Biology

Mabel Goh (7)Joanna Han (8)Lim Zhi Jun (13)

Low Hui Ting (14)

Done by:

AimTo compare the rates of

transpiration of the upper and lower surfaces of a leaf.

ApproachWe plan to measure the amount of water vapour on the We plan to measure the amount of water vapour on the

toptopand base of the leaf using a cobalt chloride paper of sameand base of the leaf using a cobalt chloride paper of samesizes.sizes.

We would leave the cobalt papers on the leaf covered byWe would leave the cobalt papers on the leaf covered byglass slides for 15 minutes.glass slides for 15 minutes.

Our constant variables for the experiment would be theOur constant variables for the experiment would be thetime, sunlight, wind velocity, humidity of the surroundingtime, sunlight, wind velocity, humidity of the surroundingenvironment.environment.

Key Variable

sControlled variables: • time• sunlight• wind velocity• humidity of the surrounding environment

Dependent variables:• colour of the cobalt chloride paper

Apparatus

• Healthy green leaf from a living plant• 2 pieces of cobalt chloride papers of the same

sizes• 2 glass slides• 2 rubber bands

This experiment should be carried out on a warm, dry day. If the atmosphere is damp, e.g. early in morning, the experiment may not be carried out successfully since the cobalt chloride papers will turn pink the moment they come into contact with damp air. In this case, cobalt thiocyanate can be used.

Use a plant with stomata mainly on the lower surface of the leaves. Cobalt chloride paper is blue when dry and turns pink when it comes into contact with water vapour.

Descriptions

1. Cobalt chloride paper at the bottom of the leaf will turn pink.

2. In the same amount of time, the cobalt chloride paper at the top of the leaf will turn purple.

3. This is so because there is more stomata at the bottom of the leaf. More water vapour escapes from the bottom of the leaf.

4. Hence, the cobalt chloride paper at the bottom of the leaf turns from blue to purple to pink whereas the cobalt chloride paper at the top of the leaf turns from blue to purple only, within the same amount of time.

Observations

CoCl2. 2H2O + 4H2O CoCl2 . 6H2O

CoCl2 + 2H2O CoCl2 . 2H2O(Blue to purple)

(Purple to pink)

CoCl2 + 2H2O CoCl2 . 2H2O(Blue to purple)

-The End-