WATER CASES FROM BLENDED SESSION
Take two containers of water; fill one with hot water and the other one with cold water, and put them in the freezer. The hot one would be frozen before the cold one. Why?
Water molecules chemically consist of two hydrogen molecules attached to an oxygen molecule through strong covalent bonds. Usually, the covalent bonds will soften and lengthen as they are heated. But in water, the opposite effect happens. This is because the hydrogen bonds unique properties which is the interaction between the hydrogen atoms in one water molecule and the oxygen molecule in the next water molecule. As the water molecules absorbs energy, the hydrogen bonds will stretch and causing individual water molecules to move apart from each other. So, the covalent bonds within each molecule become shorter and harder. This is what happens when water freezes.
More importantly, the rate at which the energy released in these shrunken covalent bonds is dependent on how much energy was initially stored. Effectively, hot water has energy wound up like a spring which gets released when you begin to cool it, allowing it to cool and freeze faster.
Since then, a number of hypotheses have been proposed as explanations which are :-
1. Convective heat transfer. When a liquid is heated, it can form convection currents that rapidly bring the hot liquid to the surface, where the heat is lost by evaporation. Convection will keep the top of the liquid hotter than the bottom, even when the temperature matches an initially cold liquid that doesn’t possess this convection cooling. This results in a faster rate of cooling that could, under the right circumstances.
2. Evaporation. A boiling or very hot liquid will lose some of its mass due to evaporation. With a lower mass, it will cool faster, possibly giving a “boost”. The evaporation alone could not account for all of the rate of cooling of the hot liquid. Hot water evaporates. Less water left behind means less water to freeze.
3. Degassing. the effect depended strongly on the amount of gas dissolved in the water. When the water was purged of air and carbon dioxide, the time to freeze became proportional to the starting temperature. The presence of gas was substantially slowing the rate of cooling, and that the heated water was purged of it.
4. Supercooling. Supercooling also known as undercooling is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid. Liquids begin to crystallize into a solid at the freezing point with the help of impurities around which the crystals can nucleate. In the absence of such impurities, the liquid can be cooled below its normal freezing point while remaining a liquid — it is “supercooled”. Auerbach suggests that the cold water will supercool to a lower temperature than the hot water, thus giving the hot water an “edge”.
5. Distribution of solutes. The solutes present in the cold water slow the freezing process, but also that those solutes get driven from the freezing water into the as-yet unfrozen water, slowing the process further.
More importantly, the rate at which the energy released in these shrunken covalent bonds is dependent on how much energy was initially stored. Effectively, hot water has energy wound up like a spring which gets released when you begin to cool it, allowing it to cool and freeze faster.
Since then, a number of hypotheses have been proposed as explanations which are :-
1. Convective heat transfer. When a liquid is heated, it can form convection currents that rapidly bring the hot liquid to the surface, where the heat is lost by evaporation. Convection will keep the top of the liquid hotter than the bottom, even when the temperature matches an initially cold liquid that doesn’t possess this convection cooling. This results in a faster rate of cooling that could, under the right circumstances.
2. Evaporation. A boiling or very hot liquid will lose some of its mass due to evaporation. With a lower mass, it will cool faster, possibly giving a “boost”. The evaporation alone could not account for all of the rate of cooling of the hot liquid. Hot water evaporates. Less water left behind means less water to freeze.
3. Degassing. the effect depended strongly on the amount of gas dissolved in the water. When the water was purged of air and carbon dioxide, the time to freeze became proportional to the starting temperature. The presence of gas was substantially slowing the rate of cooling, and that the heated water was purged of it.
4. Supercooling. Supercooling also known as undercooling is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid. Liquids begin to crystallize into a solid at the freezing point with the help of impurities around which the crystals can nucleate. In the absence of such impurities, the liquid can be cooled below its normal freezing point while remaining a liquid — it is “supercooled”. Auerbach suggests that the cold water will supercool to a lower temperature than the hot water, thus giving the hot water an “edge”.
5. Distribution of solutes. The solutes present in the cold water slow the freezing process, but also that those solutes get driven from the freezing water into the as-yet unfrozen water, slowing the process further.