Simple Question: Soap & Water

Whenever I read instructions which calls for cleaning w/ Soap & Water, I always wonder exactly what kind of soap does the manfacturer mean? Walking down the cleaning products aisles of our local supermarkets one view Hand Soap, Dish Soap/Detergent, Tub & Tile Disinfectant, Laundy Detergent, Car Wash and so on - but never one product just labeled ‘Soap’.
I presume they mean ‘Multi-use Cleaner’ (liquid cleaners like Mr. Clean, which I have a bottle of in my kitchen), but even that seems to have different sub-categories.
What is the general agreed upon meaning of ‘Soap’, as in clean w/ Soap & Water…

First off, there is a difference between “soap” and “detergent.” I’ll let someone better-versed explain it.

But in the meantime, perhaps you could give us some examples of these items that are supposed to be cleaned with “soap and water.” It would be best if you could type in the exact wording.

What’s the context? Hand soap is for washing your hands, laundry detergent is for washing laundry, etc. What is it that you’re trying to wash?

Well, over the years there have been many different items, all requesting ‘soap & water’ - I was wondering if there was one particular default ‘popular’ meaning, as opposed to specific cleaning products for specific uses.
For example, to clean resin kit parts (for a model, say), wash in ‘soap & water’ - OK, this soap can be dish washing liquid, but would that be the default meaning?

No, it can’t. Soap means soap, not detergent. You want a plain hand soap with no additives, perfumes, moisturiser, or anything else. The brand like that here is Sunlight soap.

In North America, you want Ivory soap - the white bar; no scent, no additives. There are some fabrics that it’s the only safe thing for. liquid “soaps” and detergents are different animals altogether from soap, per se, but IANA chemist, so you need one of those to explain it. There are many other things that Murphy’s Oil Soap (which is a liquid) is the best choice for.

I do know that soap is manufactured from the reaction between fat (usually animal fat) and lye. Linkie. But to explain the significance of it all, we need that chemist. :slight_smile:

What they said. Bar soap, white soap flakes, glycerin soap, but not detergents (though Zest might work), & definitely not Mr. Clean.

Didn’t we do this recently? All soaps are detergents, but only some detergents are soaps.

Depending on what specifically you’re washing and your water quality, a non-soap detergent may be a better choice than a soap, since, you know, soap scums, and non-soap detergents generally don’t.

>First off, there is a difference between “soap” and “detergent.”

There is a large category of compounds and mixtures that is used for cleaning, cosmetics, compounding foods, lubrication, and other purposes, based on organic molecules having perhaps dozens of atoms, whose shape is longish rather than spherical and whose two ends have different kinds of affinities for other substances. They congregate at the interfaces between dissimilar substances. So, for instance, if you have oil and water that you try to mix together, the oil and water have little affinity for each other, and their molecules congregate only with their own kind; but if you add these molecules, having one end with a big affinitiy for water, and another end with a big affinity for oil, they will collect at the oil-water interface. They make the interface have less energy per area, that is less surface energy or surface tension. They are therefore called “surface active agents” or “surfactants” for short.

The old way of making surfactants, known long before molecular theory explained why it was useful, was to react hydrocarbons with alkalies. So, you prepare a solution of lye (sodium hydroxide) and you add fat or oil to it, and you get soap. This process is called saponification. If your alkalai is based on sodium, you get a “hard soap”, and if your alkalai is based on potassium, you get a “soft soap”.

Enter chemical synthesys. You can create all kinds of nice tidy consistent surfactant molecules in various chemical reactions. A very common one is a 14 carbon alkane chain with a hydrophylic end (meaning “water loving” or having an affinity for water). I don’t remember what the hydrophyllic end actually is - sorry, I never took any college chemistry and my highschool chemistry teacher got fired, and that was before we left Viet Nam. Anyway, surfactants with 14 C chains are called “laurel”, as in “sodium laurel sulphate” surfactant in your shampoo. Triton X-000 is a syrupy amber liquid that seems to me to be the ubiquitous plain surfactant every lab has on hand.

So, surfactants do all kinds of nice things. In washing, they change the interfacial energies at all the interfaces so that the water-solid interface and the water-oil interface pry their way between the oil and the solid, isolating the oil into little floating beads in a process called “roll up”. In compounding of things like foods, they make floating beads of oil stable in a water environment, and when that is their purpose they are called “emulsifiers”. In cosmetics, they may be used to form a somewhat continuous barrier on the skin that inhibits the evaporation of water, leaving the skin puffier, and when that is their purpose they are called “emollients”.

Functionally, the average soap or detergent is going to work approximately the same.

I have seen the sort of instruction you are referencing and agree it’s a common one. I have taken the purpose of it to be not just a positive instruction–“Use soap and water”–but also an implied negative one–“Do not use other cleaning agents (organic solvents, bleach, etc).”

