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Name: debbie
Status: educator
Grade: K-3
Location: TX
Country: N/A
Date: 1/24/2006

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 up.

Greg (Roberto Gregorius)

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