To test a leaf for starch, the leaf is boiled in water to (i) __________. It is then boiled in methylated spirit to (ii) __________. The leaf is dipped in warm water to soften it. It is placed in a petri dish, and (iii) __________ solution is added. The region of the leaf which contains starch, turns (iv) __________ and the region which does not contain starch, turns (v) __________.

(i) KILL THE CELLS (and stop all chemical activity) (ii) REMOVE THE CHLOROPHYLL / decolourise the leaf (iii) IODINE (potassium iodide, KI/I₂) (iv) BLUE-BLACK (blackish blue / dark blue) (v) YELLOWISH BROWN (reddish brown / yellow / golden brown)

One mark per blank. Learn the test as a SEQUENCE WITH REASONS, because any step may be asked on its own:

1. BOIL IN WATER — to KILL THE CELLS, stopping all further chemical change, and to soften the leaf and break the cell walls so that alcohol and iodine can penetrate.
2. BOIL IN METHYLATED SPIRIT — to DISSOLVE OUT THE CHLOROPHYLL. A green leaf would hide the colour change entirely; decolourising it makes the result visible.
3. DIP IN WARM WATER — to SOFTEN the leaf again, since alcohol makes it brittle.
4. ADD IODINE — BLUE-BLACK where starch is present, YELLOWISH-BROWN (the colour of iodine itself, unchanged) where it is absent.

ONE SAFETY POINT examiners like: alcohol is INFLAMMABLE, so the leaf is heated in a WATER BATH and never directly over a flame.

Note that blank (v) is not "no colour" but the colour of the IODINE itself — the solution simply stays as it was.