M2.2.4 II - Hypothesises

Question 1

Distinguish hypothesis, theory and model

Answer 1

Hypothesis: enables predictions, to be tested by investigations
* necessary to develop a theory

Theory: a hypothesis that has been confirmed through repeated testing

Model: idealised depictions to initially understand complex situations.
* Representative of a hypothesis.

Question 2

Jan Baptista Van Helmont

Investigation, results, validity

Answer 2

17th century scientist
Prevailing theory: plants grew by eating soil

Investigation: weighed a willow tree and its soil, then let it grow for 5 years.

Results: the soil remained the same weight

Conclusion/Findings: the tree grew by drinking water, all plant matter came from water

Evaluation: incorrect conclusion - initial testing that soil provided matter for the plant, but then concluded that water provided matter. Should have instead concluded that very little plant matter came from soil.
Validity: mass of leaves not included, no control
Reliability: unable to determine because method was only conducted once.

Question 3

Joseph Priestley

Investigation, results, validity

Answer 3

18th century (1771)
Investigation: placed mint in a transparent closed space with a candle that burned out the air, and went out.
* After a 27 days, relit the candle (using sunbeam and mirror) and it burned well.
* Repeated with mice and a candle - with plant survived, the other died.

Results: jar with plant survived.

Findings/Conclusions: hypothesised that plants restore the air what breathing animals and burning candles removed. Later concluded by Lavoisier (1778) to be oxygen.

Evaluation: The candle and mouse “injured” the air, and the plants were responsible for replacing these factors.

Assess: well designed, contributed to the eventual understanding of photosynthesis.

Question 4

Jan Ingenhousz

Answer 4

Investigation: placed water plants in a transparent container. One container was in light, the other in darkness. A test tube was set up to collect gas bubbles. Bubbles were collected from the light container, but not from the dark container. This shows that plants need sunlight to photosynthesise. Green parts of plants were where the gas collected.

Validity: good, he controlled variables with light and dark exposure.

Reliability: good, multiple repeats of the method yielded similar results.

Accuracy: good, his results added a new dimension of depth to Priestley’s findings (that plants need sunlight to produce oxygen, which is what “restores” the air)

Cons:
- Didn’t account for the light-independent reaction. Assumed photosynthesis was only light dependent reaction