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Thus if 333,7 kilojoules must be added to a kilogram of ice to melt it , 333,7 kilojoules must be removed to freeze the one kilogram of water . Liquid to vapour and vapour to liquid .
Changing a liquid to a vapour ( like boiling water ) is called evaporation .
Changing a vapour to a liquid like the steam coming out of the kettle changing back to water on the wall ) is called condensation .
Both the heat to be added to evaporate one kilogram of liquid and the heat to be removed to condense a kilogram of vapour are called heat of vaporisation .
Some solids ( e . g . dry ice ) change directly from a solid to a vapour . This process is called sublimation and the latent heat required is called “ heat of sublimation ”.
Thank you for the question Stefan , I will go into this in more detail in the next issue .
REFERENCES : 1 . ACRA 2 . SANS 10147
Thank you for all your questions . Send your problems ( and sometimes your creative solutions ) to acra @ netactive . co . za with ‘ Solutions Page ’ in the subject line . You may include pictures . RACA
Some typical values of the aforementioned are : Latent process
Melting ice to water or freezing water Vaporizing or condensing of water Sublimation of dry ice
Heat to be added or removed 333,7 kJ / kg 2 257 kJ / kg @ 100 ° C 577,0 kJ / kg
( Dry ice sublimates at -78 ° C and in addition to the 577 kJ , a further 63 kJ / kg is required to increase the temperature to 0 ° C . Thus the total useful heat removed is 640 kJ / kg .)
" Heat , when added to or removed from a substance can either change the temperature of that substance or change its phase ."
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RACA Journal I August 2022 www . refrigerationandaircon . co . za