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Little scientists will learn about the nature of liquids and the interactions of liquids with other materials. They will learn that some materials are impervious to liquids, some are absorbent, and others are semi-permeable, like an eggshell.
In the vinegar/highlighter solution, the egg expands slightly inside its shell because its membrane is semi-permeable. Semi-permeable means it allows something to pass through it, through a process called osmosis.
Osmosis is the movement of a liquid, like water, across a membrane. Membranes like to be balanced on both sides.
The vinegar solution is mostly water with a little vinegar and ink in it, while the membrane has egg white and yolk inside (protein with a little water). The glowing water molecules travel from the vinegar into the egg to try to balance the concentrations. The egg expands and glows! The expanded egg also becomes firmer and can bounce.
When the egg starts to absorb the vinegar solution, the acetic acid in the vinegar breaks down the egg’s calcium carbonate shell, producing tiny carbon dioxide gas bubbles.