What is Best Temp H2O for Rinsing Dishes? Also, Efficacy of Soap Suds.

  1. Jul 2, 2013 #1
    What is the best temperature water for rinsing dishes?

    I've googled this question, and gotten many answers, very few of which have any rational basis for them whatsoever. For example, some people say that it's better to use hot water, because the soap dissolves better in hot water. However, it's not at all clear to me that dissolving is the underlying mechanism for getting rid of the soap, and none of the answers I've seen (including here on PF) speak to that.

    Before I go on, let me say that quickness of drying is not on my radar screen here. I totally recognize that hot water, having more highly excited molecules, will evaporate quicker than cool water, given the same environment.

    I do know this: if I have just emptied a bowl of soapy water into my sink, and I'm trying to rinse the soap down the drain, cool water seems to work much faster than hot. Indeed, hot water seems to make a lot more suds than cool water.

    So, first: what is the underlying mechanism by which water removes soap? Is is sheer mechanical action, where the water simply displaces the soap out of the way? Or is it chemical (dissolving)?

    Second: what temperature water would make this underlying mechanism work faster?

    Third: Suppose the answer to my Second Part is cool water. What guarantee would I have that there isn't a soap residue left on my dishes?

    My second main question is this: is the efficacy of soap (that is, its ability to remove dirt) at all related to the amount of suds present?

    I hope that I have put this thread in the right place. My apologies if not. I am not a chemist, so please aim your explanations accordingly.

    Thank you!
  2. jcsd
  3. Jul 5, 2013 #2
    First: mechanical action is the primary factor, diffusion has too long of a time scale to be relevant.
    Second: cold water is better for removing a surface layer of soap/dirt on the dish because cold water is more viscous so it will have a greater sheering action. On the other hand, hot water has lower surface tension and more easily spreads over the surface of the dish whereas cold water is likely to bead up and not interact with the soap on the surface.
    Third: use a chelating agent with your soap: citric acid, like in shower cleaners. I haven't personally tried this to test its efficacy, but it could be viable. It's easy anc cheap to buy powdered citric acid in bulk from ebay.

    It's not a straightforward answer of hot vs. cold as you said, since dirt is much more soluble in hot water. As someone who does research on soap films, it's my opinion that scrubbing with baking soda is more effective than using soap ;]

    In case you're not aware, most dish soaps include enzymes that are good at removing things like egg from dishes, which can be helpful in many situations. Also dish soaps are typically about 5% ethanol for sanitizing (some use triclosan). Unless your dishes are absolutely filthy and greasy, half of the cleaning action can be accomplished by simply wiping them with your hand and some water. The widely held belief that soap's surfactants do the major part of the cleaning seems questionable. Mechanical action is the major factor, so suds, which are not very viscous, have little effect on the cleaning process and are just a side product of using surfactants. Overall, it's my feeling that hot water is still better, but if you plan to rinse dishes with soap and then water without any other mechanical action, you'll probably have soap scum regardless of water temperature.
  4. Jul 5, 2013 #3
    Wonderful! Thanks for all the info.

    I am confused on one point, though. For most of your post, you seem to be saying that cold water is more effective in lots of ways. But then, at the end, you say that it's your feeling that hot water is still better. How did you arrive at that decision? How are you weighting the factors?

    Again, many thanks.
  5. Jul 5, 2013 #4
    Hot water just feels soothing to the hands, which I like!
  6. Jul 5, 2013 #5
    Ha! So one more question: you say suds are not really any indicator of cleaning action. But they would be a good indicator of the presence of soap, right?

    I live in Houston, TX, USA, and it's summer here. So cool water is much more soothing to me right now. And if you tell me that cool water rinses dishes just as well if not better than hot water, I'm all for cool water. Now if I can just convince my wife of this...
  7. Jul 8, 2013 #6


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    Staff: Mentor

    As far as solubility and chemical reactions go, most proceed noticably faster in hot water ....... :wink:
  8. Jul 8, 2013 #7
    Yes, that is a very common answer on the Internet. However, it does not tell me a lot of more fundamental things such as do I want the chemical reaction to occur? If you read the above posts, you will see that colliflour believes that sheer mechanical action is mostly responsible for removing soap. I know for a fact that if I'm rinsing my sink out, cold water works considerably faster than warm, at least for removing suds. Hence my latest post: are suds a good indicator of the presence of soap? We can all think for half a second to realize that lack of suds is not a good indicator of the lack of soap.

    Now, if you want to argue that the primary mechanism responsible for removing soap during rinsing is chemical and not mechanical, I'd be interested to hear your argument.
  9. Jul 8, 2013 #8

    I like Serena

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    Homework Helper

    I know it's very difficult to get old food stuff off with cold water, while with hot water it almost slides off on its own.
    Furthermore, soap is designed to interact with dirt and fat, which works better at higher temperatures.
    I think you won't be able to get fat off dishes with cold water.
    Finally, high temperature will kill most bacteria.

    When I start thinking about bacteria and food stuff that are too small to see, I think I prefer hot water.
    Afterward one might use cold water to get rid of the suds.
  10. Jul 8, 2013 #9
    In general, the presence of suds is a good indicator of the presence of soap. Technically it is an indicator of a chemical which modifies the surface tension, many chemicals fall into this category (some sugars, etc) but mainly this category is surfactants from soap. Other components have the opposite effect of reducing suds even when there is a surfactant (soap) present. An example of this is alcohol. Lack of suds means that there is either not enough agitation/scrubbing to produce bubbles, or something is acting to rapidly burst the bubbles, such as alcohol.

    My argument for mechanical action over dissolving is that you're scrubbing. One surface touches another and what is stuck to the surface is mechanically moved off it and left floating in the surrounding water or (disgustingly) stuck to the sponge. Dissolving is more important in rinsing, but since the water is generally turbulent, not a smooth cohesive flow, you're dealing with a lot of mechanical action by the water. If a contaminant is stuck to the surface of a dish when you start rinsing, either the water's bulk velocity will be great enough to grab it and take it off the dish, or it will slowly dissolve into the stream of water. In this situation, the water grabbing the contaminant from the surface will be more effective with cold water, but the dissolving part will be better with hot water. Now speaking from experience as a chemical engineer i can say that in the rinsing situation the time scale for diffusion is long versus turbulent time scales. If you're waiting for diffusion (dissolving) to wash something off the surface of a dish, you'll probably be waiting for a matter of minutes holding that dish under the stream of water. Of course truth be told with an actual dirty dish scenario there are a huge variety of possible types and arrangements of contaminants on a dish surface and many types of surfaces so there is some guessing involved. If you're dealing with a dish that has a significant solid stuck on layer that you think will take a long time to get off, hot water could be more useful since you'll need the help of diffusion, and since you can rely on thermal expansion to separate the two dissimilar layers. Overall i believe dissolving has a smaller effect than mechanical action, so the benefits of cold water through decreased viscosity especially could in theory win over hot water's decrease in diffusion time.
  11. Jul 8, 2013 #10
    Right; hot water for the initial washing. That much is clear. I've always found hotter water more effective for washing. However, the subject of this thread is rinsing soap off of clean dishes, which is not necessarily the same thing.
  12. Jul 8, 2013 #11
    Well, I think you've convinced me that cold water is better for rinsing soap off of clean dishes, and you've shown me the underlying mechanism. So, thanks very much for all your efforts!

    Naturally, as I just replied to I like Serena, hot water is better for washing, and I intend to continue doing that, as I always have. But being able to use cold water for rinsing will be a boon in the heat of Houston.
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