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Explaining why salt and sugar disappear when dissolved in water.

Discussion in 'Science' started by TCSC47, Oct 28, 2016.

  1. TCSC47

    TCSC47 Star commenter

    Can somebody give me a punchy short explanation as to why salt and sugar disappear when they are dissolved in water, that is suitable for a yr 10 or 11? Not just the ionising bit, but also why they no longer interact with light.

    Cheers.
     
  2. Moony

    Moony Lead commenter

    They don't disapear, they break down into individual molecules which are too small to be seen. These molecules form a mixture with the water molecules, the salt/sugar is the solute, the water is the solvent and in total they are a solution.
     
    TCSC47 likes this.
  3. TCSC47

    TCSC47 Star commenter

    It was the "too small to be seen" that I couldn't come up with. I was thinking too deeply into light wave lengths and such, and my explanation would have been OTT. But this makes it simple and accessible.

    Such is the skill of a teacher that non teachers don't always figure.
     
  4. Moony

    Moony Lead commenter

    Yep, when I teach this I work with a sugar cube, I draw it in a beaker/cup with water and I stage it out using dots to show the particles. Depending on how bright the kids are I talk about it being a polar substance and water being a polar solvent.
     
    TCSC47 likes this.
  5. phlogiston

    phlogiston Star commenter

    I would add that the attractive forces between the separate ions and water are stronger than either the attractions between the ions or between the water molecules (same withthe sugar molecules).
     
    TCSC47 and Moony like this.
  6. steve_cooke

    steve_cooke New commenter

    I'm not sure that's true. Isn't dissolving salt endothermic?
     
  7. phlogiston

    phlogiston Star commenter

    Once you get into the complexities of entropy, life becomes complex. Steer well clear of ammonium nitrate. Then the explanation gets tied up with measuring the energy and the ways of arranging the system.
     
  8. cellerdore

    cellerdore Occasional commenter

    Be careful using the word "disappear" as children of that age think that it vanishes. A nice experiment is to make a saturated salt solution and then leave it in a cup with a piece of egg shell. After a few days, the salt will start to precipitate out of solution and the children can see that it was there all along. If you use distilled water, you can grow some lovely crystals from this and kids love it!
     
    Lara mfl 05 likes this.
  9. sm3llyc4t

    sm3llyc4t New commenter

    Agree with avoiding saying it 'disappears' - Had a chat with some Y7s and one couldn't understand why the mass of the salt water was greater than the mass of the water alone... 'But Miss, the salt has dissolved away, how does it weigh anything?' - Definitely a good demo if you can show the salt 'coming back out of the solution' to prove that it was always present, just too small to see.
     
  10. bogstandardcomp

    bogstandardcomp New commenter

  11. NewToTeachingOldToMaths

    NewToTeachingOldToMaths Lead commenter

    In other words, they don't actually disappear; they only appear to disappear. :D

    (Why am I reminded of the question "Is that a real optical illusion, or does it just look like one?" :rolleyes: )


    Oh bless. I think this would be the time to switch from discussing water as a solute to discussing tea as a solute. Put sugar in your tea and it "dissolves away completely" ... but it's "still there", as you know from the fact that it still has the quality of sweetness, which it adds to the tea. Accidentally add salt rather than sugar to your tea, and it too "dissolves away completely" ... but when you come to drink your tea you DEFINITELY know it's "still there"!!!

    (Don't try this at home, boys and girls ... )

    So the quality of mass, like the quality of sweetness (or saltiness) continues to exist "be there" even after the salt or sugar has dissolved ...
     
    Last edited: Dec 11, 2016

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