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quote:Originally posted by jacei think this is wrong as i diid an experiment on it and the boiling point went downjack
quote:Originally posted by rosyUr, the change in boiling and freezing point observed should be as Micky says but it's not actually to do with the density... it's thermodynamics (what isn't!)The reason more salt dissolves in water than sugar isn't down to the particle size its to do with the different interactions between the particles, with each other in the solid and with the water molecules and between the water molecules.
quote:Originally posted by rosyThe boiling point is expected by theory to increase, because the disorder (entropy) of the solution is increased by the presence of solutes and so the dilute solution is favoured over the more concentrated solution and the gas at a slightly higher temperature than the pure solvent.
quote:Originally posted by rosyWhat was the boiling point of water with no solute (Jack or Meg)?
quote:Question - When the water is close to the boiling point happen something strange to me: if I put salt in the water it suddenly boils (on the contrary if put the salt before it takes longer to reach the boiling point). Why?---------------------------------------Davide,In order to facilitate boiling, a nucleation point, a point where bubbles can begin to form, is helpful. Typically, a nucleation point is something that has a large surface to volume ratio, like a porous stick, or grains of sand, rice grains, bits of rocks, or broken pieces of porcelain - and as you found out, salt. You can try this same experiment with such materials - but be VERY careful that you do not scald yourself!As to why adding salt before the water comes to a boil seems to prolong the advent of boiling - there is a colligative property known as "boiling point elevation". It is also possible that the addition of salt can change the specific heat of the system so that the saline solution has a higher heat capacity then an equivalent amount of water ---- but I really think these effect are too small for the amount of salt that you are mixing in the water to be noticeable. More then likely, this is more a perception issue then a real objective observation.Greg (Roberto Gregorius)====================================================================All powders carry some trapped air between grains.This air acts as nucleation points for liquid converting to vapor.If the liquid is superheated (hotter than boiling,but not hot enough to start it's own microscopic bubbles a few molecules wide),then suddenly providing new nucleation sites can cause extremely sudden boiling.Heating clean water in a clean container,or water heated in its volume rather than at a surface,(microwave ovens do that),often develops some degree of superheating before steady boiling begins.That makes heated water dangerous at that stage.To bypass the superheated-liquid stage, it is customary to, before very hot, put in a fewdry "boiling-stones" of some inert and porous substance (carbon, stone, or DuPont's Teflon (TM)).Their job is to keep some trapped air bubbles available to the liquid at all times.If you do this to both solutions I think the difference in their behavior will decrease.If you boil the water with boiling stones in it, then cool the water, then heat again,the boiling stones will not help the second boil as they do the first.During the first boil, the air in the bubbles are replaced by steam.When the liquid is cooled, these steam-bubbles can collapse out of existence,and all parts of the stone's surface are wetted completely.So, for the second boil they provide no air-bubbles, no nucleation-points.These stones have no effect.At these times you may throw in a few new (dry) stones, and they will serve.Later, after they are removed and dried, they can all be used again.
quote:The addition of solute affects the boiling point, freezing point, and vapor pressure of the solution, in general raising the boiling point, depressing the freezing point, and lowering the vapor pressure (see Raoult's law). A number of substances (acids, bases, and salts) exhibit characteristic behavior in aqueous solution. These substances dissociate in water to form positive and negative ions that enable the solution to conduct electricity. Such solutions are called electrolytic (see electrolyte). The addition of some solutes to a solvent will raise the temperature of the solution, while others may lower the temperature and still others will have no noticeable effect. This behavior depends on the heat of solution of the solute in the given solvent. The heat of solution, i.e., the amount of heat given off or absorbed during the process of solution, is equal to the difference between the energy that must be supplied to break up the crystals of the solute and the energy that is released when the solute particles are taken into solution by the solvent (see enthalpy). If the heat of solution is negative (i.e., more energy is required to break up the crystal than is released in forming the solution), then the temperature will decrease; if the heat of solution is positive, the temperature will increase.
