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Help! Salol cooling curve shows temperature increase!

Discussion in 'Science' started by RoseyGrz, Nov 15, 2011.

  1. I'm doing changing state with year 8 pupils and they recorded the temperature os liquid salol as it cooled and turned liquid. Most pupils didn't get great results so I'm using my own so they can make a graph next lesson.
    However my results show the temp decreasing from around 65degrees to around 22degrees then rising to a peak at 35degrees whilst it freezes - is this what should happen?
    I thought because making intermolecular bonds releases energy this would be right but now I'm starting to doubt the results.
    I'm a trainee and being observed ont he lesson where I cover this so really need to get it right.
    If anyone could help that would be amaizing
    Thank you
     
  2. MarkS

    MarkS New commenter

    It should happen at a constant temperature - research 'Latent heat of fusion'.
    I've never seen a temperature increase during this kind of practical, and can't honestly think of an explanation...I think you should do what all good scientists do and fudge the results!!!
    Mark
     
  3. The answer is super-cooling; if you allow a liquid to cool without any vibration, then the temperature will drop well below the accepted freezing point - any vibration will cause the temperature to rise rapidly to the accepted freezing point. This super-cooling phenomenon is also seen with water; water can be cooled to -20C without freezing, then the smallest vibration will cause freezing to occur.
    A potentially serious situation arose when my class was finding the boiling point of acetone, one girl heated the acetone in a hot water bath THEN inserted the thermometer, the acetone was at a temperature well above the boiling point (super-heated), insertion of the thermometer initiated boiling - and the acetone exploded in to her face, (and she was wearing goggles over her hair).
     
  4. I thought salol froze at about 42 deg C - I suspect you may be attempting to accelerate the cooling by using an ice bath, is that the case?
    If you cool clean salol in a very clean test tube then supercooling may occur (the liquid remains liquid at below the freezing point because of a lack of foci for crystals to form). Allow the salol to cool naturally and don't be too fussy about cleanliness.
    Forming the nucleates of crystals is endothermic however the growth of crystals is exothermic.
    Some commercially available hand-warmers use a supercooled melt which is encouraged to crystallise by means of a mechanical 'clicker' releasing heat and warming the melt up to nearer to its normal freezing point of temperature.
    Warn your students NOT to attempt to remove the thermometers from the salol else you will end up with a few broken thermometers.
    Good luck
     
  5. Thanks for that, I'll add that to our Health and Safety stuff.
    I've noticed a similar thing when I've been heating mugs of water in a microwave. I've found if I use a very clean mug and take care to place it in exactly the centre of the microwave, then if I careful remove it from the microwave after heating and drop a spoonful of sugar in, it erupts hot water all over the kitchen table --- great fun!
     
  6. Use stearic acid instead. It gives fairly reliable results.
     
  7. How do you know they didn't get good results? Have you "read" all of their values, or did they plot graphs as they went along ["temp 0-100 on y-axis, time 0-20 mins on x-axis: take measurement, record, plot" - they can do that in 1 minute easily and gives less time for talking! You can assess their plotting skill as you wander round.]
    Did anybody get good results? Could you praise them and use their results? If they haven't already done so, can they plot their own results and evaluate their "science experimenter" skills, by comparing each others graphs?
    Salol is unlikely to give you a horizontal section - this only happens with pure substances, and yours will be fairly contaminated by now, probably. You should, however, see a "knee-shaped" curve, although the knee section may be quite short.
    Remember, the pupils may not know how to take temperature readings correctly: I got fed up of seeing my pupils stop the clock, lift the thermometer out of the material, pick up a pen, read the thermometer, write the answer, put the thermometer back in, re-start the clock. [​IMG] It is difficult to stir salol as it solidifies, so there may be gaps between the glass bulb and the material, which can introduce errors.
    As a trainee, set yourself a target of having integrity: NEVER fiddle your results! Explain anomolies, look critically at where your answers deviate from the theory, consider un-accounted for variables. Then expect the same from your students.
     
  8. phlogiston

    phlogiston Star commenter

    I think you must share my year 7 class!
    P
     
  9. As the real Old Cheese would say to us in the fourth form physics class when plotting a cooling curve for molten napthalene in a boiling tube: "keep stirring kid - don't stop until the napthalene is solid and you have a nice hole in it to keep your thermometer in".
    So - to get a good demonstration of the latent heet of solidification without the complications of supercooling - you must keep stirring (kid)!
    O.C.
     
  10. Just want to thank you all for your help and thoughts, I am going to attempt to explain super cooling for year 8s tomorrow then. Wish me luck.
     
  11. WHY? You report their results as not good but only mention temperature rises when you did it. Now that you've had a chance to get it sorted, demonstrate a GOOD experiment and get them to plot the results, explaining why you got the curve.
    Unless they are extremely bright, I don't think you'll be doing them any favours. From an exam standpoint, they'll not be asked about it before A level, if then. After all, you seem to have got this far without meeting it! [​IMG]
    As a trainee, you should be evaluating your performance against your teaching intentions. What do you want them to know?
    My thoughts would be: 1) objects hotter than their surroundings lose heat (I wouldn't mention three mechanisms!) . 2) The rate of cooling is higher when the temperature difference is high (they could use this to predict / recall how a beaker of water cools - smooth curve towards ambient) . 3) Salol's temperature drops, levels out, then drops again . 4) We know energy was being lost from the material even though the temperature was "level" because it was hotter than its surroundings but we observed that the material was changing from a liquid to a solid (Q: how secure are they with atomic/molecular structure?) . 5) Changing state involves a change in energy to reformulate the bonds (we supply energy to melt ice / boil water; this is the reverse) . SKILLS: use a thermometer and stopclock carefully; record data in an appropriate format with headings and units; plot points accurately on (own choice) well-scaled axes with correct labelling and units; interpolate sensibly to avoid anomalous values and generate a smooth curve; use kinetic theory to describe observations.
    If I delivered all of that, I'd be reasonably happy. If I could add an evaluation and redesign of an experiment, and even use data-logging to acquire more information, I'd be very happy.
    One key thing to remember is that learning should be appropriate to the needs of the students, moving them forward and encouraging them to want to know more. Sometimes we don't tell them the whole story, but use a spiral curriculum to return to important topics and expand their understanding as they mature.
    Good luck. Please let us know how it went.
     
  12. mrswainwright

    mrswainwright New commenter

    I've done this expt with stearic acid and found it quite messy. Am trying salol now, it's working beautifully. Half a boiling tube full, started off by sitting the boiling tube in a beaker of boiling water. Stirring and taking temp every minute, starts to solidify after a few minutes and is slowly solidifying over 10 mins or so. The temperature DOES go up a smidge as it solidifies (dropped to 39, rose to 41 and has stayed there). I think this is because it's the least energetic molecules that are joining the solid, leaving the more energetic ones still in the liquid that you are taking the temperature of. It's going to make a lovely graph, with a nice long flat bit.

    I'm going to start by asking them to sketch the shape they think the graph will be, then get them to draw up some axes and plot accurately as the cooling progresses. I agree entirely with the previous post in principle but wouldn't hope to get all of that out of one activity!
     

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