Heating Lemon Juice
Why is lemon juice clear on paper, but when heated turns brown?
Lemon juice contains compounds that are easily oxidized (reacts with air).
Heating the lemon juice, especially on a substrate such as paper where the
much of the juice is exposed to the air, will speed up the oxidation
(heating causes most reactions to speed up). The product of the oxidation is
darker in color (brown) then the starting compounds. Hence the "invisible
ink effect". If you were to let a piece of paper impregnated with lemon
juice just sit out it will eventually turn brown -heating just speeds things
Greg (Roberto Gregorius)
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Update: June 2012