contrary to what seems to be common sense, heat does drive off gases dissolved in water. hot water will dissolve solids easier than cold water… sugar dissolves easily in hot coffee but not iced tea… gases on the contrary can be driven off by heat… oxygen depletion in cold trout streams happens when the stream water gets to warm due to drought or prolonged hot weather. It will kill the fish… just google the effect of heat on dissolved gases in water.
Removal of dissolved oxygen (DO) from water can be achieved through physical means such as thermal degassing, vacuum degassing, and countercurrent exchange. In thermal degassing, water is heated to release dissolved oxygen in air bubbles. Vacuum degassing uses a vacuum pump to extract DO.Feb 7, 2023
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One last comment on deionized water. Deionized water whether by filtration thru adsorbents of ions or via reverse osmosis DOES NOT removed dissolved gases, organic materials, including some pesticides, TCE (trichloroethylene) and other small molecule solvents. In that regards, distilled water is more pure.
Sorry but I mistakenly deleted my own post.
There is a thermodynamic reason why hot water does not dissolve gases. The entropy of a gas in gas phase is greater than in the dissolved liquid phase… as the temperature of water is increased, some of the heat energy (enthalpy) is removed by entropy effects…by driving off dissolved gases. Gasses rather be free moving than constrained by a solution…therefore gases exsolve first…The Effects on the Solubility of Gases
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As with all processes under constant pressure and constant temperature, dissolving a solution into solution will occur only if ΔGtotal<0Δ������<0.
ΔGsol=ΔHsol−TΔSsol<0(1)(1)Δ����=Δ����−�Δ����<0
For dissolving solids in liquids, ΔSsol>0Δ����>0, but for dissolving gases solutes, the entropy of solution is negative (ΔSsol<0Δ����<0) since the entropy of the gas phase solute is appreciably greater than the entropy of that solute in solution. Consequently, the only way that ΔGsol<0Δ����<0 for a dissolving a gas in solution is if the solution process is exothermic (i.e., ΔHsol<0Δ����<0). This occurs due to the enthalpy differences from making and breaking intermolecular interactions in the solvent and solution. There are three basic steps involved in dissolving a solute from a condensed state (or a non-ideal gas) into a solution each with a corresponding enthalpy change.
What all of that gobbledegook means is that hot water dissolves less gas than cold. When warmed or heated, it dissolves even less… the reason is thermodynamic, but can be understood easily by analogy: when water is heated under atmospheric pressure, it will boil and the temperature will remain steady at 100% until all of it boils off… water is being turned into steam, a gas… it takes not only temperature but added heat to boil off all the water in a container as the temperature of the container will not go above boiling (100 degrees C) until all of the water is gone… if gasses had a higher solubility in water, they would not escape unless they were boiled off along with all the water. The temperature of boiling,. 100 degrees C., would be increased from that of pure water, if that were the case… salts on the other hand have a higher solubility in water than water itself… boiling off water from a salt solution (any kind of salt) requires a temperature above 100 degrees C. The temperature of boiling increases as the salt concentration increases until all of the water is finally boiled off…
if distilled water is not exposed to air during manufacturing and bottling, it will contain no dissolved gases but will be pure water… If it’s heated slowly until it boils, no bubbles will form, until steam bubble finally form as the water boils. For water exposed to air, including distilled water that is aerated, gas bubbles will form as the water approaches boiling temperature. The gas bubbles are mostly oxygen from the air… Carbon dioxide is very soluble in water but the amount in air is only 400 parts per million. Oxygen is the bulk of dissolved gas. Nitrogen is poorly soluble in water and does not enter water appreciably until the air pressure is highly elevated… three times or greater than atmospheric pressure at sea level…these gas bubble will either stick to the sides of a pot or rise up to the surface as the water gets hotter but is still not at boiling… tap water starts to do this at about 80 degrees C… as the water starts to slowly boil, more bubble form… the steam bubbles at the bottom of the pot where the heat is being applied will form and collapse and disappear, only to re-form again as the whole water column heats to boiling… once at boiling point, they will no longer collapse, but bubble up as steam…the story of dissolved salts are the opposite… salts form ions when dissolved… these ions bind the water together with hydrogen bonds… getting salts separated from water requires a higher boiling temperature to break the hydrogen bonds and boil the water… the temperature of boiling increases as the salt concentration increases as more and more water is boiled away… until all of the water is gone and dry salts are left behind in the pot…The reverse is also true… as salt raises the temperature of boiling, it also depresses the temperature of freezing… the reason why salt solutions or crushed salts are used to de-ice roads…
Hi,
interesting…i wonder if thst is why they say to put salt in when boiling water for pasta…or potatoes…
julie
adding salt will raise the temperature of boiling water. the amount added to pasta would not be signifant… a lot of salt would. raising the pressure makes much more of a difference. pressure cookers take advantage of that property. and can go up to 250 degees at two times atmospheric pressure. given that 212 degrees is the temperature water boils at sea level aire pressure, the rise in temperature is significant. pressure cookers important at high altitude. at 10,000 at 194 degrees, 180 at 14,000 ft.
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I should have added to my last post that pressure is how reverse osmosis works also. The RO membrane blocks salt ions (all salts, not just table salt) from passing through, biut not water. a salt solution has osmotic pressure. It will draw free water into the solution. If the membrane is not expandable, the pressure inside will rise… RO pressurizes the untreated water within the membrane compartment, reversing the osmotic pressure gradient and pushes out free water/
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