• Question: This may sound like a silly question to ask, and maybe more to do with chemistry, however I have been wondering about what happens when different materials, particularly paper, get wet. What happens on a molecular level that makes things go darker when they are wet? How do the molecules interact to make this happen? And why does paper go slightly transparent when it is wet, and then slightly brittle when it is dry after being wet? Is it something to do with density? Or something completely different?

    Asked by slunnun to James, Marcus, Martin, Rob, Suzanne on 12 Mar 2012.
    • Photo: Marcus Gallagher-Jones

      Marcus Gallagher-Jones answered on 10 Mar 2012:

      I’m not incredibly sure on this one but I will have a go at parts of it.
      I think you are approaching the right area with the term density though. In the case of paper it is made up of very small fibres made up mostly of a sugar called cellulose. Each cellulose molecule is linked to the next to form long chains.

      These bonds between these sugars are very sensitive to a process called hydrolysis, from “hydro” meaning water and “lysis meaning to loosen or detach. When you add water to paper you interfere with the bonds holding fibres together causing the area to expand. Once it has expanded it will allow more light to pass through, hence becoming transparent. Once it dries I imagine that the fibres do not pack as neatly as they were originally. The closeness of one fibre to another contributes quite a lot to papers tensile strength (ability to with stand being pulled or stretched), and so if they are not so close together then they will no longer be so resistant.

      I don’t know if the same is true in the case of things becoming darker but I’d imagine so.

    • Photo: James Boone

      James Boone answered on 10 Mar 2012:

      I’ll try and answer the transparent bit. Normally, paper is rough on the surface. When light hits paper, it is reflected in all directions (called diffuse scattering) and this is why normal paper doesn’t look shiny (if the light were reflected without this scattering, the paper would act more like a mirror).
      Paper is also full of holes because it is made up of fibres all tangled together so light is scattered all the time as it tries to go through the paper. When you put water or oil on paper, it fills all these little holes and allows the light to pass through without being bounced off all the empty spaces inside. The light then travels much straighter through the water or oil which is why it looks like you can almost see through it.

    • Photo: Martin Zaltz Austwick

      Martin Zaltz Austwick answered on 12 Mar 2012:

      This isn’t a silly question, it’s a really good one! And quite hard to answer.

      I think these explanations are very good actually. Light often gets reflected at the boundary between two materials – like the reflection from a pond or swimming pool. When there are lots of boundaries, light tends to get bounced around a lot and things look cloudy. E.g. If you get a glass of water out the tap, it can look very cloudy because of the tiny bubbles of air and the light bouncing between them.

      I think that’s what happens in paper, and then when it gets wet the whole thing is full of water and fewer reflections occur, so it looks less cloudy because the light can travel in a straight line.

    • Photo: Robert Thompson

      Robert Thompson answered on 12 Mar 2012:

      In science there are no silly questions, only silly people who don’t ask questions. It’s a very good question my favorite part of science is understanding how light interacts with different materials. I don’t think I could explain any better than the answers you have already.

    • Photo: Suzanne McEndoo

      Suzanne McEndoo answered on 12 Mar 2012:

      This is a really cool questions, and thanks to Marcus, James and Martin for their answers. I can’t think of anything to add to what they said.

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