The Cycle of Proof:
Designing Experiments
The Cycle of Proof:
Designing Experiments
Designing Experiments:
Daily Learning Goals
The student will be able to formulate scientific
questions and hypotheses. (A1.1)
The student will be able to identify independent,
dependent, and controlled variables. (A1.5)
The Problem
The Problem or Question that prompts an
experiment should question the relationship
between variables. It should be specific.
The Problem
The Problem or Question that prompts an
experiment should question the relationship
between variables. It should be specific.
For example:
NO: “What affects the rate of a chemical
reaction?”
The Problem
The Problem or Question that prompts an
experiment should question the relationship
between variables. It should be specific.
For example:
NO: “What affects the rate of a chemical
reaction?”
YES: “How does the concentration of the
reactants affect the time for a chemical reaction
to complete?”
The Hypothesis
The Hypothesis should then state the believed
relationship between the variables in the Problem.
The Hypothesis
The Hypothesis should then state the believed
relationship between the variables in the Problem.
Problem: “How does the concentration of the
reactants affect the time for a chemical reaction to
complete?”
The Hypothesis
The Hypothesis should then state the believed
relationship between the variables in the question.
Problem: “How does the concentration of the
reactants affect the time for a chemical reaction to
complete?”
Hypothesis: “As the concentration of the reactants
increases, the time for the chemical reaction to
complete decreases.”
Variables
A variable is anything in an experiment that may
be changed.
Most experiments change only two:
 the independent variable, which is the one
changed by the experimenter
 the dependent variable, which is the one for
which the experimenter measures the response
Variables: Example
Problem: “How does the concentration of the
reactants affect the time for a chemical reaction
to complete?”
What is the independent variable?
What is the dependent variable?
Variables: Example
Problem: “How does the concentration of the
reactants affect the time for a chemical reaction
to complete?”
What is the independent variable?
the concentration of the reactants
What is the dependent variable?
the time for the reaction to complete
Constant Variables
All other variables are controlled and are known
as constants because they do not change.
What would need to be kept the same each time
when measuring the time for the reaction?
Constant Variables
All other variables are controlled and are known
as constants because they do not change.
What would need to be kept the same each time
when measuring the time for the reaction?
the volume/mass of the reactants
the temperature of the reactants
the laboratory equipment
etc.
Constant Variables
These values of these constants need to be
specified when the procedure is written.
Constant Variables
These values of these constants need to be
specified when the procedure is written.
E.g., “5 mL of hydrochloric acid at room
temperature was added to a test tube. . . .”
Writing Procedure
Remember that however the instructions are
given in a procedure, in the lab report ,
everything must be rewritten in the past tense,
passive voice/third person.
Directions such as:
“Measure the _________ . . .”
become
“The __________ was measured . . .”
Writing Procedure
Simply leaving off the pronoun does NOT make a
sentence passive voice/third person.
“Measured the _________”
still has an invisible [I/We] at the start:
“[I/We] measured the __________”
Writing Procedure
Simply leaving off the pronoun does NOT make a
sentence passive voice/third person.
“Measured the _________”
still has an invisible [I/We] at the start:
“[I/We] measured the __________”
The Independent Variable
To see significant changes in the dependent
variable, the independent variable must be
changed significantly during the experiment.
E.g. the experimenter might use concentrations
of:
4%, 6%, 8%, 10%, and 12%
NOT 10% and 11%
The Independent Variable
To see significant changes in the dependent
variable, the independent variable must be
changed significantly during the experiment.
you should choose at least 5 values with at least a
factor of 2 difference between the largest and smallest
Experimental Error
Why should the changes be large?
Experimental Error
Why should the changes be large?
Not only because you want to see the trend
clearly on the graph but also because you don’t
want the experimental uncertainty in your
measurements to be larger than any changes.
Experimental Error
Why should the changes be large?
Not only because you want to see the trend
clearly on the graph but also because you don’t
want the experimental uncertainty in your
measurements to be larger than any changes.
And all measurements will have some
experimental uncertainty or “experimental
error.”
Experimental Error
Experimental errors are
NOT human errors and NOT mistakes.
When asked for sources of experimental error,
NEVER put “we might have measured the time
wrong.”
Measuring something “wrong” is NOT
experimental error.
Experimental Error
Experimental errors are limitations on the
accuracy and precision of measurements.
E.g. reaction time when measuring something
with a stopwatch
Experimental Error
We expect our measurements, even if they are a
little off because of error, will be distributed
randomly about the actual value:
Experimental Error
We expect our measurements, even if they are a
little off because of error, will be distributed
randomly about the actual value:
which is why we do multiple trials of any
measurement and average the results.
(Sometimes we may use a class set of data.)