A performer drops a lump of dry ice into a bowl of warm water and thick white fog billows out immediately. No puddle of leftover liquid ever forms from the dry ice itself before it disappears. Dry ice is frozen carbon dioxide. What state change is the dry ice actually going through?
- It sublimates, turning directly from a solid into a gas without ever becoming a liquid in between
- It melts into liquid carbon dioxide, which then evaporates too quickly for the eye to notice
- It reacts chemically with the water, producing an entirely new gas as a by-product
- It freezes the surrounding water solid, and it's actually that new ice disappearing, not the dry ice
Why A? And why not the others?
Correct answer: A. It sublimates, turning directly from a solid into a gas without ever becoming a liquid in between
Solid carbon dioxide sublimes at around -78.5 degrees Celsius under normal atmospheric pressure, meaning it passes directly from a solid to a gas without ever forming a liquid in between - the warm water simply speeds up this change and the visible 'fog' is water vapour condensing in the cold carbon dioxide gas, not the carbon dioxide itself changing colour. The option describing melting into a liquid is wrong because carbon dioxide only exists as a liquid at pressures far higher than normal atmospheric pressure, which is exactly why no puddle of liquid CO2 is ever seen forming during this demonstration. The option describing a chemical reaction is wrong because no new substance is created; the carbon dioxide molecules stay chemically the same throughout, only their physical state changes. The option describing freezing the surrounding water is wrong because the visible fog and disappearing solid are explained entirely by the dry ice itself changing state, and any ice that might form on the outside of the bowl is a separate, much smaller effect that doesn't account for the dry ice vanishing.
Source: BBC Bitesize GCSE Combined Science: Particle model -- states of matter and sublimation