Exercises: Hypotheses, Theories, and Laws

In your own words, define a scientific hypothesis. Then explain what makes a hypothesis "testable," and give one example of a testable hypothesis about plant growth.

Decide whether each statement describes a scientific hypothesis, a scientific theory, or a scientific law. Write one short sentence to justify each choice.

1. a) Cell theory states that all living things are made of cells and that cells come from pre-existing cells.
2. b) "If I water bean plants with salt water, then they will grow shorter than plants watered with fresh water."
3. c) The law of conservation of mass states that matter is neither created nor destroyed in a chemical reaction.

Mateo watches a pond for two weeks and records the number of frogs he sees each day, the water temperature, and the weather. He does not change anything about the pond. Is this a descriptive, comparative, or experimental investigation? Explain your answer in one or two sentences.

Many people say, "It's only a theory," when they doubt an idea. Explain why this use of the word "theory" is different from how scientists use it. In your answer, describe how a scientific theory differs from a hypothesis, and give one example of a well-supported theory in biology.

Aaliyah wants to know whether the brand of fertilizer affects the height of tomato plants. She sets up 12 identical pots with the same soil and the same type of tomato seedling. Four pots receive Brand A, four receive Brand B, and four receive only water. All pots get the same sunlight and 100 mL of water every two days. After 30 days she measures plant height in centimeters.

1. a) Identify the independent variable and the dependent variable.
2. b) Identify the control group.
3. c) List two constants (controlled variables).

A student asks each of the following questions. For each one, state whether it can be answered by a scientific investigation, and if not, explain why.

1. a) Do goldfish eat more food in warm water than in cold water?
2. b) Which flavor of ice cream is the most delicious?
3. c) Does the amount of light affect the rate of photosynthesis in elodea?

For the questions that are testable, rewrite one as an "If... then..." hypothesis.

Priya hypothesizes that a new sports drink improves reaction time. She recruits 10 volunteers. All 10 drink the sports drink and then take a computer reaction test. Their average reaction time is 0.21 s. She concludes that the drink improves reaction time.

1. a) Identify at least three flaws in her experimental design.
2. b) Redesign the experiment so it supports a valid conclusion. Name the independent variable, dependent variable, control group, and at least three constants.
3. c) Explain why increasing the number of volunteers would strengthen the conclusion.

For centuries, scientists have refined the explanation for how species change over time. Evolution by natural selection is supported by evidence from fossils, DNA comparisons, and direct observation of populations such as bacteria that become antibiotic resistant.

1. a) Explain why this explanation is called a theory and not a law.
2. b) Describe how a theory differs from a law in what each one does for science.
3. c) Argue what would happen to a well-supported theory if new evidence contradicted it. Use the idea that scientific knowledge is tentative but reliable.