What is a hypothesis?
Posing a question

Proposing a hypothesis
A tentative explanation
Why need a hypothesis?
If the hypothesis is correct,
what predictions can be made?

Design experiments or
plan observations.
If the hypothesis is correct,
we should observe xxx when
a certain test is conducted
[predictions]

This guides us to plan
observations or design
experiments
What hypothesis should be put forward
for the following investigations?
降落傘的大小如何影響其下降速度？
降落傘的形狀如何影響其下降速度？
• A hypothesis is NOT needed in such
cases!
• An ‘expected’ result from the
investigator will affect the objectivity
of observation
水的溫度與受熱時間的關係？
• A hypothesis is NOT needed in such
cases!
• An ‘expected’ result from the
investigator will affect the objectivity
of observation
水的溫度與受熱時間的關係？
用玻璃瓶蓋著燃燒
的蠟燭，你會觀察
到什麼？
為什麼火焰會熄滅？
A question that asks “why”
usually requires a hypothesis.
A question that asks “what”
usually requires making objective
observations. 
No hypothesis is needed.
Do we need to suggest a hypothesis
for the following questions?
What is the W.P. of potato cells?
Why does a potato become more
soft after storing for a long time?
Do we need to suggest a hypothesis
for the following questions?
• What is the optimum temperature
for salivary amylase?
• Why is salivary amylase still very
active at 70oC?
Do we need to suggest a hypothesis
for the following questions?
• What is the effect of light intensity
on the rate of photosynthesis?
• Why does the rate of photosynthesis
becomes constant with increase in
light intensity when the light intensity
is high?
What food constituents are present in
peanut, banana, grape, potato and biscuit?
Hypothesis:
I think that peanut, banana, grape,
potato and biscuit all have proteins
and lipids.
What food constituents are present in
peanut, banana, grape, potato and biscuit?
Hypothesis:
I think that peanut, banana, grape,
potato and biscuit all have proteins
and lipids.
A hypothesis Not needed!!
What is the respiration rate of mealworms?
Hypothesis:
I think that the respiration rate
(oxygen uptake) is constant.
What is the respiration rate of mealworms?
Hypothesis:
I think that the respiration rate
(oxygen uptake) is constant.
A hypothesis Not needed!!
What is the transpiration rate of a
plant outside and inside the laboratory?
Hypothesis:
The plant outside the lab should have
a higher transpiration rate than the
plant inside the lab.
What is the transpiration rate of a
plant outside and inside the laboratory?
Hypothesis:
The plant outside the lab should have
a higher transpiration rate than the
plant inside the lab.
A hypothesis is NOT needed
Why is the transpiration rate of a plant
differ outside and inside the laboratory?
Hypothesis:
Why is the transpiration rate of a plant
differ outside and inside the laboratory?
Hypothesis:
The light intensity is higher outside
the lab.
•The CO2 conc is higher outside the
lab.
•The UV light intensity is higher
outside the lab……….
In the Coleus leaf, more stomata are present
on the lower epidermis than on the upper
epidermis.
What is the significance of this uneven
distribution of stomata?
Hypothesis:
In the Coleus leaf, more stomata are present
on the lower epidermis than on the upper
epidermis.
What is the significance of this uneven
distribution of stomata?
Hypothesis:
• To reduce the rate of water loss due to
transpiration when sunlight shines directly
from above.
• To allow a faster rate of photosynthesis as
more carbon dioxide can be taken in.
If a potted plant is placed outside the
laboratory for two days without any water
supply, it becomes wilted. A similar plant
inside the laboratory will not show wilting.
How would you explain these observations?
Hypothesis:
If a potted plant is placed outside the
laboratory for two days without any water
supply, it becomes wilted. A similar plant
inside the laboratory will not show wilting.
How would you explain these observations?
Hypothesis:
• The plant outside the laboratory becomes
wilted because it has a higher transpiration
rate due to the windy conditions outside.
• The plant outside the laboratory becomes
wilted because it is hotter outside.
Potatoes stored for a long time
becomes soft to the touch.
Why does this occur?
Hypothesis:
Potatoes stored for a long time
becomes soft to the touch.
Why does this occur?
Hypothesis:
• The longer the time of storage, the
more water is lost through transpiration.
• The cell membrane is damaged during
storage. So the cells lose their turgidity.
Posing a question (Why …?)

Proposing a hypothesis
A tentative explanation
A tentative idea that generates
predictions – for testing the idea.
A tentative idea that generates
predictions – for testing the idea.
Examples
• Light intensity affects photosynthesis
• Water is necessary for seed
germination
Posing an ‘testable’ question

Is it necessary to put
forward a hypothesis ?

Designing experiment/observation
Designing experiment/observation
Biological principle

Designing method of investigation

What is the independent variable?
How to change it?

What is the dependent variable?
How to measure it?
What are the assumptions made?
Designing experiment/observation
What are the controlled variables?
Any assumptions made?
Is it necessary to set up a control
experiment?
If yes, how to do it?
What are the assumptions that you need to make in
order to draw a valid conclusion from your results?
To measure transpiration rate using a bubble potometer:
rate of water absorption =
To find the W.P. using the % change in mass method:
change in mass of potato cylinder 
To find the growth rate of a plant by measuring the
change in dry mass:
To compare the sugar content of green, red and dark
grapes using Benedict’s test:
What are the assumptions that you need to make in
order to draw a valid conclusion from your results?
To measure transpiration rate using a bubble potometer:
rate of water absorption = rate of transpiration
To find the W.P. using the % change in mass method:
change in mass of potato cylinder  amount of water
uptake/loss by osmosis
To find the growth rate of a plant by measuring the
change in dry mass:
change in dry mass  amount of cytoplasm formed
To compare the sugar content of green, red and dark
grapes using Benedict’s test:
amount of reducing sugar  total amount of sugar
Assumptions made in the controlled variables?
Effect of light intensity on photosynthesis:
rate of respiration remains constant in different light
intensity; gas bubbles are identical in size
To compare the vitamin C content of different food
samples using DCPIP:
To compare the rate of anaerobic respiration of yeast
using different substrates:
Assumptions made in the controlled variables?
Effect of light intensity on photosynthesis:
rate of respiration remains constant in different light
intensity; gas bubbles are identical in size
To compare the vitamin C content of different food
samples using DCPIP:
Each drop of solution has the same volume; the end
points are the same
To compare the rate of anaerobic respiration of yeast
using different substrates:
air temperature & pressure unchanged; the yeast
samples are identical