Ordinarily, water has many nuclei in the form of microscopic impurities. But if the water is purified, as it is in most bottled waters, it will not contain the nuclei needed to begin the crystallization process.
Thus, purified water will remain a liquid even well below its freezing point. Place your purified water in the freezer and set a timer for ninety minutes. For the best chance of success, we recommend using several water bottles at once and laying them on their sides. After your timer goes off, check the water bottles for ice crystals — these crystals may be small and easy to miss. If no ice crystals appear, check the bottles again every fifteen minutes.
Depending on your freezer and the size and shape of your water bottles, this may take as long as three hours. Once ice crystals appear in one of your bottles, check the others. If they all have ice crystals, write down the time — when you repeat the experiment, remove the bottles fifteen minutes sooner. Once you have carefully removed the bottle s from the freezer, simply give it a quick jolt and watch the water freeze right before your eyes!
When water becomes a solid, it releases heat, warming up its surroundings. This makes freezing an exothermic reaction. Textile Research Journal , 87 19 , Freezing-enhanced reduction of chromate by nitrite. Accelerated redox reaction between chromate and phenolic pollutants during freezing. The Chemical Record , 17 4 , Kunov-Kruse , Peter L. Chemistry - A European Journal , 22 33 , Ray , H.
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Bansal , K. Kinetics of atmospheric oxidation of nitrous acid by oxygen in aqueous medium. For purposes of this discussion, processes that require or give off heat will be limited to changes of state, known as phase changes, and changes in chemical constitution, or chemical reactions.
Changes of state involve a solid melting, a liquid freezing, a liquid boiling or a gas condensing. When steam, which is gaseous water, condenses, heat is released. Likewise when liquid water freezes, heat is given off.
In fact heat must be continually removed from the freezing water or the freezing process will stop. Our experience makes it easy for us to realize that to boil water or any liquid and thereby convert into a gas, heat is required and the process is endothermic.
It is less intuitive to grasp that when a gas condenses to a liquid, heat is given off and the process is exothermic. Perhaps it is easier to explain an exothermic phase change using the following argument. Liquid water had to have energy put into it to become steam, and that energy is not lost. Instead, it is retained by the gaseous water molecules.
When these molecules condense to form liquid water again, the energy put into the system must be released. And this stored energy is let out as exothermic heat.
The same argument can be made for the process of freezing: energy is put into a liquid during melting, so freezing the liquid into a solid again returns that energy to the surroundings. Like phase changes, chemical reactions can occur with the application or release of heat. Those that require heat to occur are described as endothermic, and those that release heat as exothermic.
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