Beth Butel, of Ilam School, asks :-

Is there anything colder than liquid air?

John Campbell, a physicist at the University of Canterbury, responded.

Yes, quite a few things are colder than liquid air. Solid air is one.

Beth, we need to set up a scale to compare the temperatures of different objects. The one we use every day is the Celsius scale which is named after Anders Celsius of Sweden. On this scale water freezes to ice at 0 degrees Celsius and water boils to steam at 100 degrees Celsius.

The coldest temperature in a house is in the freezer and is about 18 degrees Celsius below zero. This is usually written as -18C. On this scale when we cool the nitrogen gas in air it finally becomes a liquid when cooled down to -196C.

The lowest temperature possible is minus 273 degrees Celsius. To avoid using minus numbers scientist often use another scale named after Lord Kelvin who was a famous British physicist. On this scale the absolute lowest temperature is set at zero degrees Kelvin, and the temperaure at which water solidifies into ice is fixed at 273 degrees Kelvin. This is usually written as 273K. On this scale nitrogen liquifies at 77K and the liquid becomes solid at 63K.

Only 3 light gases remain as gases below that temperature. Neon, a gas used in red neon signs and lasers, liquifies at 27K, hydrogen liquifies at 20K and the lowest temperature for a gas is helium which liquifies at just over 4K.

Scientists can use tricks to get to even lower temperatures. For example, if we pump the gas away from above liquid helium it cools to below 2K. If a man-made form of helium atoms are used (called He$^3$ isotope) the temperature can be reduced to 0.3K. I do this regularly to cool a very sensitive heat detector and so the lowest temperature produced in New Zealand is in my laboratory.