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I boiled apples in water until tender. I took apples out, then added lemon and sugar to water in pot and boiled it. When I poured it over the apples it made a nice thickened gel. By morning it's all watery. Why?

11

Sugar is hydrophilic, meaning it's attracted to water. Your water soaked/softened apples likely had a higher moisture content compared to your syrup. The water migrated out of the apples and equalized the sugar concentration. This process is referred to as osmosis.

Next time, try boiling the apples in that sugar syrup instead of just water.

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    @briantist Actually, the explanation is almost entirely wrong. First, "hygroscopic" refers to substances that absorb water vapour from the air, which isn't likely to be what's happening and which contradicts the rest of the answer. Second, it's not sugar moving from the syrup into the apples, but water moving from the apples into the syrup. – David Richerby Dec 27 '18 at 21:10
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    This is the issue with the upvote system and the HNQ. An answer with a fancy word which looks plausible at first glance gets many upvotes, even when it makes no sense – Tim Dec 27 '18 at 22:15
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    @Tim On the other hand, HNQ was the only reason I found this. – David Richerby Dec 27 '18 at 22:33
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    The part about boiling the apples in the syrup next time is exactly right. – JPmiaou Dec 27 '18 at 23:50
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    @DavidRicherby I suggested that edit around the time of your comment. It sat in the edit queue for too long. This is what i thought was the intent of his answer. He, for instance, thought the word for hydrophilic was hydroscopic which is a very common mistake (hygroscopic was a correction by someone else). – KMB Dec 28 '18 at 20:00
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The syrup has very high sugar content and the apples much lower. The syrup pulls water out of the apples by a process known as osmosis, which tries to equalize the sugar concentration on each side of the permeable membranes of the apples' cells.

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    The other answer clearly had the right idea, but is wrong in both the word choice and the way it is described. It is still more descriptive than this answer. If someone knows what osmosis is, they understand that is what he was going for. If they don't, your answer doesn't help them understand what is going on. (Note that I did upvote because you are right.) – KMB Dec 27 '18 at 21:22
  • @KMB I added a bit more detail. – David Richerby Dec 27 '18 at 22:35
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    I upvoted because this is clearly the best answer, but "The syrup pulls water out of the apples" is a bit misleading, because the syrup isn't doing anything. As you mention, it's basically just that the system is tending towards uniformity/homogeneity. It's basically statistical mechanics. – Faheem Mitha Dec 27 '18 at 23:25
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Some of the water content of the apples has migrated from the apples to the surrounding syrup. This is due to an effect called osmosis. The apple, like all living things is made out of cells that have (among other things) water inside them. The cell walls are semipermeable, meaning small molecules like water can pass through them, but larger molecules like sugars cannot. All dissolved substances (sugars, salt, etc.) attract water to themselves, so if you expose the apples to a highly sugary solution (like syrup), the water will be pulled from the apples into the syrup.

  • I think it unlikely that the apple cell walls are intact after boiling. You don't need to invoke osmosis to explain the water migration - simple diffusion is enough. – user20637 Dec 29 '18 at 15:53
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    @user20637 minor nitpick, it's the cellmembranes, not cell walls that prevent diffusion. Nevertheless, those are probably also toast after boiling. – JAD Dec 29 '18 at 16:43
  • @JAD Hat-tip for the accurate nitpick. I think you agree it's not osmosis. I wouldn't be surprised if syneresis also made a contribution- so water would migrate from boiled apples even in the absence of sugar syrup - but I'm not sure. – user20637 Dec 29 '18 at 17:11

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