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I can't think of anything else to say. All I can find via Google is that it's because it's harder to create lather with hard water, but that doesn't seem very convincing to me.

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Good question. If it’s called hard water because it’s hard to get a lather up, shouldn’t its antonym then be called easy water? There must be more to it than that. As a kid, I used to think was because hard water left a hard mineral residue, whereas soft water would dry into a soft, smooth ring. But you really don’t notice it that much from just one drying, only with many, many, many dryings. –  tchrist Feb 12 '13 at 21:57
This has a “why” to it, but when you boil it all down, the residue it leaves is not all that much different from the rest of them, really. More from Cecil & Harwich. –  tchrist Feb 12 '13 at 22:13
The very first sentence on Wikipedia says Hard water is water that has high mineral content (in contrast with "soft water").... –  Izkata Feb 13 '13 at 2:48
@Izkata That completely misses the point. The question is not one of what but of why. –  tchrist Feb 13 '13 at 11:39
@Izkata: Your point would make sense if it were called mineral water, rather than hard water. Besides which, sodium carbonate is also a "mineral" - and by most definitions it's a "solid", meaning it's "hard". But if you add more of that, you get soft water. So your "because it has more minerals in it than soft water" doesn't really "explain" anything - and as tchrist says, I'm not asking what is it?, I'm asking why it's called hard/soft. –  FumbleFingers Feb 13 '13 at 16:24

7 Answers 7

Hard water contains minerals that actually harden. Over time, they form stains and crusts on the pipes and vessels they contact. If you look at the inside of pipes that have carried hard water for decades, there are thick stonelike coatings.

Soft water does not exhibit such behavior.

See a discussion of hard water deposits and scale here.

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Or at least, not as much. But yes, I'm 99% sure that this is the simplest and most correct answer so far.. –  Izkata Feb 13 '13 at 2:46
It would be great to have a reference for it. So we know it is not just somebody's guess (logical as it is). –  GEdgar Feb 13 '13 at 15:28
@GEdgar See edit above. –  bib Feb 13 '13 at 16:11
Unfortunately, the reference does not claim to explain use of the word hard. –  GEdgar Feb 13 '13 at 16:43
@GEdgar I have seen it explained on commercial sites, such as this one, but not more authoritative references, except obliquely. –  bib Feb 13 '13 at 22:05

Hard water contains >160ppm of minerals (typically calcium compounds) and actually feels harder when drunk than soft water does (<160ppm). The earliest use in OED below would seem to support that origin rather than it being "hard" to form a lather.

14. a. Applied to water holding in solution mineral, especially calcareous, salts, which decompose soap and render the water unfit for washing purposes.

1660 F. Brooke tr. V. Le Blanc World Surveyed 18 : The water was sharp and hard, but nothing brackish.
1756 C. Lucas Ess. Waters i. 83 : Hard waters are the best for builders and plasterers.
1805 W. Saunders Mineral Waters 305 : A very hard water, curdling soap, and possessing a large portion of selenite and earthy carbonats.
1849 R. T. Claridge Cold-water Cure (1869) 85 : Hard water makes the skin rough, but soft water, on the contrary, renders it smooth.

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Other OED entries that somewhere (usually in the citations) mention either of hard water or soft water are: bottom, bunny, calciphilous, charge, detergent, feel, frappé, hard, lather, lean, lime, liquid, mixer, overrun, precipitator, quick, rather, sequester, soap, soapless, soft, soften, stare, staring, steeper, strangle, wash, water, and water sapphire. –  tchrist Feb 12 '13 at 22:17
Wines, for example, can be called "soft". Generally the opposite of "sharp/acid". I'm not sure I've ever heard "hard" being used metaphorically in terms of the taste of anything. So I don't know what you mean by "actually feels harder when drunk". –  FumbleFingers Feb 13 '13 at 2:04
Oddly enough, Brooke's usage is from a translation of a French work, where the phrase is L'eau en est aspre, & aucunement salée. Aspre is now obsolete but clearly seems to come from Latin asper meaning harsh or as Brooke wrote sharp. The word hard seems to have been wholly invented. –  Xerxes Feb 13 '13 at 3:12
@FumbleFingers This isn't necessarily related to taste, but I've heard the term hard liquor to refer to drinks with especially high alchohol content. –  Peter Olson Feb 13 '13 at 3:34
@Peter Olson: Also, hard cheese is older and thus drier than soft cheese. So in all these contexts, water/cheese/booze is harder when less "watery". In fact, I guess life is hard without water. –  FumbleFingers Feb 13 '13 at 4:45

