Hard Water - Calcium and Magnesium ions in water, causes scum deposits and scaling

Have you ever wondered why the ring forms around your bath? Why the porcelain hand basin looks like it always needs cleaning? Why your hair feels dirty even when it has just been washed? There's a good reason for those outcomes and the cause is related to the chemistry of the water. Let's start with soft water, then work our way to understanding the term 'hard water' and what it means to us.

Rain Water

Pure water is very difficult to maintain in that condition, simply because gases from the atmosphere (carbon dioxide, oxygen, nitrogen oxides, sulphur dioxide) dissolve in the water and change the chemical make-up of the water. This difference varies with location and proximity to oceans, industry, cities and other contributors.

Rainwater is not pure water because it has the gases and a range of other particles in the atmosphere dissolved in it, which may include sea spray from the oceans, nitrate from the electrical activity of lightning (nitrogen gas converted to nitrate), nitrogen and sulphur gases from pollution, dust, pollens, bacteria and numerous other compounds.  Compared to water that has run over the ground, rainwater is reasonably clean.  But you need to collect it without making it any more contaminated and therein lies a problem.

So rainwater essentially is water with some minor impurities, with an electrical conductivity  (EC) less than 0.01 dS/m and a pH about 4.5 to 5.5 (that's slightly acid). Close to industrial areas, the rain may be more acid (lower pH) and can cause serious environmental problems.

Now let the rain fall on your roof.  What sort of roof is it?  Concrete tiles, terracotta tiles, galvanised iron, Zincalume sheeting, anodised aluminium, timber, fibrous cement, asbestos cement, lead flashings, a chimney from a wood fire, a TV aerial that is a great perch for resting birds, gutters that accumulate leaves and decay to form beaut compost?  Think about it!  How clean is your roof?  Well, whatever is on the roof may end up in your rainwater tank and the chemicals will contaminate the water with both organic and inorganic components.

So the EC will be elevated a little, but probably still less than 0.05 dS/m and a pH a little higher around 5-6.

For the story on water hardness, we need to consider only calcium and magnesium. Hardness is an aesthetic quality of water that is used to describe the ease with which soap forms a lather. We can drink extremely hard water but it may not make a good cup of tea or coffee.

Soft Water
There's not going to be very much calcium or magnesium from the dust and leaves or even from the roof tiles, and none from the steel or aluminium sheeting.  So the rainwater will be soft. There will not be enough calcium or magnesium to form a scum with ordinary soap and certainly not enough to need a special detergent.  Sometimes from new concrete water tanks there may be minor amounts of calcium leaching from the concrete in the first year, but certainly not levels that would stop the water being called 'soft'.

In soft water, ordinary soap lathers quickly and easily, and the soap is easy to rinse out of clothes in the laundry or from your hair and skin under the shower. Your hair feels clean and not 'clingy'.

A problem with very soft water is that because of its purity, the copper pipes used in the house will start to dissolve in the water.  The green mark on the porcelain under the dripping tap is an accumulation of copper from the pipes. The cleaner the water the more corrosive it is.

Hard Water
We're not talking about frozen water (ice). Calcium and magnesium ions  (these are both positive ions) in water join with the soap to form as insoluble curd and reduce the effectiveness of the soap as a washing agent. The real problem comes when you try to wash the curd out of the clothes because as an insoluble material it sticks to the fibres of the clothes, does not re-dissolve and is left behind after rinsing.  So you may have added dirt to the clothes (in the form of these insoluble calcium and magnesium compounds) rather than cleaning them. Clothes may feel damp even though they are dry, and do not have a 'crispness' after washing. Over time, this soap curd will give a dull yellow colour to white materials.

In the bath or shower, soap is difficult to get a lather with hardwater and instead of feeling slippery it seems difficult to wash. Your hair, in particular, starts to matt together and it's difficult to run your fingers through your wet hair.  When your hair dries it feels sticky and clings together because the calcium and magnesium compounds have not been removed from the hair fibres but are acting like glues. Your skin may be clammy because the calcium/magnesium precipitate absorbs moisture, and may also leave the skin drier than usual.

In the bath, there are tell-tale signs of the high water mark and the hand basin gets a greasy film over the surface of the porcelain. It requires cleaning with some of the special products because this scum is insoluble in water.  Shower screens and tiles get a matt appearance (dull) and appear to have a coating that will increase in thickness over time, unless removed.  It is usual to use a proprietary product to remove this 'grunge'. These products are usually acidic to dissolve the calcium and magnesium precipitate.

Inside pipe, hard water leaves an insoluble deposit of calcium and magnesium carbonates and over time may completely clog up the pipe.  In hot water services, in the electric kettle or coffee making machines, the calcium deposits on the heating element and can build up to a stage that the water is no longer in contact with the element, the element overheats and explodes.

How do measure water hardness?
There are some quick and simple test kits available from Global Water Solutions.  In our laboratory, the water is analysed for calcium and magnesium and a calculation is made. Hardness can be reported for calcium hardness, magnesium hardness or total hardness (the sum of the calcium and magnesium hardness).  Total hardness is reported in milligrams per litre of calcium carbonate equivalent (mg/L CaCO₃).

Some imported water softeners refer to hardness in obsolete German units of "degrees of hardness" (^odH).  To convert ^odH to total hardness in mg/L CaCO₃, divide by 0.14.  See references to other units at http://en.wikipedia.org/wiki/Hard_water)

Classification of  total hardness
The Australian Drinking Water Guidelines (2004) state that there are no health guideline values for total hardness, but an aesthetic value is suggested at 200 mg/L CaCO₃ (which is reasonably hard water).  Very hard water is likely to cause scale  (insoluble calcium and magnesium compounds) to form on the inside of pipes and boilers.

    Soft   < 60 mg/L CaCO₃
    good quality  60-200 mg/L CaCO₃
    increasing scaling problems 200-500 mg/L CaCO₃
    severe scaling >500 mg/L CaCO₃

Other classification systems are intermediate to the Australian Drinking Water Guidelines (2004)and refer to various scales of total hardness as:

    soft                                 <17 mg/L CaCO₃
    slightly hard                     17-60 mg/L CaCO₃
    moderately hard              60-120 mg/L CaCO₃
    hard                                120-180 mg/L CaCO₃
    very hard                         >180 mg/L CaCO₃