Why Soap Makes Scum Before It Makes Lather

Because one tap delivers soft water and the other hard water, and hard water destroys soap before it can foam.

Hard water contains dissolved calcium and magnesium salts. Soap reacts with those first, forming an insoluble, greyish, sticky curd — the scum you see on a bucket rim or a bathroom tile. Only once every last calcium and magnesium ion has been used up does any soap remain to make lather.

So nothing is wrong with the soap. The water is consuming it. This page covers the fourth part of the ICSE Class 8 Chemistry chapter on water: identifying hardness, its two kinds, how each is removed, and what water pollution does.

The lather test, which needs nothing but soap solution and two test tubes.

Take equal volumes of the two water samples. Add soap solution drop by drop, shaking after each drop, and count the drops needed to produce a lasting lather.

- Soft water lathers readily, with few drops and no scum.
- Hard water needs many more drops, and forms a white or grey scum before any lather appears at all.

The number of drops is therefore a rough measure of how hard the water is, which makes this a comparison test as well as an identification.

What causes hardness. Dissolved salts of calcium and magnesium:

- Bicarbonates — $\text{Ca}({\text{HCO}}_3{)}_2$ and $\text{Mg}({\text{HCO}}_3{)}_2$
- Sulphates — ${\text{CaSO}}_4$ and ${\text{MgSO}}_4$
- Chlorides — ${\text{CaCl}}_2$ and ${\text{MgCl}}_2$

All six are calcium or magnesium salts, and no other metal is involved. That is why hardness is often described simply as the presence of calcium and magnesium ions.

Where they come from. Rain water is soft, because evaporation left every dissolved salt behind. It becomes hard on the ground: as it percolates through rocks such as limestone, chalk, gypsum and dolomite, it dissolves them. Rain water is naturally slightly acidic from dissolved carbon dioxide, and that acidity is what attacks the limestone.

So which waters are soft and which hard. Rain water and distilled water are soft. River, well, spring and sea water are hard, sea water being hardest of all. A well sunk into limestone gives harder water than one in granite, which is why hardness varies so much between neighbouring towns.

The misconception to clear. Hardness has nothing to do with how the water feels, its temperature or how much of it there is. It is purely about which salts are dissolved — which is why the lather test, and not touch or taste, is the test.

Temporary hardness can be removed by simply boiling the water. Permanent hardness cannot.

Temporary hardness is caused by the bicarbonates of calcium and magnesium — $\text{Ca}({\text{HCO}}_3{)}_2$ and $\text{Mg}({\text{HCO}}_3{)}_2$.

These are unstable to heat. Boiling breaks them down into insoluble carbonates, which settle out and can be filtered off:

$$\text{Ca}({\text{HCO}}_3{)}_2\to {\text{CaCO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$$

$$\text{Mg}({\text{HCO}}_3{)}_2\to {\text{MgCO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$$

Since the calcium has left the solution as a solid, the water is now soft. It is also called carbonate hardness.

Permanent hardness is caused by the sulphates and chlorides of calcium and magnesium — ${\text{CaSO}}_4$, ${\text{MgSO}}_4$, ${\text{CaCl}}_2$ and ${\text{MgCl}}_2$.

These are stable to heat. Boiling does nothing at all to them; the water is just as hard when it cools. Removing them needs a chemical method, because something must be added to convert them into insoluble carbonates. It is also called non-carbonate hardness.

How to identify which you have. Boil a sample and test it again. If the lather test now shows soft water, the hardness was temporary. If it is still hard, the hardness was permanent — or at least partly so.

Most natural water contains both, so boiling usually improves it without softening it completely, and the remaining hardness is the permanent part.

The link that makes this predictable. Temporary hardness yields to heat because a bicarbonate is unstable and decomposes, exactly as limestone does in the chapter on decomposition reactions. Permanent hardness resists because a sulphate or chloride is stable and has no decomposition to offer. So the classification is really a statement about the salt, not about the water.

Four methods, and which ones work depends on the kind of hardness.

Boiling — temporary hardness only. As shown above, the bicarbonates decompose to insoluble carbonates which are filtered off. Cheap, simple, and useless against permanent hardness. The carbonate left behind is exactly the scale that furs up a kettle.

