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Thread: The new Cricketweb Science thread

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    U19 Captain R_Powell_fan's Avatar
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    The new Cricketweb Science thread

    Just thought that resurrecting this thread was a good idea....

    My Question:- Water Filters, three stage Reverse Osmosis? Can anybody try to explain what is meant by " Reverse Osmosis" ??
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    Hall of Fame Member luckyeddie's Avatar
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    OK, let's start with the term 'Osmosis'. This involves the passage of a solvent through a 'semi-permeable membrane' at molecular level.

    Take a volume of some solid (solute, eg salt) which is dissolved in a solvent (eg water). Cover it with a semi-permeable membrane (which is a fancy name for a fine seive) and turn it upside down, plunging it into a larger volume of the same solvent (a big bucket of water) with a greater concentration of solute(salt). Dip the tube into the bucket until the levels of the liquids (in tube and bucket) coincide.

    Osmosis is the phenomenon where the two volumes of solvent try to equalise each other out as far as concentration of solute (their respective saltiness) goes.

    Now the semi-permeable membrane (I prefer the term 'molecular sieve') will allow the smaller water molecules through, whilst trapping the larger salt ones. The water molecules on both sides of the barrier (the membrane) are in constant motion, so there's a tendency for some to penetrate through the sieve from either direction. The salt molecules cannot - they're too large, so they inhibit the flow from the salty side to the less-salty side (block the sieve, so to speak).

    Eventually, the concentration of salt in water on both sides of the barrier become equalised. The water level in the tube will slowly rise.

    Incidentally, it's the reason that drinking sea water will kill you. The salty water in your stomach drags the less-salty water from the rest of your body and you de-hydrate.

    Anyway, reverse-osmosis is a clever marketing use of a scientific term - like all marketing terms it's 'bad science'. The only way to REVERSE osmosis is to have lots of pressure on one side of the membrane to drive the solvent molecules through from the 'more salty' side to the 'less salty' side.

    It isn't the reverse of osmosis at all. It's a pressure-sieve.

    Three-stage reverse-osmosis - three sieves, each with smaller pores than the previous (down to about 4 Angstroms)

    It does work, though. Cheers - enjoy the water.
    Last edited by luckyeddie; 21-06-2003 at 04:58 AM.
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    PY
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    Osmosis was the basis of my GCSE Biology coursework couple of years ago. Well boring.

    With my basic knowledge, surely the pressure would equalise anyway after a while with the pressure-sieve idea and once pressure has equalised normal osmosis occurs.

    Textbook definition of diffusion is:

    "A substance moving from an area of its high concentration to an area of its low concentration through a semi-permeable membrane"

    Now I remember why I dropped Biology.:rolleyes:
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    Hall of Fame Member luckyeddie's Avatar
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    Originally posted by PY
    With my basic knowledge, surely the pressure would equalise anyway after a while with the pressure-sieve idea and once pressure has equalised normal osmosis occurs.

    That's why the principle of 'reverse osmosis' is, in fact, nothing to do with osmosis at all.

    The respective pressures never get a chance to equalise because the pressure on the high concentration side is kept artificially high by the use of a peristaltic pump - and the purified solvent is then 'tapped off' from the low concentration side.

    The artificial pressure differential is maintained in order to resist natural osmosis.


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    Soutie Langeveldt's Avatar
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    Originally posted by PY
    Osmosis was the basis of my GCSE Biology coursework couple of years ago. Well boring.
    Yeah that same coursework that ive just done..... Notes went in a big oil drum and were blown up using god knows what chemicals last night....
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    Hall of Fame Member luckyeddie's Avatar
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    Originally posted by Langeveldt
    Yeah that same coursework that ive just done..... Notes went in a big oil drum and were blown up using god knows what chemicals last night....
    Ah, the school science lab.

    Where I learnt how to make nitrogen tri-iodide.

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    Request Your Custom Title Now! Mr Mxyzptlk's Avatar
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    Originally posted by PY
    Textbook definition of diffusion is:

    "A substance moving from an area of its high concentration to an area of its low concentration through a semi-permeable membrane"
    Au contraire. Osmosis is through a semi-permeable membrane. In diffusion, such a membrane is absent.
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    Hall of Fame Member luckyeddie's Avatar
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    To clarify this, in Osmosis, the solute (the stuff that's dissolved') doesn't move anywhere, but the solvent does.

    Diffusion is a totally different animal - and does, as has been mentioned, involve the migration of a substance through another substance (or barrier), but the standard definition given is not complete. It's not 'fluids' in the normal 'liquidy' sense, but in the 'fluid dynamics' sense. Misleading, huh?

    Examples :

    Gas in a tyre.
    Air can 'bleed' through rubber (your tyre will lose pressure over time even if 'perfectly' sealed). This can be clearly demonstrated with a child's helium balloon. It will deflate in 5-10 days - no problem. Even the metallised ones.

    Solids in polythene.
    Get a polythene bag, fill it with a brightly-coloured solid like turmeric (I first noticed this phenomenon with a drug called 'Danthron' when I worked in the Pharmaceutical industry 30 years ago). Seal the bag. Leave it a couple of weeks - the outside of the polythene pag will become stained yellow and you will be able to transfer this on to your hand quite easily. I was able to demonstrate this a few years ago with saffron - but that's a bit more expensive.

    Incidentally, did you know that glass is not a true solid at room temterature? Nor is pitch. Both will flow over time. A window over 100 years old is always thicker at the bottom and thinner at the top.

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    Hall of Fame Member luckyeddie's Avatar
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    Originally posted by marc71178
    Science? Boring? Never.

    Oh, all right then.

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    U19 Captain R_Powell_fan's Avatar
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    Another Topic:-

    ESP or "Extra Sensory Perception", and the idea that all Human beings have it, but some people have better control over it, clairvoyance, telepathy....etc.

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    Request Your Custom Title Now! Mr Mxyzptlk's Avatar
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    I don't believe in natural telepathy

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    Hall of Fame Member luckyeddie's Avatar
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    Originally posted by R_Powell_fan
    Another Topic:-

    ESP or "Extra Sensory Perception", and the idea that all Human beings have it, but some people have better control over it, clairvoyance, telepathy....etc.
    The James Randi Educational Foundation has put up a prize of one million dollars for ANYONE who can prove any evidence of paranormal activities.

    The original prize of 10 thousand dollars was offered many years ago (at least 15) - and the standard answer from charlat sorry, clairvoyants, was that 'it wasn't worth giving up their time'.

    Five years ago, the prize was increased to a million dollars.

    Thus far, no-one has been able to even pass one of the preliminary tests.

    I can't understand why I even bothered typing this reply out.

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    Eyes not spreadsheets marc71178's Avatar
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    Originally posted by Mr Mxyzptlk
    I don't believe in natural telepathy
    I knew you were going to say that yesterday

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    Hall of Fame Member luckyeddie's Avatar
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    Originally posted by marc71178
    I knew you were going to say that yesterday
    Wait until Wednesday - I predict that DD will be claiming a million bucks

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