In order to prove that carbon dioxide is necessary for (a) __________, a potted plant is placed in dark for 48 hours to (b) __________ the leaves. A part of a leaf is inserted into a conical flask containing potassium hydroxide solution. This is to absorb (c) __________ from the flask. The plant is then placed in sunlight for a few hours. The experimental leaf is tested for starch. The part of the leaf that was inside the conical flask turns (d) __________, whereas the part of the leaf outside turns (e) __________ in colour.

(a) photosynthesis (b) destarch (c) carbon dioxide (d) brown (e) blue-black

One mark per blank. Read the experiment as a CONTROLLED test and every blank follows.

(b) DESTARCH — 48 hours in the dark uses up the starch already stored, so that any starch found at the end must have been made DURING the experiment. Without this step the result would prove nothing.

(c) CARBON DIOXIDE — potassium hydroxide is a strong alkali that absorbs CO₂, so the part of the leaf inside the flask is deprived of it while everything else stays the same.

(d) BROWN and (e) BLUE-BLACK — iodine turns blue-black only where starch is present. Inside the flask there was no carbon dioxide, so no photosynthesis, so no starch: the leaf stays BROWN (the colour of iodine itself). Outside the flask CO₂ was available, so starch formed and that part turns BLUE-BLACK.

THE LOGIC OF THE WHOLE EXPERIMENT: the two parts of the SAME leaf differ in one factor only — carbon dioxide. Since only the part with CO₂ made starch, carbon dioxide must be necessary. Using one leaf for both halves is what makes it a fair test.