Q:

Agar gel vs aqueous solution

Can someone tell me why - in experiments to observe corrosion of iron nails - we often use a preparation of agar gel (with $\ce{NaCl}$, phenolphthalein, etc.) instead of an aqueous solution?

A:

If you are trying to observe corrosion in a specific area, agar is a convenient way to prevent convection currents from obscuring what caused the corrosion and for localizing the indicator.

