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