Clark's method — temporary hardness only. A calculated quantity of slaked lime, $\text{Ca}(\text{OH}{)}_2$, is added:

$$\text{Ca}({\text{HCO}}_3{)}_2+\text{Ca}(\text{OH}{)}_2\to 2{\text{CaCO}}_3+2{\text{H}}_2\text{O}$$

Both the calcium already in the water and the calcium added come down as insoluble carbonate, which is filtered off. The word calculated matters: too much slaked lime would itself make the water hard again, so the quantity must match the hardness present.

Washing soda — both kinds. Sodium carbonate, ${\text{Na}}_2{\text{CO}}_3$, converts the offending calcium and magnesium salts into insoluble carbonates while leaving harmless soluble sodium salts behind:

$${\text{CaSO}}_4+{\text{Na}}_2{\text{CO}}_3\to {\text{CaCO}}_3+{\text{Na}}_2{\text{SO}}_4$$

$${\text{MgSO}}_4+{\text{Na}}_2{\text{CO}}_3\to {\text{MgCO}}_3+{\text{Na}}_2{\text{SO}}_4$$

$${\text{CaCl}}_2+{\text{Na}}_2{\text{CO}}_3\to {\text{CaCO}}_3+2\text{NaCl}$$

$$\text{Ca}({\text{HCO}}_3{)}_2+{\text{Na}}_2{\text{CO}}_3\to {\text{CaCO}}_3+2{\text{NaHCO}}_3$$

Note these are all double displacement reactions producing a precipitate, which is the reaction type from the chapter on chemical reactions.

Ion exchange — both kinds, and the method used industrially. The water is passed through a column of a resin or zeolite, sometimes called permutit, which carries loosely held sodium ions.

As the hard water flows through, the resin swaps its sodium ions for the calcium and magnesium ions in the water. The calcium and magnesium are retained on the resin and sodium enters the water, and sodium salts do not cause hardness.

When the resin is exhausted it is regenerated by flushing it with strong brine, which reverses the exchange and restores its sodium. This reusability is why ion exchange is used for boiler feed water and in domestic softeners.

Which method for which question. If the hardness is temporary, boiling or Clark's method will do. If it is permanent, only washing soda or ion exchange will work. And distillation removes every dissolved salt, so it gives the softest water of all — but it is far too expensive for anything but laboratory use.

Hard water has genuine advantages as well as drawbacks, which is why softening is not always the right answer.

Advantages:

- It tastes better than soft water, because of the dissolved salts.
- It supplies calcium, useful for bones and teeth.
- It is used in brewing and tanning, where the salts are needed.
- It is less likely to dissolve lead from old pipes, because a thin protective carbonate layer forms inside them.

Disadvantages:

- It wastes soap and leaves scum on clothes, tiles and bathtubs.
- It deposits scale in kettles, geysers, boilers and pipes, which wastes fuel and can eventually block or burst a pipe.
- Food, especially pulses, takes longer to cook in it.
- It spoils the colour and texture of washed fabrics.

Water pollution is the contamination of water by substances that make it unfit for its intended use.

Its main causes:

- Domestic sewage — untreated drains carrying human waste and detergents
- Industrial effluent — factory discharge containing acids, alkalis, dyes and heavy metals such as lead and mercury
- Agricultural run-off — excess fertilisers and pesticides washed off fields by rain
- Oil spills from ships and pipelines
- Thermal pollution — hot water discharged from power stations and factories
- Immersion of waste and idols in rivers, and open bathing and washing at riverbanks

Its effects:

- Waterborne diseases — cholera, typhoid, dysentery, jaundice, from sewage-contaminated drinking water
- Eutrophication — fertiliser run-off feeds an enormous growth of algae, which then dies and decays, using up the water's dissolved oxygen and suffocating fish
- Death of aquatic life, and thermal pollution doing the same by a different route, since warm water holds less dissolved oxygen as the solubility section showed
- Accumulation of heavy metals and pesticides in fish, and so in the people who eat them
- Loss of usable fresh water, from the very small share the first part of this chapter described

The point worth drawing out. Hard water is naturally occurring and merely inconvenient; polluted water is caused by us and genuinely dangerous. Confusing the two is a common error — hardness is dissolved calcium and magnesium from rock, and no amount of hardness makes water unsafe to drink.