Which chemical property explains why certain invisible inks become visible when exposed to sunlight?

• A. Electrical conductivity
• B. Thermal conductivity
• C. Photochemical reaction
• D. Density

Answer: (C)

Verified step by step guidance

1. Identify the nature of the problem: It asks for the chemical property that explains why certain invisible inks become visible when exposed to sunlight.
2. Understand that invisible inks that become visible under sunlight do so because of a chemical change triggered by light energy, not by physical properties like conductivity or density.
3. Recall that a photochemical reaction is a chemical reaction initiated by the absorption of light, which can cause a substance to change color or become visible.
4. Compare the options: Electrical conductivity and thermal conductivity relate to the movement of electrons and heat, respectively, and density relates to mass per volume; none of these explain the visibility change caused by sunlight.
5. Conclude that the correct chemical property is a photochemical reaction, as it directly involves light-induced chemical changes that reveal the ink.