Water forms the basis of most cleaning processes. As it seeps through the ground, magnesium and calcium ions dissolve in it. Water containing dissolved minerals is known as hard water. Dissolved minerals reduce the efficiency of cleaning processes and additional cleaning agent is required to compensate for this. Water softeners such as Calgon? may be used to reduce the effect of hard water on appliance efficiency and improve cleaning by removing or masking the effect of dissolved ions. Links to the curriculum
This section contains information relevant to the following areas of your chemistry curriculum:
？ Group 2 elements
？ Acids and bases
In the western world, we think of our tap water as being pure. And it is in the sense that it is safe to drink and contains no dangerous micro-organisms - viruses and bacteria - which could make us ill. However, in the chemical sense of being a single substance, it is not pure at all. It will contain materials dissolved from the ground, through which it has seeped between falling as rain (when it has effectively been distilled) and arriving in a reservoir from which it enters the distribution system.
Explain why rainwater is ‘effectively distilled’.
Even rainwater is not chemically pure when it falls. Suggest what else may be dissolved in it.
These dissolved materials can make a great deal of difference to water's ability as a cleaning solvent - for its use in washing machines and dishwashers, for example. Water containing dissolved minerals is termed ‘hard’ and water softeners such as Calgon? may be used to reduce the effects of this and improve cleaning.
Hard water has two effects on the cleaning efficiency of washing machines and dishwashers. Firstly, inorganic compounds may deposit on the electric elements used to heat up the cleaning water, thereby reducing efficiency. These compounds may also deposit on the articles being cleaned. Secondly, metal ions in the hard water react with soap and certain other detergents to form scum, which reduces cleaning power, and deposits on the items to be cleaned. The solution to these problems is to use more cleaning agent.
This is why laundry detergent manufacturers recommend different levels of detergent depending on the water hardness: the higher the water hardness, the more detergent is required, see Figure 1.
Figure 1: Manufacturers recommend using more detergent in hard water areas
The use of a water softener can reduce the amount of detergent required by up to 40%. The taste of water
Dissolved materials also make a significant difference to water's taste. If you live in a hard water area, you may never have tasted pure water. If you get the opportunity to taste some distilled or deionised water at school, college or elsewhere try it (with suitable hygiene precautions). You may be surprised.
Note: Do not taste water that has been treated with Calgon? or other water softening products.
How water becomes hard
Figure 2 shows the water cycle. The time taken between water falling as rain and entering a reservoir can vary from zero (rain falling directly into the reservoir) to thousands of years. So, there may be plenty of time for material to dissolve, even if the rate of dissolving is slow. What materials actually dissolve will depend on the nature of the rock through which the water percolates. Of most importance from the point of view
2+2+of cleaning are dissolved metal ions, and of these the most relevant are Ca, Mg and, to
3+a lesser extent, Fe.
Figure 2: The water cycle
When rainwater seeps through limestone rock, it dissolves some of it. This is the origin of limestone caves. Limestone is calcium carbonate, which is pretty insoluble in pure water, but rainwater naturally contains dissolved carbon dioxide which makes it acidic. The following reaction occurs, dissolving limestone as calcium hydrogencarbonate (calcium bicarbonate).
2+-CaCO + HO + CO Ca + 2HCO ?H –ve 3(s)2(l)2(aq)(aq)3(aq)
A similar reaction can occur with magnesium carbonate.
Rainwater is naturally acidic because of dissolved carbon dioxide, which is normally present in the air and takes part in the equilibria:
+-HO + CO HCO + H HCO 2(l)2(aq)3(aq)(aq)23(aq)
The acidity of rain is increasing because the level of carbon dioxide in the atmosphere is rising as we burn more fossil fuels. The presence of gases such as sulfur dioxide and nitrogen oxides, also produced by burning fuels, adds to the acidity.
A second source of hardness is the dissolution of other calcium and magnesium salts, such as chlorides and sulfates, in rainwater. Here acidity does not play an important role. One common rock is anhydrite, calcium sulfate:
2+2-CaSO Ca + SO 4(s)(aq)4(aq)
Temporary and permanent hardness
2+-CaCO + HO + CO Ca + 2HCO ?H -ve 3(s)2(l)2(aq)(aq)3(aq)
is exothermic in the forward direction, and Le Chatelier's principle predicts that at high temperatures, the equilibrium will move to the left.
This is what happens inside kettles, dishwashers and washing machines and results in the formation of a deposit of calcium carbonate on their heating elements, Figure 3. This is usually called limescale in everyday language. Limescale reduces heat transfer from the element to the water and makes the appliance less efficient.
Figure 3a: The effects of limescale: a new heating element
Figure 3b: The effects of limescale: a heating element with limescale deposit It can also deposit on clothes making them feel rough. Figure 4 shows the effect of deposition of limescale on fibres of clothing.
Figure 4a: Scanning electron microscope images of clothing fibres with limescale deposits
Figure 4b: Scanning electron microscope images of clothing fibres without limescale deposits Because the calcium ions are removed from solution (as solid calcium carbonate) by heating the water, the type of hardness where the negative ion is the hydrogencarbonate ion is called temporary hardness.
