calcium carbonate

Calcium carbonate is the white, insoluble solid that forms when carbon dioxide reacts with calcium hydroxide (lime water) — its tiny particles are what make the once-clear solution turn milky.

• This cloudiness is actually a simple, reliable test for carbon dioxide: if a gas turns lime water milky, carbon dioxide is present.
• The reaction happening is: calcium hydroxide + carbon dioxide → calcium carbonate + water — a compound forming fresh from a gas dissolving into a solution.
• Calcium carbonate isn't just a lab curiosity — it's the main compound in chalk, limestone, and marble, and even in eggshells and seashells.
• Because it doesn't dissolve in water the way calcium hydroxide does, it separates out as tiny suspended particles, which is exactly what scatters light and makes the milky appearance.