper understanding.
Famous scientific discoveries have resulted from hypotheses that turned out to be wrong. The process of testing and disproving is what makes science reliable.
Practice Activities
- Give students a question ("Does the temperature of water affect how fast sugar dissolves?") and have them write a hypothesis before conducting the experiment.
- Practice the if-then format with non-science predictions: "If I sleep more, then I will be less tired." Discuss whether this is testable.
- After an experiment, compare the hypothesis to the results. If they match, explain why. If they don't, discuss what might have caused the unexpected result.
- Show students a hypothesis and ask: "Is this testable? Is it based on prior knowledge? How would you design an experiment to test it?"
Frequently Asked Questions
What is a hypothesis in science?
A hypothesis is a testable prediction about the outcome of an experiment. It is not a random guess - it is an educated prediction based on what you already know or have observed. A hypothesis is always written so that it can be tested and potentially disproven. Example: 'If plants receive more sunlight, then they will grow taller than plants that receive less sunlight.'
How do you write a hypothesis?
The most common format is the if-then statement: 'If [independent variable is changed in this way], then [dependent variable will change in this way].' A complete hypothesis also sometimes includes 'because': 'If plants receive more water, then they will grow taller, because plants need water to grow.' The 'because' forces students to connect their prediction to prior knowledge.
What is the difference between a hypothesis and a theory?
A hypothesis is a specific, testable prediction for a single experiment. A scientific theory is a well-substantiated explanation of a pattern in nature, supported by extensive testing, multiple lines of evidence, and repeated confirmation over time. Calling something a 'theory' in science actually means it is strongly supported - the opposite of how 'theory' is used in everyday speech.
Free Hypothesis Worksheets
Curriculum-aligned printable worksheets for 2nd – 5th Grade. Download free.