Ashley Finch, of Balclutha Primary School, asks :-

Who discovered dry ice and how?

Dave Warren, a chemist at the University of Otago, responded.

Dry ice is the solid form of carbon dioxide gas (chemical formula CO2). The first report of dry ice was made in 1834 by a French chemist called Charles Thilorier. He is given credit for being the person who discovered carbon dioxide gas and whilst he was experimenting with his new discovery Thilorier found that carbon dioxide could be turned into a liquid when it was pressurized.

Later, he observed that when he opened the lid on a pressurized metal container of liquid carbon dioxide it quickly evaporated and left behind a very cold block of solid ice that then slowly disappeared as it warmed up. However, unlike the ice that you find in a freezer at home the solid that Thilorier discovered turned straight into a gas and left behind no liquid (which led to the name that it was later given -‘dry ice’).

The change that Thilorier observed is called sublimation by chemists and dry ice is quite unusual because not many common substances sublime; solids usually melt to a liquid state and then evaporate into a gas.

Dry ice sublimes at minus 78.5 degrees Celsius and because of its extreme cold it was first sold in 1925 by the Dry Ice Corporation of America as a way of refrigerating railway trucks on long journeys. For many years dry ice was only used as a refrigerant, for example ice cream parlours around the world used dry ice to keep the ice cream cold until electrical freezers became common in the mid-twentieth century. These days it is rarely used to freeze food but has many other uses, such as in artificial ‘smoke’ machines for stage shows, to create artificial snow, in food preservation, for the freezing of warts and as a cleaning agent to remove grease and paint from metal surfaces.