Photosynthesis

Science — Second Preparatory

Experiments and worked examples

Comparison aspect	Main idea	Example	Keywords
Experiment 1: detecting starch in a leaf.	Keep a plant in the dark for 48 hours to use up the starch in its leaves. Then expose one leaf to light after covering part of it with black paper. Boil the leaf in alcohol to remove the chlorophyll then add iodine solution. A blue-black colour appears only in the part that was exposed to light. This shows that photosynthesis happens only where light reaches the leaf	—	Keepplantdarkhours
Experiment 2: detecting oxygen.	Place a water plant	Elodea in water containing dissolved carbon dioxide and expose it to light. Gas bubbles appear from the cut stem. A glowing splint brought to the mouth of the tube bursts into flame which shows that the gas is oxygen	PlacewaterplantElodea

Key points

Example 1What mass of glucose is formed from 88 g of carbon dioxide?

Solutionmolar mass of CO₂ = 44 g, so the number of moles = 88 ÷ 44 = 2 mol.

From the equation6 mol of CO₂ give 1 mol of glucose, so 2 mol of CO₂ give 1/3 mol of glucose.

Mass of glucose = (1/3) × 180 = 60 g

Example 2What mass of oxygen is formed in the previous example?

Solutionfrom the equation, 6 mol of CO₂ give 6 mol of O₂, so 2 mol of CO₂ give 2 mol of O₂. Mass of oxygen = 2 × 32 = 64 g

Checktotal mass of reactants = 6 × 44 + 6 × 18 = 372 g, and total mass of products = 180 + 6 × 32 = 372 g.

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