The reasonable possibilities would seem to include:

  1. Hard water comes from the ground, which is hard, soft water from rain. (All the more reasonable when we consider times when people had less of an understanding as to why some ground-sourced water is soft).

  2. It's hard to get up a lather in hard water. (Conversely, it's easy, not soft to do so in soft water).

  3. The residue left behind by evaporating hard water, is hard in the physical sense.

  4. Vegetables, particularly hard legumes, do not soften as quickly in hard water. In particularly hard water, they may not soften at all unless you use baking soda (which results in the sort of soft flavour and nutrition-free vegetables that was so beloved of a previous generation, in some parts of the world). Blanching in very hard water can even harden soft fruit and vegetables.

  5. While some drinks benefit from hard water (it benefits some styles of beers, and hurts others), generally soft is preferred for tea-making. It's hard to make a good cup of tea with it.

Other qualities seem less likely; that it can sometimes taste nasty but the best-tasting waters are hard, that the waters of spas long considered to have health benefits are particularly hard, that it messes up modern central heating systems.

An interesting commentary on hard water from 1751, Francis Home's An Essay on the Contents and Virtues of Dunse-Spaw argues:

To be ſoft is one of the chief properties in the offices of common life. The hardneſs of water, I find, depends moſtly, if not entirely, on ſea ſalt : for water, that eaſily diſſolves ſoap, is hindered from doing it by a mixture of ſea ſalt ; even after ſoap is diſſloved, the equal ſolution is curdled by an addition of that ſalt. As the ſame effect happens betwixt ſalt of tartar and ſea ſalt, the curdling of ſoap muſt, ſtill be owing to the action of theſe two.

The interesting thing is how wrong they are! Now, they're clearly on the right mark in noting that salt of tartar and sea salt both have the hardening effect, but he's blaming sodium chloride for the hardness common to water, and not suspecting that it could be another salt again.

He defines "soft water" thus:

The criterions of ſoft water are, that it brews well; boils peaſe without making them hard; diſſolves soap equally; and washes dirt entirely out of linnen.

As we would expect, having found he does not have a clear picture of the cause of hardness, he focuses on the effect not the cause.

It would seem from this, and other texts of the time that scaling wasn't much considered. That soft water from rain sources was used in some cooking tasks could explain this, as it would reduce the amount of scaling witnessed, but also complicate the question of which water was resonsible. This also in itself speaks to the most likely origin of the word; he was not the only person to note the effect on "peaſe", but it was well-known to cooks (and remains culinary knowledge to this day).

(Alas I cannot find references for the belief that hard water blocks "pores" in vegetables, nor the advice to soak beans in soft but cook them in hard for better colour or favour, though I'd like to as the foodie in me likes the idea of the dedication that uses both well-water and rain-water for the same item [just add some salt! Jeez!]).

In all, while I could just about credit the ground = hard possibility, it seems that the effect on vegetables is the most likely source:

Soft water makes vegetables softer on boiling and soaking than hard, and very hard water can even harden some.