quote:What's wrong with this experiment for measuring boiling point elevation? Hi, I'm doing a lab on the effect of salt on the boiling point of water... The procedure is to fill a metal pot 3/4 full with water, add salt, place a thermometer in for 5 minutes, then place the pot on the stove and begin heating the pot. I was hoping you could give me some information on what happens. Gianna Massouras quote:Gianna, There are some flaws in this experiment that you'll have to correct if you want to use the results to show how salt affects the boiling point of water. The molecules in the liquid are in constant motion. They constantly collide with each other and with the walls of their container; every now and then, a collision will cause one of the molecules to be ejected from the liquid into the air above it, creating a vapor pressure above the liquid. Heating the liquid increases the average speed of the molecules, increases the number of molecules ejected, and increases the vapor pressure. A liquid boils when its vapor pressure becomes equal to atmospheric pressure. Low atmospheric pressure causes the boiling point to go down; high pressure drives it up. Atmospheric pressure varies a bit from day to day, depending on the weather, and it varies from place to place, depending on the altitude. So it's quite possible that you'll get a boiling point for the salt water that's less than 100°, which seems to contradict what your textbook predicts (namely, that salt increases the boiling point temperature). You can look at the effect of salt without the pressure effect if you measure the boiling point of the water before adding the salt. That way you can compare the boiling point of the water and the solution at the same pressure. Another thing that affects the vapor pressure of the water is the relative number of water molecules in the solution. The higher the percentage of water, the more molecules will escape into the vapor, and the higher the vapor pressure will be. A salty solution has a lower percentage of water molecules than pure water does. So dissolving salt in water decreases the vapor pressure of the water. The more salt you dissolve, the lower the vapor pressure of the water becomes. You'll have to heat the solution to a higher temperature than before to get its vapor pressure equal to atmospheric pressure You must carefully measure the amount of salt you use to be sure that it's salt and not something else that is affecting the boiling point. The boiling point goes up by roughly 1°C for every mole of NaCl per liter- it's a very small effect, and one you're likely to miss completely if you're sloppy or your thermometer isn't very precise. Finally, make sure that the temperature you're measuring is really the boiling point. The first bubbles that appear aren't steam- they're dissolved air, which comes out of solution as the temperature rises. You won't be at the boiling point until the bubbles forming at the bottom rise all the way to the top and burst on the surface. Don't wait too long; as the liquid boils, some will evaporate, increasing the salt concentration. The boiling point temperature will slowly climb as the experiment proceeds! Author: Fred Senese senese@antoine.frostburg.edu
quote:Gianna, There are some flaws in this experiment that you'll have to correct if you want to use the results to show how salt affects the boiling point of water. The molecules in the liquid are in constant motion. They constantly collide with each other and with the walls of their container; every now and then, a collision will cause one of the molecules to be ejected from the liquid into the air above it, creating a vapor pressure above the liquid. Heating the liquid increases the average speed of the molecules, increases the number of molecules ejected, and increases the vapor pressure. A liquid boils when its vapor pressure becomes equal to atmospheric pressure. Low atmospheric pressure causes the boiling point to go down; high pressure drives it up. Atmospheric pressure varies a bit from day to day, depending on the weather, and it varies from place to place, depending on the altitude. So it's quite possible that you'll get a boiling point for the salt water that's less than 100°, which seems to contradict what your textbook predicts (namely, that salt increases the boiling point temperature). You can look at the effect of salt without the pressure effect if you measure the boiling point of the water before adding the salt. That way you can compare the boiling point of the water and the solution at the same pressure. Another thing that affects the vapor pressure of the water is the relative number of water molecules in the solution. The higher the percentage of water, the more molecules will escape into the vapor, and the higher the vapor pressure will be. A salty solution has a lower percentage of water molecules than pure water does. So dissolving salt in water decreases the vapor pressure of the water. The more salt you dissolve, the lower the vapor pressure of the water becomes. You'll have to heat the solution to a higher temperature than before to get its vapor pressure equal to atmospheric pressure You must carefully measure the amount of salt you use to be sure that it's salt and not something else that is affecting the boiling point. The boiling point goes up by roughly 1°C for every mole of NaCl per liter- it's a very small effect, and one you're likely to miss completely if you're sloppy or your thermometer isn't very precise. Finally, make sure that the temperature you're measuring is really the boiling point. The first bubbles that appear aren't steam- they're dissolved air, which comes out of solution as the temperature rises. You won't be at the boiling point until the bubbles forming at the bottom rise all the way to the top and burst on the surface. Don't wait too long; as the liquid boils, some will evaporate, increasing the salt concentration. The boiling point temperature will slowly climb as the experiment proceeds! Author: Fred Senese senese@antoine.frostburg.edu
quote:Originally posted by ukmickyHi rosy [] As salt desolves in water some of its molecules break up and some of the hydrogen bonds break apart and become sodium hydroxide and hydrogen chloride, this requires energy which i assume is taken from the water in the form of heat . Now as water boils away and salt crystal are formed is this process reversed and if so is energy once again taken from the water to fuel this process or is energy released back into the water as everything reforms.Or have i got it wrong again, go easy on me Michael