Water

Lessons

Senior 1 · Chemistry

Water

5 lessons

Lesson 3 of 5

Categories of Water: Hard and Soft, Fresh and Salt, Drinking and Waste

25 min

Soap that will not lather

Wash your hands with the same soap in rainwater and then in water from some boreholes, and you notice a difference at once. The rainwater lathers easily. The borehole water gives little lather and leaves a grey, sticky film on the basin, and a kettle filled from it slowly grows a hard white crust inside. The water looks equally clear in both cases. What is dissolved in it that you cannot see?

Categories of water

Water is sorted into categories by what is dissolved or suspended in it and by what it can be used for. One sample can belong to several categories at once: rainwater is fresh, soft and, after treatment, drinkable.

Table 3.1: Categories of water and their characteristics

Category	Main characteristics	Examples
Fresh water	less than about 1 g of dissolved salts in each litre; tastes fresh	rain, rivers, lakes and groundwater; all of Rwanda's lakes and rivers
Salt water	about 35 g of dissolved salts, mostly sodium chloride, in each kilogram; salty taste; not drinkable without treatment	sea water in the Indian Ocean, the sea nearest to Rwanda
Hard water	contains dissolved calcium and magnesium ions; lathers poorly with soap; may form scale when boiled	some borehole and well water
Soft water	contains few calcium or magnesium ions; lathers easily	rainwater, distilled water
Drinking (potable) water	clear, colourless and odourless; free of germs and of harmful amounts of chemicals	treated tap water; boiled water (Lesson 4)
Waste water	water that has been used and contains soap, food, faeces, oils or chemicals	grey water from washing, sewage from toilets, effluent from factories

Definition

Hard water: water that does not easily form a lather with soap because it contains dissolved calcium ions, $\mathrm{Ca^{2+}}$ , and magnesium ions, $\mathrm{Mg^{2+}}$ . Soft water contains few of these ions and lathers easily. Example: rainwater is soft; water from a well dug through limestone is usually hard.

Experiment: the lather test

Put 10 cm³ of each water sample into a separate test tube. Add soap solution one drop at a time from a dropper, stopper the tube and shake it for 10 seconds after each drop. Count the drops needed to give a lather that lasts for 30 seconds. Then boil a fresh portion of each sample for five minutes, let it cool, top it up to 10 cm³ with distilled water, and repeat.

Figure 3.1: The lather test. Hard water needs much more soap to give a lasting lather.

Warning

Safety. Use a test-tube holder and let boiled samples cool before you stopper and shake them: a stoppered tube of hot water can push its stopper out. Never drink laboratory water samples, even if they look clean.

What makes water hard?

Rain dissolves a little carbon dioxide as it falls, which makes it very slightly acidic. When this water soaks through rocks such as limestone (calcium carbonate), it slowly dissolves them and carries away calcium ions:

$\mathrm{CaCO_3(s) + H_2O(l) + CO_2(g) \rightarrow Ca(HCO_3)_2(aq)}$

Water flowing through rocks that contain gypsum (calcium sulfate) or dolomite (calcium magnesium carbonate) picks up calcium and magnesium in the same way. So whether your water is hard depends on the rocks it has passed through: groundwater from wells and boreholes is more likely to be hard than rainwater or river water.

Soap reacts with the calcium and magnesium ions to form an insoluble grey-white solid called scum:

soap + calcium ions → scum (insoluble)

Until all these ions have been used up as scum, no lather can form. That is why hard water wastes soap.

Definition

Temporary and permanent hardness: temporary hardness is caused by dissolved calcium and magnesium hydrogencarbonates and is removed by boiling; permanent hardness is caused by dissolved calcium and magnesium sulfates and chlorides and is not removed by boiling. Example: water hardened by limestone has temporary hardness; water hardened by gypsum has permanent hardness.

Removing hardness

Boiling removes temporary hardness. The hydrogencarbonate breaks down, and insoluble calcium carbonate settles out as a white solid, scale (or limescale), on the inside of kettles, pans and pipes:

$\mathrm{Ca(HCO_3)_2(aq) \rightarrow CaCO_3(s) + H_2O(l) + CO_2(g)}$

Adding washing soda (sodium carbonate) removes both kinds, because it turns the dissolved calcium into insoluble calcium carbonate:

$\mathrm{CaSO_4(aq) + Na_2CO_3(aq) \rightarrow CaCO_3(s) + Na_2SO_4(aq)}$

Distillation and the ion-exchange columns in water softeners also remove both kinds.

