Freezing water, a simple task?
It is known that the melting and freezing point of water is 0 °C (273.15 K) [1,2]. For that reason, when we started the experiments in the “Activity Test” group, we expected ice to form in plastic and/or glass tubes below 0 °C. However, after numerous attempts, we were surprised that the water remained liquid. In one of the most recent experiments, new glass tubes were used, placing 1 mL of distilled water in three tubes and 5 mL in three others. After being in the recirculating bath for more than 1 hour at -5 °C and not perceiving any physical change, a thermometer was introduced to verify the temperature of the liquid inside the tube. When we removed it, we could see the formation of small ice crystals. Within minutes, all the water had frozen.
How are these results explained? To begin with, the phenomenon we expected to occur in the tests performed is homogeneous nucleation; that is, the aggregation of water molecules without the help of another substance . However, it has high activation energy and occurs spontaneously at temperatures close to -40 ° C [3,4]. In the range from 0 to -38 or -40 °C, liquid water is called supercooled water and is in a metastable state [1,4]. To accelerate and promote nucleation, nucleating agents are added, in which the initial aggregation of the water molecules is carried out [1,3]. In the presence of this class of catalysts, the process is called heterogeneous nucleation . For this reason, in the next experiments tests with different nucleating agents will be carried out, so that they simulate the action of the ice-nucleating active (INA) bacteria present in plants.
1. Lundheim R. Physiological and ecological significance of biological ice nucleators. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences. 2002 Jul 29;357(1423):937-943.
2. Castellan GW. Fisicoquímica. 2a ed. México: Addison Wesley Longman; 1998. p. 305-306.
3. Montes JM, Cuevas FG, Cintas J. Ciencia e Ingeniería de los Materiales. Madrid: Ediciones Paraninfo; 2014. p. 183.
4. Hirano SS, Upper CD.  Bacterial nucleation of ice in plant leaves. In Methods in enzymology 1986 Jan 1 (Vol. 127, pp. 730-738). Academic Press.