Hard Water - What is it?

Water is a very good solvent - meaning it readily reacts with many different elements in order to form complex compounds. Typically rainfall hits the ground as carbonic acid - water falls as pure water, but reacts with carbon dioxide in the atmosphere to form H2CO3.

This acid rainfall washes over limestone (Calcium Carbonate - CaCO3). The alkaline limestone reacts with the acidic water to form Calcium Bicarbonate, a soluble salt, which is then carried through the entire water supply chain until it reaches your tap.

Magnesium, typically present in smaller quantities, also reacts in an identical way due to it having the same valency as Calcium (+2 positive i.e. cation which reacts identically with acid rain/carbonic acid).

Invisble in water, it presents no health risks - in fact - there is some evidence to suggest the trace amounts of calcium and magnesium may even be beneficial, however, the structure of the compound isn't that stable - and heating the water to 70 degrees plus, quickly breaks it's bonds.

When heating both Calcium and Magnesium bicarbonate the water is driven off as H20 vapour and the carbon dioxide is driven off as a gas. Following the steps, you've therefore gotten rid of the initial rainfall, then the CO2 which turned it into acid rain, hence you are simply left with the limestone on it's own - hence why it precipates back to it's original solid form.

Limescale forms in plates due to crystalisation. Basically as the water heats up you start to find CaCO3 precipitating in the solution as small isolated molecules. As these move in the suspension they bump into other identical molecules and are attracted together. The bond holding this formation is usually only a weak attraction, such bonds are easily disturbed. As the amount of Calcium Carbonate increases eventually enough manage to group together before breaking up, and a protocrystal forms - this is basically a group which is of such a size that it stabilises in the water. Once you have a nucleation site such as this other free calcium carbonate is attracted to it, and crystal literally starts to grow.

In the UK the water is generally considered to be very hard. The west country admittedly has softer water than the Eastern and Northern areas, however, there are relatively few places that do not exhibit 100ppm hardness in the UK (Cornwall/Devon and Wales are about the only places with truly soft water below 50ppm).

Calcium carbonate, limescale, is a particularly good insulator of heat. As well as being a problem aesthetically, and physically (i.e. blocking pipes, steam nozzles etc), it's tendancy to coat heat exchangers means elements need to work harder to perform their function. Over time this has the effect of increasing the power consumption of the device (so a kettle, boiler etc will become less efficient), but also (and perhaps more importantly for expensive equipment) it increases chance of component failiure due to the additional stresses.