A cut hot-water pipe whose inside is lined with a thick crust of scale. Scale narrows the pipe and, because it conducts heat badly, wastes fuel in boilers and kettles. Photo: Александр Юрьевич Лебедев, public domain, via Wikimedia Commons

Table 3.2: Hard and soft water compared

Property	Hard water	Soft water
Dissolved calcium and magnesium ions	many	few
Lather with soap	poor; needs much soap	good; needs little soap
Scum	forms	does not form
Scale on boiling	forms if the hardness is temporary	does not form
Taste	often preferred: it tastes of its minerals	can taste flat
Effect on health	supplies some calcium and magnesium; no convincing evidence of harm	no calcium or magnesium from the water
Effect on pipes and kettles	scale blocks pipes and wastes energy	no scale, but can slowly dissolve metal pipes

Worked example

Problem. Figure 3.2 shows a class's results for five water samples. Which samples are soft? Which have temporary hardness, which permanent, and which both?

Figure 3.2: Drops of soap solution needed for a lasting lather, before and after boiling.

Worked example

Solution

A and B need only 2 or 3 drops, before and after boiling: they are soft.
D needs 20 drops, but only 5 after boiling: boiling removed most of its hardness, so it is mainly temporary hardness.
E needs 18 drops and still 17 after boiling: boiling hardly helped, so its hardness is permanent.
C drops from 12 to 6: boiling removed part of the hardness, so it has both kinds.

Answer: soft: A, B; temporary: D; permanent: E; both: C. Always compare the two bars for each sample, not the heights of different samples.

Worked example

Problem. On boiling, 162 g of calcium hydrogencarbonate gives 100 g of calcium carbonate scale. A borehole water contains 0.243 g of calcium hydrogencarbonate in each litre. A school kitchen boils 40 L of it every day. What mass of scale forms in 5 days?

Solution.

Scale from one litre: $0.243\,\mathrm{g} \times \frac{100}{162} = 0.150\,\mathrm{g}$
Water boiled in 5 days: $40\,\mathrm{L} \times 5 = 200\,\mathrm{L}$ .
Scale: $0.150\,\mathrm{g} \times 200 = 30\,\mathrm{g}$ .

Answer: 30 g of scale, enough to coat the bottom of the pan. That is why kitchens with hard water have to descale their boilers.

Warning

Common mistake: thinking that hard water is dirty or unsafe. Hardness is caused by dissolved minerals, not by dirt or germs. Hard water can be perfectly clear and safe to drink, and soft water can be full of germs.

Tip

"Boiling beats bicarbonate." Boiling removes hardness caused by hydrogencarbonates (temporary). Sulfates and chlorides stay (permanent) and need washing soda.

FuseSchool: Hard and soft water (4 min). Watch for how scum forms and why it wastes soap.

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Practice

Name the two ions that make water hard.
Give one example of soft water and one of salt water.
Why does hard water waste soap?
Write the equation for the removal of temporary hardness by boiling.
A water sample needs 16 drops of soap before boiling and 16 after. What kind of hardness does it have, and how could it be softened?
Give one advantage and two disadvantages of hard water.
What is the difference between waste water and salt water?
A family boils water with 0.162 g of calcium hydrogencarbonate per litre. What mass of scale forms from 50 L? (162 g gives 100 g of scale.)

Show answersHide answers

Calcium ions, $\mathrm{Ca^{2+}}$ , and magnesium ions, $\mathrm{Mg^{2+}}$ .
Soft: rainwater or distilled water. Salt: sea water.
The soap reacts with the calcium and magnesium ions to form scum, and no lather forms until all those ions have been used up.
$\mathrm{Ca(HCO_3)_2(aq) \rightarrow CaCO_3(s) + H_2O(l) + CO_2(g)}$
Permanent hardness, because boiling did not help; soften it with washing soda (sodium carbonate), by distillation, or by ion exchange.
Advantage: it tastes better or supplies calcium. Disadvantages: it wastes soap and forms scum; it forms scale in kettles and pipes.
Waste water has been used and carries soap, food, faeces or chemicals; salt water is natural water with a lot of dissolved salt, such as sea water.
$0.162 \times \frac{100}{162} = 0.100\,\mathrm{g}$ per litre, so $0.100 \times 50 = 5.0\,\mathrm{g}$ .

Sources and credits

Hard water (its causes, temporary and permanent hardness, scum and scale, softening, and its health effects), Wikipedia contributors, Wikipedia, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
Image: Limescale-in-pipe.jpg by Александр Юрьевич Лебедев, via Wikimedia Commons, in the public domain.
Image: Limescale from the Kettle.jpg by Alexander Davronov, via Wikimedia Commons, licensed under CC BY-SA 4.0.
Video: Hard & soft water | Environmental Chemistry | Chemistry, FuseSchool - Global Education, embedded from YouTube.
The lather-test results in Figure 3.2 are illustrative figures made up by Ganzaa for teaching.
The diagrams, worked examples, Practice items and exercises were written and drawn by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Soap that will not lather

2Categories of water

3Experiment: the lather test

4What makes water hard?

5Removing hardness

Soap that will not lather

Categories of water

Experiment: the lather test

What makes water hard?

Removing hardness