In other words, it’s safe and appropriate to clean with a soap/detergent of your choice. Avoid using WD-40.

Any kind of soap will do. In most cases I’d use dish soap because of a less tendency to leave scummy residue. As others have mentioned, most sorts of household detergents are also soaps (except ammonia which IIRC nonetheless turns grease into soap).

(bolding mine)

Could you give a specific example of such a fabric? I’m honestly curious as to what and why this might be.

But you didn’t tell us about the water. What kind of water are you using? Fresh water? Salt water? Hard water? Soft water? Hot water? Cold water? How do you expect us to answer a question without the important facts?
:smiley: Hey, I’m just messing with you. This is the Straight Dope. You can’t ask a simple question around here. Ask how much is 2+ 2 and you’ll get a treatise on mathematics.

I think Cosmic Relief’s answer was the most direct. Use whatever soap best fits the job. i.e.: to remove latex paint from your hands use a bar of hand soap. To remove it from your paintbrush use a liquid soap.

It is true that by definition soaps are all made this way. When you get into the liquid soaps though they usually aren’t soaps but detergents. Sodium laurylsulphate is a detergent and is made by the “esterification”* of the alcohol with sulfuric acid. In a general sense I would always pick one of the sulfates over soap. Soap tends to form insoluble products with hard water so has a tendency to leave a residue also known as soap scum.

  • Technically it is a dehydration reaction that is very similar to an esterification.

IANAC but I believe this is incorrect; soap and detergent are two different things, distinguished by the chemical processes used to make them. Soap, like the fat & lye-based one like Grannie Clampett used to make, is not a detergent. **Napier **described detergents (such as sodium laurel sulphate), and that post does a good job of making the distinction, although the post does not actually use the word “detergent.”

Soaps and detergents are both, however, surfactants.

>Sodium laurylsulphate is a detergent and is made by the “esterification”* of the alcohol with sulfuric acid.

Thank you, Warm. It sounds like you know what you are talking about and maybe even have studied chemistry.

BTW, I think small articles like parts and little electronic gadgets say that “soap and water” bit so they can blow off the legitimate need to be able to clean products. Soap and water isn’t effective on lots of the things that get on little parts and gadgets, like label adhesive. Worse, the big challenge becomes getting rid of the soap by diluting and washing.

I like isopropanol - like in “rubbing alcohol” - for many of these situations. There is almost no little gadget that isopropanol will hurt, and it is very easy to apply in a limited way. You don’t have to clean it off; there is no residue from its drying. The premoistened felt squares packaged in foil, sold in drug stores for diabetics to sterilize injection sites, are great for this, and they are amazingly convenient.

Admittedly, isopropanol sometimes has no effect on label adhesive, and for that and other situations I wind up with Goof Off on a Q-Tip, Goof Off in a cup, kerosene, a nice light petroleum naptha, various ketones, dichloromethane, a 55 gallon drum of halogenated solvents (I got pulled out of one of these, half full, as a teenager, while cleaning a vacuum pump). There are all sorts of options, each with some spectrum of surfaces it will ruin.

>the post does not actually use the word “detergent.”

Yeah, actually I was a bit uncertain what it meant. A wikipedia article says it means “cleaning agent” and could be surfactant but could also be any combination of various other things. What I gather is that anything used as a cleaning agent is by definition a detergent, except that we don’t usually use the word when referring to some cleaning agents (such as ammonia solution for window cleaning, or such as chromic acid for cleaning lab glassware, or such as paint thinner for degreasing surfaces before painting).

That’s right. We have common speech and technically accurate descriptions. In common speech, it’s okay to say, “it’s just a theory!” But in technical contexts, “theories” are distinct from hypotheses. I suppose I could make the argument that ocean water can be a detergent too, but I’m not sure I’d really try to push it that far.

You have this backwards. Most household “soaps” are actually detergents.

Except for bathtub bar soap, very few things you might find in the “cleaners” aisle of the supermarket are soap-based.

Well, the answers so far have been informative, and I’m glad that at least one poster (Chief Pedant) has seen the same sort of instructions. I’m positive though that bar soap is definitely not what the manufacturer had in mind, since that’s pretty much a pain to make a nice soaking solution with (soap flakes…I have to see if they’re even sold around here - haven’t seen any in awhile, but then again I wasn’t looking).
I guess the easiest solution is to keep using what I have been using, which is first soaking the parts (resin, metal, plastic) in a dish soap and hot water ‘bath’ [‘soft’ tap water, Peanuthead], and then rinsing w/ cool water.
But while that’s covered, not sure what to do with certain machines and outdoor furniture, etc. - I distinctly remember a vaccum cleaner which stated clean the outside [plastic] cover w/ ‘Soap & water’.