Experimental Design

The Recipe for a Controlled Experiment

What is the first step of experimental design?

• Write A Testable Hypothesis!

–Allows the scientist to identify variables

–Suggests an experiment

Step Two of Experimental Design

• Assign the variables

• Variable: a component of the experiment which may change– the analyst controls this change or

measures this change

• Independent variable

• Dependent variable

Independent Variable

• #1 controlled variable

• Set (assigned) by the analyst

• Only one independent variable may be “set” or changed at a time

• “The Treatment” applied to a group

• The hypothesis states this variable will create a predictable result.

• X-axis of graphs

Dependent Variable

• Measured by the analyst• Allowed to vary freely in relation to the

independent variable• May or may not show a change in response to

the “treatment”, which is the independent variable– i.e. The dependent variable is affected by the

independent variable (unless there is no effect!).

• Y-axis of graphs

The Constants

• Any component of an experiment which is not the independent or dependent variable must remain the same for all control and data trials.

• The analyst must carefully design all aspects of the experiment, so that the conditions of experimentation are controlled.

The Constants• If all aspects of the experiment are not

constant, then the results may not be confidently assigned to the independent variable. The conclusion may be invalid. The analyst cannot determine if the independent variable affected the dependent variable from the data and results.

• The analyst must imagine the test conditions that he or she would like to create.

The Control• The Control is a “neutral trial” used as a

baseline for comparison for the data trials.• Often the first trial performed• May be repeated and averaged• No treatment is applied to this trial• Zero result or no effect expected• Examples:

– No peppermint for students taking a math test– UV spectrophotometer control standards and

temperature fluctuation

Data Trials• Trials performed to yield measurable results

(observed or numeric) in the dependent variable• Treatment (independent variable) controlled by

the analyst and intended to yield data (collected in a table)

• Data trials must be repeated (magic # = 3, minimum) and the results are often averaged

• Multiple subjects, animals, or plants = multiple data trials

• Data is usually further processed and interpreted by calculations and displayed visually as results (statistics, graphs, etc.)

• Interpreted Data = Results

Design a Controlled Experiment

How does salt affect the

boiling point of water?

– Design a controlled experiment addressing this question.

– Carefully list and assign all components of the experiment in your notebook.

Design a Controlled Experiment

Hypothesis:

If salt does not change the boiling point of water, then salted water will boil at 100° C.

Independent variable:

salt

Dependent variable:

boiling temperature of water

Design a Controlled ExperimentConstants:

Same volume of water

Same type of water: distilled water (no salt)

Same type of salt (Sodium chloride, NaCl)

Same amount of salt OR add increasing amounts to look for a relationship

Same beaker

Same hotplate

Same thermometer or same calibration

Same analyst

Design a Controlled Experiment

The Control:

Boil distilled water

No salt added (no treatment)

Average the boiling temperatures from three repetitions.

Recored the raw data and averages.

What would be the expected average boiling point for plain water?

100° C

Design a Controlled Experiment

The Data Trials:

Add 2 g NaCl salt to 200 mL distilled water.

Stir to dissolve.

Average the boiling temperatures from three repetitions.

Record the raw data and averages.

Design a Controlled Experiment

Review the hypothesis:

If salt does not change the boiling point of water, then salted water will boil at 100° C.

Think about the data and results:

What type of data will support your hypothesis?

What type of data will not support your hypothesis?

What really happens?

http://en.wikipedia.org/wiki/File:Freezing_point_depression_and_boiling_point_elevation.png

A liquid boils at a temperature when the vapor pressure of the liquid equals the surrounding pressure. Adding salt lowers the vapor pressure, so more energy is required to raise the vapor pressure. Therefore the boiling point is elevated.

Salt interferes with crystal formation of the solid.

How is this applicable to the real world?• Boiling Point Elevation:

Salt is added to boiling water to raise the boiling point and cook food faster, but…

• Freezing Point Depression:

• Adding salt to walkways and roads to prevent ice formation

• Antifreeze in car engines (water and ethylene glycol in radiator)

• The ocean does not freeze and icebergs only form at arctic latitudes

• Some cold water fish and hibernating amphibians use this concept to avoid freezing during hibernation (sugars)