Oxidation
Oxidation is a chemical reaction where a substance combines with oxygen, like when lemon juice on paper turns brown from heat.
Cut an apple and leave it on the counter. The white flesh slowly turns brown as oxygen in the air reacts with the apple's surface. This is oxidation. The same thing happens when iron rusts or butter goes rancid — oxygen bonds with the material and changes it chemically.
Explaining oxidation by grade level
You can write a secret message on paper using lemon juice. The writing is invisible at first. When a grown-up holds the paper near something warm, the message turns brown. That happens because the juice reacts with the air and changes color. The heat speeds up this change so you can see it.
Projects that explore oxidation
Some metals rust while others stay shiny for years. When a substance combines with oxygen, that chemical reaction is called oxidation — and metals vary widely in how fast it happens. You immerse small squares of copper, aluminum, iron, and zinc in tap water, salt water, and lime juice, then check each surface after two days. Some pieces corrode; others do not.
Lemon juice dries clear and invisible on paper. When you apply heat with a hair dryer, the heat causes the lemon juice to oxidize — combining with oxygen and darkening. The once invisible message appears as brown text on white paper.