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 independent variable, which is the one changed by the experimenter the dependent variable, which is the one for which the experimenter measures the response 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.)