I'm studying biology and this came up in the context of passive transport. I understand why, in general a gas moves down a concentration gradient (the random movement of gas particles and probability). But I don't understand why the concentration of some solute (ions, salt, etc) would affect the movement of water. Why does water move from an area with a lower concentration of solute to an area with a higher concentration of solute?

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    $\begingroup$ In general chemical reactions have a forward reaction and a backwards reaction. The equilibrium is established where the two rates of reaction are equal. $\endgroup$ – MaxW Aug 27 '18 at 23:31
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    $\begingroup$ The driving force is the same. Usually, in the cases you're describing, the solute is prevented from moving, so the water moves instead. Whereas in the general case, both the solvent and the solute freely disperse to provide a relatively uniform concentration. $\endgroup$ – Zhe Aug 27 '18 at 23:49
  • $\begingroup$ It might help to think of a lower concentration of water in the area of a higher concentration of solute $\endgroup$ – GreenSmurf Aug 28 '18 at 15:10
  • $\begingroup$ Think of it this way: water moves both ways, but there is more of it on one side, hence the flow from that side to another will be greater than the reverse flow. $\endgroup$ – Ivan Neretin Aug 31 '18 at 8:45

There are two possibilities. The first is that it is simple dilution, but for more likely is that osmosis is occurring. In this case the concentrated solution and the water, or a dilute solution, are separated by a semi-permeable membrane, i.e. one that allows passage of small water molecule but not large solute ones. To lower the overall energy (as represented as the chemical potential) water moves through the membrane from the dilute solution to the more concentrated one. In a cell this can lead to the cell swelling and possibly bursting if left uncontrolled. There is some more explanation in the answer to this question Entropy as the driving force for osmosis


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