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Thread: Cold water/ hot water

  1. #1 Cold water/ hot water 
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    Can anybody tell me what the scientific principles and evidence are behind the fact that we wash with hot water, or at least have been "trained"(lol) to use hot water instead of cold water. Would cold water do just a good job, would it clean aswell? would using hot water on say something that is usually cleaned with cold water , you car, for example give it a better clean?
    Would shaving with cold water not do a better job, and would showering or bathing in a cold bath (not for medicinal properties) but too clean as usual as you would with hot water, clean you any better or any worse, or would it be the same. Is the structure of water molecules all the same hot or cold and even if they are not, does it make a diference in cleaning dirt of a body or lavering soap up to work better?
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  3. #2  
    Bullshit Intolerant PhDemon's Avatar
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    This is off the top of my head but the main thing a small temperature change (from say cold water tap to hot shower) is solubility and reaction kinetics. The structure of water molecules is the same at all temperatures (it is always H2O), temperature just affects how fast the molecules move (if there are no phase transitions).

    For washing, soap will react with and dissolve muck better at higher temperatures, if the temperature is high enough (like in steam cleaning) you sometimes don't need soap at all as the muck will dissolve in the hot water without soap (not recommended for people though .
    For shaving, don't know, I've had a beard since I was 17.


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  4. #3  
    ▼▼ dn ʎɐʍ sıɥʇ ▼▼ RedPanda's Avatar
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    I thought that cleaning with hot water simply helps emulsification and is more comfortable for us. (Cold showers are not my favourite thing.)
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  5. #4  
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    Hot water is mainly to increase the effectiveness of the soaps. In this regard it makes a significant difference in the reduction of bacteria on the skin. Cool mist cleansing was experimented with by R Buckmister Fuller and found to be very effective. Apparently the increased surface area of the water particls in a very fine mist under moderate air pressure cleaned the skin well using only about a pint of water to clean an adult head to toe. I imagine the effect is sort of like a low pressure sand blasting with water droplets instead of sand. He discovered this in the navy when he noticed that a half hour of standing on the deck of a destroyer cruising through north atlantic fog melted engine room gerease right off.
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  6. #5  
    Forum Radioactive Isotope MagiMaster's Avatar
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    I've actually heard that the difference between hot tap water and cold tap water (when used with soap) is insignificant. I'd be interested to see some actual tests to determine whether or not that's true. (I can't remember now whether or not that was only in regards to bacteria or if it included oil/grease as well.)
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  7. #6  
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    Quote Originally Posted by MagiMaster View Post
    I've actually heard that the difference between hot tap water and cold tap water (when used with soap) is insignificant. I'd be interested to see some actual tests to determine whether or not that's true. (I can't remember now whether or not that was only in regards to bacteria or if it included oil/grease as well.)
    Interesting, but do you have a source for the idea that cold water is just as good? I'd have thought PhDemon must be right about solubility at least. Shaving with cold water always seems painful, possibly because the viscosity of the lubricating film provided by shaving soap would be higher. I also have the feeling hot water softens the bristles more - or more rapidly - than cold.

    Also if you try to wash up plates in tepid water you find the grease does not come off as well.

    So all in all I am a bit sceptical that hot water is used only for comfort rather than effectiveness.
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  8. #7  
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    it depends on the job say i throw ice on a hot pan.. thermal shock breaks down caked on carbon
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  9. #8  
    Bullshit Intolerant PhDemon's Avatar
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    Just more made up bullshit from fiveworlds.
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  10. #9  
    Forum Radioactive Isotope MagiMaster's Avatar
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    Quote Originally Posted by exchemist View Post
    Quote Originally Posted by MagiMaster View Post
    I've actually heard that the difference between hot tap water and cold tap water (when used with soap) is insignificant. I'd be interested to see some actual tests to determine whether or not that's true. (I can't remember now whether or not that was only in regards to bacteria or if it included oil/grease as well.)
    Interesting, but do you have a source for the idea that cold water is just as good? I'd have thought PhDemon must be right about solubility at least. Shaving with cold water always seems painful, possibly because the viscosity of the lubricating film provided by shaving soap would be higher. I also have the feeling hot water softens the bristles more - or more rapidly - than cold.

    Also if you try to wash up plates in tepid water you find the grease does not come off as well.

    So all in all I am a bit sceptical that hot water is used only for comfort rather than effectiveness.
    Unfortunately I don't. Also, as far as I remember, that was only in relation to hand washing. I don't remember hearing anything either way about shaving or washing other things.
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  11. #10  
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    I really depends on what you're washing and what happens next. For washing your body there's not much of a problem, because the surface of your skin is always being renewed and there's always some friction between your skin and your garments - so anything that's missed will eventually come off. Either at a future washing or rubbed off while the skin is more or less dry.

    For clothes and linen it's a very different matter. Because hot water sets protein and because fabrics are permeable. So any fabric that has food spills on/ in it along with any sweat, blood or other body excretions needs to be soaked or washed/rinsed in cold water to dilute any protein content in gravy or blood. Only then should hot water be used to get rid of any oil or grease.

    For a car? There are all sorts of things in the dirt on a car. One of the preventive measures is to keep as many surfaces as possible as well polished / coated as possible so that as little material as possible can actually get into the surface. Just as a highly starched linen item like a tablecloth will allow dirt and some liquids to just run off the surface of an item that would otherwise soak it up, a well polished metal surface will do the same for bugs, road grime, dust and other materials - they can be rinsed or washed off the surface rather than scrubbed out of the substance. Same principle applies to wood furniture - a high polish repels liquids and allows dust to be merely dusted off the surface rather than penetrating or sticking to the grain of the wood.
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