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Water's triple point is the one combination of temperature and pressure at which ice, liquid water and water vapour can all exist together in balance: 0.01 °C at a pressure of about 612 pascals, roughly 0.6% of normal air pressure at sea level.

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Most substances have their own triple point, and it shows that freezing and boiling depend on pressure as well as temperature. Below the pressure of water's triple point, ice cannot melt; it turns straight into vapour, which is how freeze-drying removes water from foods and medicines. Because the triple point is so exact and easy to recreate in a sealed glass cell of pure water, scientists used it to define temperature itself. From 1954 the kelvin, the scientific unit of temperature, was fixed so that water's triple point was exactly 273.16 kelvin, which put 0 °C at 273.15 kelvin. In 2019 the kelvin was redefined using a constant of nature, the Boltzmann constant, which links temperature to the energy of moving particles. The triple point is now a measured value, though still almost exactly 0.01 °C, and laboratories still use triple-point cells to check thermometers.

Türkçe oku
Suyun üçlü noktasında buz, sıvı su ve su buharı dengede, bir arada durabilir. Bu yalnızca tek bir sıcaklık ve basınçta olur: 0,01 °C'de ve yaklaşık 612 paskalda, yani deniz seviyesindeki hava basıncının yüzde 0,6'sı kadar bir basınçta.

Maddelerin çoğunun kendine özgü bir üçlü noktası vardır; bu nokta, donmanın ve kaynamanın yalnızca sıcaklığa değil basınca da bağlı olduğunu gösterir. Basınç suyun üçlü noktasındakinin altındaysa buz erimez, doğrudan buhara dönüşür; gıdalardan ve ilaçlardan suyu uzaklaştıran dondurarak kurutma yöntemi buna dayanır. Üçlü nokta çok kesin olduğu ve saf su dolu kapalı bir cam hücrede kolayca yeniden elde edilebildiği için bilim insanları sıcaklığı onunla tanımladı. 1954'ten başlayarak bilimsel sıcaklık birimi kelvin, suyun üçlü noktası tam 273,16 kelvin olacak biçimde belirlendi; böylece 0 °C de 273,15 kelvine denk geldi. 2019'da kelvin, sıcaklığı hareket eden parçacıkların enerjisine bağlayan bir doğa sabitiyle, Boltzmann sabitiyle yeniden tanımlandı. Üçlü nokta artık ölçülen bir değerdir ama hâlâ neredeyse tam 0,01 °C'dir; laboratuvarlar termometreleri denetlemek için bu cam hücreleri kullanmayı sürdürüyor.

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Background reading: Triple point
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