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The bit about vegetables not softening so much when boiled in hard water seemed so odd to me I asked my friend who's a professional chef. He tells me adding salt makes them soften quicker, but doesn't know if this might be caused by the salt increasing the boiling point of the water. About then we got sidetracked by the problem of making a decent cup of tea up a mountain... –  FumbleFingers Feb 14 '13 at 0:01
Having cooked lentils in a city with very hard water, I can assure you that without a little soda you can cook them for a day and they will still be crunchy. –  Andrew Lazarus Feb 15 '13 at 23:12

The earliest reference I could find (not including that Brooke reference, which I think is erroneous) is 1712 in the The Natural History of Northamptonshire by John Morton. He says:

The common springs that have a little, slight Tincture of Vitriol, which with us are usually called Hard and harsh Waters, are vulgarly known to be such by their not taking so speedy or so lasting a Lather with Soap; by their turning and curdling of Milk in case a smaller Quantity of Milk is infus'd upon a much larger Quantity of the Boyling Water; by their requiring a larger Quantity of Malt to make the Drink, that's brew'd with it, of equal Strength to that which is made with softer Waters; and by their not boyling Pease, and other Garden Stuff so quickly or so soft and tender, as do those of a milder Nature. A Person of a nice Palate may distinguish them by the Tast; And to one who washes with them, whose Skin is of a finer Texture, they are sensibly harsh, and leave a little Roughness on the Hands and Face.

Harsh water actually seems to be an earlier term for the same thing, being found as early as 1667 in The History of the Royal-Society of London, for the Improving of Natural Knowledge by Thomas Sprat (earlier if you count the l'eau aspre) and being the opposite of soft water. Perhaps hard water was derived from soft water as being more clearly its opposite.

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The mention of skin, and your suggestion of soft as earlier makes me think of folklore around rain or dew on particular days. Suggesting that an association with the real benefits of soft water in these cases was long made, albeit mixed with less scientific claims about it being particularly good on Mayday morning, etc. Far from conclusive, though. –  Jon Hanna Feb 14 '13 at 11:15

There is an actual difference in the feel of hard and soft water. Soft water feels softer and flows differently from hard water.

I live in a place with very hard water. When I travel to places with soft water, I notice this difference right away when I turn on the tap.

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Flows differently? You mean it has different viscosity and/or surface tension? –  tchrist Feb 14 '13 at 16:52
@tchrist: Yes. - –  Roger Dahl Feb 14 '13 at 17:39
Like @Roger, I have lived mostly in hard water areas. Soft water feels soft and soapy, compared with hard water. My first experience of it puzzled me, because after washing my hands it felt like I could not get the soap off. Also, hard water has a kind of 'bite' to it which is lacking when drinking soft water. –  Bobble Feb 19 '13 at 18:26

The 'tough to work up a lather' sense is right, though not quite so literally. The hardness and softness refer, in this case, to the words' scientific senses- to how "hard", or "soft", the surface of the water is.

(I'm afraid this answer really is more school-level scientific explanation than erudite etymology, but here it is :-)

The conventional method -till the early part of the 20th century, at least- to determine hardness of water was to add a little soap/soap solution/soap oil to a water sample and work up a lather (typically by vigorously shaking a cone-shaped beaker to which a drop of soap liquid had been added. The practice continues in many school science fairs to date- presumably because it is simple and requires no fancy scientific equipment).

How easily -or not- water lathers depends on how hard, or soft, the water,or more precisely, its surface is. The "hardness" or "softness" of the surface of water is referred to as Surface Tension-this is what "holds water together" - an index of how adhesive/cohesive water molecules are to each other. For soap to dissolve, it has to first overcome the surface tension of water.

Surface Tension is also what enables a water-strider to walk on water (visual demo and interesting article here)

If the surface tension is high, or if the surface is "hard", the soap and the water don't readily mix, which means the water doesn't lather easily, and you have water that can be termed hard. On the other end of the hardness index/scale, obviously, we have soft, referring to how easily the soap additive breaks the surface, permeates the water, and turns into lather.

