Question: |
Why would the boiling point of a liquid (take water, for example) be elevated when a nonvolatile solute is added to it? |
Category: |
Science > Chemistry |
Keywords: |
salt, temperature, affect, increase, liquid, water, boiling point elevation |
Type: |
why |
Rating: (0 ratings) Views: 95 Discussions: 0 In Watch Lists: 1 |
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Answer:
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Vapor can form anywhere in a boiling liquid. If you watch a boiling pot of water you will see bubbles form throughout the water and on the bottom and sides of the pot. Also, if you pour a layer of oil on top of the water it will still boil in the same way, in particular at the same temperature, without having any exposed surface at all. The reason dissolved solutes increase boiling point is that the solute must come out of solution in order for the water to boil. This costs entropy (the entropy of solution). Boiling is entropically driven, hence the reduction in the net entropy gain of boiling results in a higher temperature needed for the reaction to go. To put it without jargon: for a little packet of water with dissolved salt to turn to steam the salt atoms must, in the course of their random zooming about, ALL simultaneously leave the packet. This is not a likely event. It becomes more likely as the temperature (i.e. the average speed of zooming about) becomes higher, though, and at a certain temperature above the ordinary boiling point it becomes sufficiently likely to allow boiling in spite of the handicap. You can also see that the effect will naturally increase with the concentration of dissolved solutes (i.e. the number of salt atoms per packet that must simultaneously leave). Incidentally, at the low energies involved here molecules are quite hard and space-filling to each other. You can regard them as little hard balls, which are as closely packed as they can be in the liquid state. That is why, for example, it is so very hard to compress or expand a liquid --- there's no space. This is the basis for hydraulic equipment, e.g. the brakes in your car.
Source: Argonne National Laboratory
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Why would the boiling point of a liquid (take water, for example) be elevated when a nonvolatile solute is added to it?
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