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Thread: What effect does an impurity have on the meltin point of H20

  1. #1 What effect does an impurity have on the meltin point of H20 
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    I really need a lot of help with this chemistry lab!!!!!!!!! please help me out
    Would the melting point of impure water be: lower than/the same as/higher than, the melting point of pure water????(prediction before doing the lab)
    - I used salt for the first 10 trials
    - I used antifreeze for the other 10 trials

    Describe what happened to the tap water under the beaker + explain.

    Identify two limitations in the procedure that detract from the precision of the experiment and thus the conclusions that can obtained from it.



    Additional Details
    Explanation of the lab:
    The purpose of this experiment is to determine the effect of impurities on the melting point of water.

    Procedure:
    1) half fill the 100ml beaker with table salt
    2) obtain about 100ml of antifreeze in the 150ml beaker
    3) loosely 3/4 fill the 400ml beaker with snow. do not compress
    4) record the initial lowest temperature of the snow
    5) place several drops of tap water on the table and place the 400ml beaker in the puddle
    6) add one level 5ml scoop of salt to the 400ml beaker
    7) without moving the beaker, continuously stir the salt-snow mixture with the plastic stirring rod
    8) record the lowest temperature obtained of the salt-snow mixture
    9)add a second level 5ml scoop of salt, stirring continuously, recording the new lowest temperature achieved. repeat until ten scoops have been added
    10) record all physical changes at each step and other notable observations
    11)repeat steps 3 through 10 using 10ml of antifreeze in steps 6&9 instead of 5ml of scoops of salt.


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  3. #2  
    Time Lord
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    Come on, you can rough it in the kitchen at home. If you have no salt or antifreeze use tears and liquor.


    A pong by any other name is still a pong. -williampinn
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  4. #3 Re: What effect does an impurity have on the meltin point of 
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    Quote Originally Posted by Melissa
    - I used salt for the first 10 trials
    - I used antifreeze for the other 10 trials
    What happened when you did your experiment?
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  5. #4  
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    melting point of impure and pure water or snow ,ice? i think you mean boiling point of water and melting point of ice,snow.

    as far as my knowledge says ..........no matter what you add to the water ..at 1 atmospheric pressure its boiling point will be 100 degree celsius,to increase its boiling point you have to increase the atm pressure.

    your experiment of salt and tap water ....salt and tap water will form a mixture that will reduce the temperature of the surrounding of snow and not of the snow ,so melting point of ice will not change.

    about antifreeze....it is exothermic right?

    let me know what do you think.
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  6. #5  
    Reptile Dysfunction drowsy turtle's Avatar
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    Salt only affects the freezing point of water, when in solution. Melting snow, it will make no difference to, because it cannot dissolve in a solid.

    Antifreeze will dissolve the water, on the other hand, lowering the melting point.
    "The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair." ~ Douglas Adams
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  7. #6  
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    Don't know how often it snows in your part of the UK, but hereabouts, we put rock salt on the ice, and it melts. At least, that works down to about 0 degrees F. So, yes, it reduces the melting point.

    Dissolving anything in water also increases the boiling point.
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  8. #7  
    Reptile Dysfunction drowsy turtle's Avatar
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    Quote Originally Posted by Harold14370
    Don't know how often it snows in your part of the UK
    Very rarely

    Quote Originally Posted by Harold14370
    but hereabouts, we put rock salt on the ice, and it melts.
    Hmmm, yes, I've noticed that from the odd occasion when it does snow. In theory, the salt shouldn't dissolve because the snow isn't liquid, and it's common advice to put salt out before it snows rather than after. So I'm not too sure about this one....
    "The major difference between a thing that might go wrong and a thing that cannot possibly go wrong is that when a thing that cannot possibly go wrong goes wrong it usually turns out to be impossible to get at or repair." ~ Douglas Adams
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  9. #8 Crystal Structure 
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    The freezing point of impure water is lower than that of pure water because contaminating molecules in the water help prevent it from forming crystals.

    Within most snow there will be some in crystal form and some in liquid form (unless the snow is held at a consistently low temperature). When added salt would mix with the liquid part of the snow lowering its freezing point and preventing it reforming crystals.

    The melting point can't be lowered by the salt until it has mixed throughout the water molecules. This is why you put the salt on before it snows so it has longer to mix with any snow which melts.

    If salt was mixed with the water before freezing then the melting point will be lower than ice formed from pure water.
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  10. #9  
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    Adding salt to water lowers it's melting point and raises it's boiling point.

    Adding anti-freeze to water also lowers it's melting point and raises it's boiling point.
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  11. #10  
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    It is analogous to metallic alloys in that small amount of additional material can significantly change exhibited properties. Bronze vs copper, steel vs iron, etc., including high temperature superconductors.

    Encouragement is extended to budding scientist and all dotcomrades, despite recent changes this is still pretty good forum.
    The bravest are surely those who have the clearest vision of what is before them, glory and danger alike, and yet notwithstanding go out to meet it.- Thucydides
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  12. #11  
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    My guess is that all the information her is correct and im going to indirectly quote some of the people before me about lowering and raising the boiling point of water. I would assume that if you add antifreeze to the water solution would depend on the ratio of that solution/ solvent to the amount of water. If the ratio of substance to water is so minor then no it shouldn't effect the boiling point and freezing point MUCH. It might but not to a measurable amount. Now having said that I think the closer the ratio is the more drastically you will see a difference in boiling/ freezing point. Now on the other hand if the ratio gets so diluted to the point where their is more substance then water... Then id only expect mathematically that so would the boiling point/freezing point... that is if there even is one at that point.

    I know that I remember doing this lab a few years back in high school and I vaguely remember concluding that, the more you contaminate the substance's ability to crystallize at its freezing point, the greater amount of effort required. That's obvious no surprise there and then the same goes for its boiling point if you contaminate it it all depends on the ratio. Now all of this was based off my recollection of my High School lab that i performed about 3 or 4 years ago so don't quote me on this for your own lab!


    Hope that helped and was somewhat accurate.
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