What is a Hypothesis?

As you learned in the lesson in this module titled "Types of Investigations," there are many types of investigations that scientists perform. We examined three types.

Investigation Type Purpose Hypothesis? Variables manipulated? Control?
Descriptive To draw conclusions No (but does answer a questions) No No
Comparative To determine relationships Yes Yes No
Experimental To determine a causal relationship Yes Yes Yes

Look carefully at the hypothesis and variables manipulated columns. Notice that in descriptive investigations there is no hypothesis and no variables manipulated but in comparative and experimental investigations there is both a hypothesis and variables that are manipulated.

A hypothesis is a proposed explanation for an observable phenomenon. For a hypothesis to be used in science, one must be able to test it in the form of an experiment. The hypothesis is not just an “educated guess.” Hypotheses should be based on previous observations that cannot be explained with available information. A scientific investigation is designed to test this hypothesis.

In an experimental investigation, the hypothesis should establish a cause and effect relationship. The cause is the independent variable in the investigation and the effect or result is the dependent variable and is a prediction of what the student thinks will happen.

In a comparative experiment, the hypothesis should identify the relationship between the independent and dependent variable. Often the hypothesis of a comparative experiment identifies which level or condition of the independent variable will perform the best when the dependent variable is applied.

The basic form of a hypothesis is usually similar to one of the following:

  1.     IF independent variable is applied, THEN the dependent variable will happen.
  2.     AS the independent variable changes, THE dependent variable changes. 

How to Write a Hypothesis

The first step in writing a hypothesis is to identify the independent and dependent variables in the experiment. (If you are not sure what the independent and dependent variables are, review the lesson titled “Experimental Design” in the Related Resources.)

How does the amount of fertilizer affect plant growth (height)?

Look at the scientific research question above. See if you can identify the independent and dependent variables.

Now that you have identified the independent and dependent variable, you can easily write the hypothesis. The hypothesis should be written in the form of an “if-then” or “as-the” statement linking a change in the independent variable to a predicted change in the dependent variable.

An easy way to write a hypothesis is to follow this simple formula.

If the (independent variable) (verb or verb phrase describing how the independent variable is changed), then the (dependent variable) will (verb or verb phrase predicting how the dependent variable will change).

The picture below shows the setup of the fertilizer and plant growth experiment. 

plants in an experimental investigation

Notice there are different amounts of fertilizer (zero drops, 2 drops, 4 drops and 6 drops) being applied to the plants. These are the levels or conditions of the independent variable. The conditions or levels of the independent variable will determine the verb you add to the first part of the hypothesis. In this investigation, the number of drops of fertilizer is increasing (from zero up to 6 drops) so you could use the word increases in the first part of your hypothesis.

The verb that follows the dependent variable in the second part of the hypothesis should be your prediction of what will happen to the dependent variable. So, a sample hypothesis might be as follows: 

If the amount of fertilizer is increased, then plant growth will increase.

Sometimes you will see hypothesis written as an “as-the” statement. You would write the same hypothesis but use the words “as-the” instead of “if-then.” For example,

If the amount of fertilizer applied to plants is increased, then plant growth (height) will increase.

As the amount of fertilizer increases the plant growth increases.



Writing a hypothesis for a comparative investigation can be a little more difficult than you think. Remember, the hypothesis of a comparative experiment identifies which level or condition of the independent variable will perform the best when the dependent variable is applied.

Writing a Hypothesis: Practice 1

Read the lab scenario below.

Pendulum Lab

  1.     Cut a piece of string 10 cm, 20 cm, 30 cm, 40 cm, and 50 cm.
  2.     Tie a paperclip to one end of the 10 cm string.
  3.     Tape a pencil to the end of the table.
  4.     Tie the other end of the string to the pencil.
  5.     Tape the string in place with a small piece of masking tape.
  6.     Hang three metal washers on the paperclip.
  7.     Raise the washers to be level with the edge of the table.
  8.     Start the stopwatch and release the pendulum.
  9.     Count the number of times the pendulum makes a complete trip (a period) in 10 seconds.
  10.     Record your results in your data table.
  11.     Repeat this experiment for four more trials.
  12.     Remove the pendulum from the pencil. Disassemble the pendulum by removing the washers and paper clip.
  13.     Repeat steps 2-10 with the remaining lengths of string.


Writing a Hypothesis: Practice 2

Watch the video and read the scenario to create a hypothesis for this experiment.

Source: The Extreme Diet Coke & Mentos Experiments, Zorro103, YouTube

Mixing chewy mint candies with a carbonated drink such as diet cola will produce a fountain of soda that spews from the bottle. You decided to do an experiment using diet cola and different types of chewy mint candies. You decided to use a diet soda instead of a regular soda so that your clean up is easier; it won’t have the sugary, sticky residue that comes from regular sodas. Below is the procedure and results that you observed.

  1.     Obtain mint and fruit-flavored chewy mint candies
  2.     Examine both types of chewy mint candies under a stereoscope
  3.     Draw a picture of what you see
  4.     Obtain two bottles of diet cola; both need to be the same size
  5.     Place one mint chewy mint candies into a diet cola
  6.     Place one fruit-flavored chewy mint candies into the other diet cola
  7.     Record your observations

In your observations about the chewy mint candies, you notice that there are small pits all over the surface of the chewy mint candies, but the fruit-flavored chewy candies have a smooth coating on them. In your observations, you write that the diet cola with the mint chewy candies looked like a geyser as the diet cola was streaming from the bottle. You note that the diet cola with the fruit-flavored chewy candies remained intact and unchanged.