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Natural Sciences 2º ESO

Archivo de Noviembre 2015

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Scientific method



How could I use the scientific method to solve a problem?



Let’s say I have a problem: My wallet is missing.

  • Problem: Missing wallet.
  • Data: I went to the market to buy some goods with my friend. When I reached home I noticed that my wallet was gone.
  • Hypothesis:
  • My first hypothesis was: I lost it in the market while we were buying goods.
  • The second hypothesis was: My friend got my wallet.
  • The third was: I had misplaced it in the house.
  • Experiments: I searched all over the house for my wallet, but I could not find it. Then I went to my friend´s house to ask  If he got my wallet. She answered in the affirmative. She said she found it while she was on her way home near the market.
  • Conclusion: I lost my wallet in the market and my friend found it and gave it back to me.



Example of Scientific method: Flemming’s discovery of Penicillin (1928)

(Keen observation of surroundings is the key quality of a scientist)

Flemming was working on different aspects of growth of bacterium called Staphylococcus that cause throat infection. He plated bacterial colonies on a medium in petri-plates for an experiment. Next day, he found out that his bacterial cultures were ruined by a blue green fungus, Penicillium notatum colonies. Instead of throwing away the plates, he made an important observation that bacterial colonies could not grow around this fungal colonies. He wants to know the reason. He asked why?


How Flemming solved his problem using scientific method?


 Step I: observation and defining the problem

Flemming’s observation was bacterial colonies could not grow around fungal colonies. Why not?

Step II: Formulate a hypothesis

Fungal colonies can synthesise certain chemicals that can inhibit bacterial growth (possible answer).

Step III: Testing hypothesis by conducting experiments

Experimental Design: 

  • Two culture tubes A and B with nutrient broth for bacterial growth.
  • To the first tube A(“test” tube), he added fungus and allowed to grow for few days.
  • He kept second tube, tube B as control (remained sterile).
  • After few days, he filtered the broth to remove any fungal cells and allowed bacteria to grow in both tubes A and B.

Step IV: Collect and record data.

In the A tube (test), no bacterial growth were observed.

In the B tube (control), bacterial growth was observed.

Step V: Analyse and interpret data and draw conclusions.

He concluded that in tube A, previously grown fungal cells secreted certain chemicals that inhibited the growth of bacteria. In tube B, bacteria could grow as no fungal colonies were grown before. Thus hypothesis put forward was correct.

Step VI: Report results or theorising:

Later he could isolate antibiotic penicillin from Penicillium notatum coloniesThis discovery was during Second World War. Penicillin was called “wonder drug” as millions of lives were saved from bacterial wound infection due to the discovery of this antibiotic.



Basis of life: Comparisons

Eukaryotic Cell vs Prokaryotic CellEukaryotic Cell vs Prokaryotic Cell

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