Experimental Design using Scientific Method:

When scientists begin an experiment they use the steps of the Scientific Scientific MethodMethod. You will be asked to understand many of these steps on the STAAR Test next year.

Scientific Method Scientific Method – – a logical step by a logical step by

step way of solving step way of solving problems problems

Problem: Will

Problem: Will

the amount of

the amount of

water given water given

effect plant effect plant

growth?growth?

The ProblemProblem (in the form of a question) that the scientists will try to answer, must be logical and based on data or observations.

Problem Problem - what you want to - what you want to learn or understandlearn or understand

Hypothesis: Hypothesis:

Less than Less than

500ml per 500ml per

week will week will

slow growth slow growth

of ivy plants.of ivy plants.

A prediction called a Hypothesis Hypothesis is next. It must be a testabletestable statement that you could gather data to prove. (You can’t measure which is the “prettiest”, “strongest”, or “best”.)

Hypothesis Hypothesis – – an educated guess to explain an educated guess to explain

what you observe or what might what you observe or what might happenhappen

Testable –Testable –can be proven true or false with can be proven true or false with

datadata

You must controlcontrol an experiment. You can test only one variablevariable at a time. (How does the amount of water effect plant growth? Keep the light, containers, soil and plants the same.)

Controls: Light, type of plant, amount of soil, pot, temperature.

Control Control ––what you do not what you do not

change in an change in an experimentexperiment

Variable Variable

– – what changes in what changes in an experiment an experiment

A Control GroupControl Group is the set-up that isn’t changed. (One plant gets the normal amount of water!). Which group above is the control group?

500ml 400ml 300ml

The independent variableindependent variable is the thing you change (amount of water). The dependent variabledependent variable is the result of the change you made (height of plant in cm.).

Independent Independent variable: Amount variable: Amount of water given in of water given in ml. ml. Dependent Dependent variable: Plant variable: Plant growth in cm.growth in cm.

Independent variableIndependent variable – – what I change in an what I change in an

experiment on purposeexperiment on purpose

Dependent variableDependent variable – – what changes because what changes because

of changing the of changing the independent variableindependent variable

When reporting the results, the Independent Independent VariableVariable is graphed on the X-axis. The Dependent variableDependent variable is on the Y-axis.

DRY MIX = DependentDependent Responding Variable on Y-axisManipulated IndependentIndependent Variable on X-axis

Amount of Water

Gro

wth

in

mm

The ConclusionConclusion must be supported by the data & results you gather. To be validvalid, each experiment needs a large sample sizelarge sample size(grow 2000 plants in each experiment) and must be repeated many timesrepeated many times in order to prove the Hypothesis (grow 2000 plants 100 times). If you are collecting samples, in an experiment, do it randomlyrandomly. Don’t take them all from one place. Don’t take them all from one place.

Yes! No!

Conclusion Conclusion – what you have – what you have

learned (hypo true?)learned (hypo true?)

valid valid - true, trusted, reliable- true, trusted, reliable

Check for understanding

Check for understanding by answeringthe following questions.

Remember that you are using the scientific method (scientific reasoning and investigating skills).

Which of these is a hypothesis that can beWhich of these is a hypothesis that can betested through experimentation?tested through experimentation?

F Bacterial growth increases as temperature increases.

G A fish’s ability to taste food is affected by The pH of aquarium water.

H Tadpoles’ fear of large insects increases as the tadpoles age.

J The number of times a dog wags its tail indicates how happy the dog is.

The correct answer is FThe correct answer is F. Bacterial growth can be measured. Dog happiness, fish taste, and tadpole fear cannot be measured scientifically.

The table shows times required for water to evaporate from identical containers. Which of these is the bestbest question to ask before developing a reasonablereasonable hypothesis to explain the data?

A Does a lower temperature slow the rate of evaporation?

B What is the boiling point of the water after both samples are heated?

C Why does water exist as a solid at -15°C and as a liquid at 25°C?

D How does the rate of evaporation change when a different container is used?

The correct answer is A.The correct answer is A. The table shows data taken to investigate temperature and the evaporation of water.