Experimental Design Concepts

A science experiment is designed so that only ONE variable is being tested at a time.

A VARIABLE is something that is CHANGED to study the effect it will have on the object

By changing only one variable, when you make your conclusion you can be assured that it is only that one variable that is causing the EFFECT.

Example Experiment…

FREAKY FROGS Observation Problem Hypothesis Prediction Test Prediction Conclusion

Write your hypothesis in an “if-then” format Hypothesis 1: The deformities were caused by one or more chemical pollutants in the water. If a substance in the pond water is causing the deformities, then some tadpoles will develop deformities when they are raised in pond water collected from ponds that have deformed frogs.

Write your hypothesis in an “if-then” format Hypothesis 2: The deformities were caused by attacks from parasites. If a parasite is causing the deformities, then this parasite will be found more often in frogs that have deformities.

Write your hypothesis in an “if-then” format Hypothesis 3: The deformities were caused by an increase in exposure to ultraviolet light from the sun. If an increase in exposure to ultraviolet light is causing the deformities, then some frog eggs exposed to ultraviolet light in a laboratory will develop into deformed frogs.

4. Test Predictions EXPERIMENT

Many factors… Scientists in Minnesota observed that there were many factors or things that could have caused the deformities in the frogs (chemicals, light, parasites…) To help sort things out and find the cause, they have to set up experiments that test one thing at a time

Hypothesis 3: If an increase in exposure to ultraviolet light is causing the deformities, then some frog eggs exposed to ultraviolet light in a laboratory will develop into deformed frogs.

EXPERIMENT Components Independent variable Dependent variable Constants Control Repeated trials

Independent variable (IV) the variable that is purposely CHANGED by the experiment

Dependent variable (DV) the variable that responds and is the variable MEASURED

all factors that are kept the SAME during the experiment Constant (C) all factors that are kept the SAME during the experiment

CONTROL - the standard to COMPARE the experimental effect against

Repeated trials the NUMBER of objects/organisms undergoing treatment for each value of the independent variable, or the number of TIMES the experiment is repeated

The Experiment Constants Variable Control Group leopard 100 25˚C kind of frog number of eggs temperature of water Variable UV light exposure Control Group leopard 100 25˚C 0 days Experiment Group 15 days 24 days

The Experiment 15 days 24 days

Results 15 days 24 days 0 deformed frogs 0 deformed frogs

Conclusion - Is it the Answer? Our experiment supports the hypothesis that the frog deformities can be caused by exposure to UV light Was that definitely the cause?

New Problems Finding an answer doesn’t always end an investigation. Often that answer begins other investigations. In this way, scientists continue to build knowledge. Could parasites or pollutants also be a cause? Could it be a combination of all of them? What effect does UV light have on us?

Scenarios and Tables Problem: Possible Hypothesis: IV: Treatment: # of Trials: DV: Constants:

#1 - Vitamin C in Orange Juice Description: Erica’s chemistry class was studying a unit on acids and bases. They investigated the amount of absorbic acid (vitamin C) in a solution. Erica’s laboratory group decided to experiment with the amount of vitamin C in types of orange juice. The types of orange juice they experimented with were fresh, frozen, and bottled. They collected 100 mL of fresh squeezed juice. They mixed up 100 mL of frozen juice as directed on the package. They purchased 100 mL of orange juice from the vending machine in the cafeteria. Using an established procedure, the students measured the milligrams of absorbic acid in each sample of juice.

Scenario One: The amount of vitamin C in orange juice. Problem: Possible Hypothesis: IV: Treatment: # of Trials: DV: Constants: Which type of orange juice contains more vitamin C? Types of orange juice Fresh Frozen Bottled 1 1 1 Amount of vitamin C 100 ml of orange juice, same procedure Ways that the experiment could have been improved?

#2 - Effectiveness of water sealants Jake’s family had recently built a new deck for a hot tub. He wondered which brand of water sealant would best protect the wood decking from absorbing water splashed from the hot tub. Jake performed a test by cutting small blocks from the decking material. He dipped two blocks in each of one of three major brands of sealant and allowed it to dry for two days. He massed each block. Jake placed the blocks in hot tub water overnight. He massed each block again, finding the difference in the masses (grams). He concluded the difference was the amount of water absorbed.

Scenario Two: The effectiveness of water sealants. Problem: Possible Hypothesis: IV: Treatment: # of Trials: DV: Constants: which brand of water sealant would best protect the wood decking from absorbing water Types of decking sealant Brand 1 Brand 2 Brand 3 2 2 2 Mass of wood blocks Wood type, time to dry, time in water Ways that the experiment could have been improved?

Your turn! Fill out the table for scenario 3 and scenario 4