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So, on 'http://filestore.aqa.org.uk/sample-papers-and-mark-schemes/2016/june/AQA-BL3HP-QP-JUN16.PDF', question 1.b., they show a partially permeable bag of glucose solution, in a less concentrated solution. Since sugar is more dense than water, why does the bag get HEAVIER, when there's more sugar, and less water?

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closed as off-topic by David, theforestecologist, Bryan Krause, another 'Homo sapien', Satwik Pasani May 7 '17 at 18:33

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  • "Homework questions are off-topic on Biology unless you have shown your attempt at an answer. For more information see our homework policy." – David, theforestecologist, Bryan Krause, another 'Homo sapien', Satwik Pasani
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  • $\begingroup$ Hint: the bag is partially permeable, but to what? $\endgroup$ – Devon Ryan May 1 '17 at 10:09
  • $\begingroup$ You try to explain why you find it surprising that the bag would get heavier but I still don't understand your reasoning. I don't see how you go from the density of sugar and water and how much of them there is in the bag, to being surprised the bag is heavier. Maybe you could give more details on the logical connections there? $\endgroup$ – Oosaka May 1 '17 at 10:48
  • $\begingroup$ @DevonRyan If it's not permeable to sugar molecules, then is the amount of sugar inside and out remaining the same? If so, and water is going inside the bag, then the bag is swelling. Does diffusion do this? Make things swell? My understanding was that you couldn't create pressure this way. $\endgroup$ – Piomicron May 1 '17 at 13:24
  • $\begingroup$ @RozennKeribin If you replaced the sugar in the bag with water via diffusion, the bag would be lighter. My understanding is this what is happening in the bag. Why is it becoming heavier? $\endgroup$ – Piomicron May 1 '17 at 13:25
  • $\begingroup$ @Piomicron thank you, your question makes more sense that way (you might want to edit your question to include that reasoning). I think you have it right in your answer to DevonRyan: the bag is permeable to water but not sugar. Could you elaborate on the objection "my understanding was that you couldn't create pressure that way"? Is there a specific source you are basing this on? $\endgroup$ – Oosaka May 1 '17 at 15:15
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The reason that dialysis bag A and B will get heavier is due to water diffusing into the bags in order to bring the sugar concentration on both sides of the membrane to an equilibrium. Likewise bag C and D will get lighter as water will diffuse out of the bags in order to bring the system as close as possible to equilibrium. Water diffusion is also called osmosis and can be studied closer here: https://en.m.wikipedia.org/wiki/Osmosis

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