EDIT : Here's another reference link that explains the same concept in brief-http://answers.ask.com/Science/Chemistry/what_is_water_hardness

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Does that imply pond skaters, for example, are able to grow bigger and fatter in hard water areas? :) –  FumbleFingers Feb 13 '13 at 15:15
@FumbleFingers:-) I found a wiki page, and have linked to that now, so both links in my answer are now updated. FYI. –  Autoresponder Feb 14 '13 at 10:00
The scientific way to determine temperature is to use a thermometer, but we don't have the word hot from "that which makes mercury expand in a tube". –  Jon Hanna Feb 14 '13 at 10:39
@Thanks for the kinds words. I think this one is going to be a case where all we can do is add more possibilities and then arguments and counter-arguments for and against. I'm liking Xerxes idea that "soft" came first, because the idea that rain and dew water is better for skin seems to be very old, and "soft/hard" seems to have first referred to "rain/well" before people understood why some well water is also soft. So that it's related to soft skin is not unreasonable. –  Jon Hanna Feb 14 '13 at 11:08
@Jon Hanna: Sure. I also like Andrew Leach's post. I think his interpretation is not just straight to the point, but also derives the etymology straight out of human nature, which makes it sound very plausible-"drink the water and tell if it is hard or soft." –  Autoresponder Feb 14 '13 at 11:25

Ever tried washing your hair in soft water? I did when I lived in a tiny Italian village whose source of water came directly from the nearby mountains. My hair never felt softer. And I'm pretty sure that clothes and linen felt softer too.

Now I live in a city centre, and my British kettle has that thick, hard greyish limescale; the taps have that tough residue of calcium around the spout, which is really difficult to shift, and the bathroom radiator hisses because of deposit and sludge slowly building up.

Why is water called hard or soft? The deposit of calcium or limescale is hard to the touch, and a challenge to get rid of. Whereas, soft water aptly describes the gentleness and kindness it has on our skin, hair and digestive systems. (Minestrone tastes better too, cooked in soft water, see Jon Hanna's answer.)

Moreover, think of hard liquor versus soft drinks. You would never call a pop, soda or fizzy drink (BrEng) easy, would you?

Well, that's my theory. I have no backup to support my assertions.

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Hydrochloric acid (a.k.a. 'muriatic acid') added to the water in a kettle will quickly dissolve limescale (especially when the water is heated), even when relatively dilute. (Note: the kettle needs to be rinsed afterwards, though hydrochloric acid is corrosive to unprotected tissue rather than being poisonous; it's the same acid that we all have in our stomachs.) –  Erik Kowal Sep 5 at 4:12
@ErikKowal Thanks for the tip. I've tried with vinegar, and the "anti-calcare" products in Italy but without much success. –  Mari-Lou A Sep 5 at 4:21
Yes, I too have found that other products are very feeble compared with hydrochloric acid. BTW, it is also very effective for cleaning toilets and removing limescale encrustation on taps -- though be careful using it on metals. For instance, aluminium will be corroded by it. If in doubt, try it first on a small area and evaluate the effect. Use rubber gloves -- preferably made of neoprene -- to protect your hands, and quickly rinse off any splashes on skin with plenty of cold water. –  Erik Kowal Sep 5 at 5:18
I will try it out first on the shower head, the water jets in all sorts of weird directions. Thanks again, good stuff to know. –  Mari-Lou A Sep 5 at 5:25
You will be pleasantly surprised at its effectiveness. 1) You can also speed things up, if necessary, by using a sewing pin to poke through the holes and loosen the scale. 2) An old toothbrush is also useful for removing scale, though its bristles should not be exposed to undiluted acid -- they will go stiff. 3) Finally, hydrochloric acid plus hot water and a plunger is a much more effective declogger of grease-clogged drains than products containing alkaline products like sodium hydroxide. (I've tried both.) But hydrogen sulphide gas is sometimes given off by this -- be prepared to ventilate. –  Erik Kowal Sep 5 at 6:29

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