The hardness caused by calcium (and magnesium) salts other than hydrogencarbonates is not affected by heating and is therefore called permanent hardness.
a) Write the expression for the equilibrium constant K for the equilibrium c
2+-CaCO + HO + CO Ca + 2HCO?H -ve 2(l)2(aq)(aq)3(aq) 3(s)
Remember how to deal with the concentrations of solids and pure liquids. b) What are the units of the equilibrium constant in the previous question? c) Apart from changing the temperature, what other ways can you suggest of
2+reducing [Ca]? (aq)
How hard is your tap water?
Water companies routinely monitor the hardness of water in their areas and you can find the level in your area by contacting them, by typing your postcode into http://www.homesolutionsnews.com/waterhardness/uk/popup1.jsp
Figure 5: The hardness of water in different areas of the UK (Courtesy of Caterchem UK)
On the map in Figure 5, the units of water hardness are given as ‘mg/l of calcium carbonate equivalent’. Another unit often used is ‘mg/l of calcium’.
100 mg/l as calcium carbonate equivalent is the same as 40 mg/l of calcium. Use the formula of calcium carbonate (CaCO) and the relative atomic masses (Ca = 40; 3
C = 12; O = 16) to explain why these two measurements are equivalent.
-31 mg/l is the equivalent of 1 mg dm.
The method of reporting hardness as ‘calcium carbonate equivalent’ makes the assumption that all the calcium ions in the water are derived from (or associated with) carbonate ions, ie that all the calcium is actually part of calcium carbonate.
-3If a sample of water contained 272 mg/l (272 mg dm) of calcium sulfate (CaSO), 4
how would this be reported as ‘calcium carbonate equivalent’? (Relative atomic mass S= 32)
Calgon? is formulated to soften water and reduce the effects of hardness in a number of ways that reinforce each other – they are synergistic. It prevents the deposition of limescale in four ways called technically ion exchange, neutralisation, suspension and
regrowth inhibition. Find out more about these below.
Calgon? contains compounds called zeolites. These are aluminosilicate minerals whose structures have large numbers of ‘holes’ or ‘tunnels’ giving them an enormous surface
4+area. The structures are made up of notional Si ions linked by oxygen atoms. Some of
3+the silicon ions are replaced by aluminium ions. However, aluminium forms Al ions
and so, to keep the structure neutral, each one must be associated with another, singly
+charged, ion, usually Na. These sodium ions can be exchanged for other ions such as calcium or magnesium.
3+4+Explain why aluminium forms Al ions and silicon might form Si ions.
4+The silicon atoms do not in fact form true Si ions, there is a degree of covalent
4+bonding with the oxygen atoms in the structure. Explain why true Si ions are
unlikely to form.
5+4+Explain why Si and Al will never form.
+2+2+To keep the charges balanced, two Na ions are exchanged for each Ca or Mg, Figure
6. Thus, zeolites remove the ions that cause hardness from the washing water.
3+Figure 6: Part of the structure of a zeolite showing one of the ‘tunnels’. Blue represents Al and red 4+ represents Si(Courtesy of molecularsieve.org)
Neutralisation (1 of 3)
Essentially this removes calcium ions from solution so that the equilibrium
2+-CaCO + HO + CO Ca + 2HCO 3(s)2(l)2(aq)(aq)3(aq)
cannot move to the left and deposit calcium carbonate.
Calcium ions may be removed using a complexing agent. A complexing agent is a
2+negatively charged ion that wraps itself around the Ca ion forming dative covalent (co-
ordinate) bonds with it and effectively removing it from solution. These are sometimes called ‘sequestering agents’ - sequester meaning ‘to seclude, isolate or set apart’ according to the Oxford English Dictionary.
One of the complexing agents used in Calgon? is sodium citrate (sodium 2-hydroxypropane-1,2,3-tricarboxylate).
- Figure 7: Structure of the citrate 3ion.
The structural formula of the citrate 3- ion is shown in Figure 7 and a 3D representation in Figure 8. Make sure that you can relate the two representations of the citrate ion to one another.
-Figure 8: 3D representation of the citrate 3 ion
In the 3D representation, carbon atoms are coloured grey, oxygen red and hydrogen white. You may find it helpful to make a model of the citrate ion using a ball and stick modelling kit such as Molymod? to help you understand how this complexing works.
Neutralisation (2 of 3)
3--The citrate 3- ion is often written Cit for short. The three Os can twist round to enfold
2+a Ca ion (remember that there is free rotation about single bonds). They are attracted to the ion electrostatically and can use their lone pairs of electrons to form dative (co-ordinate bonds) with it.
a) What will be the charge on the complex ion formed from the citrate ion and a calcium ion?
b) Will it be attracted to a hydrogencarbonate ion?
Would you expect the citrate/calcium ion to be soluble in water? Explain your answer.
2+The effect of complexing the Ca ion is to keep it dissolved in the water rather than
2+being deposited as calcium carbonate. Mg ions will be complexed in the same way.
222+Write the electron arrangement (1s, 2s etc) of a Ca atom and Ca ion. What
2+noble gas configuration does Ca have?
2+What empty orbitals might be used on the Ca ion to accept lone pair electrons
from the citrate ion?
-The two C-O bond lengths in the COO groups of sodium citrate are identical rather
than one shorter than the other. Explain this observation.
Considering its electronic structure and the formation of complexes, in what ways
2+a) resemble a Ca ion ?
2+b) differ from a Ca ion?
Another typical complexing agent in Calgon? is sodium tripolyphosphate, Figure 9.
Figure 9: Structure of the tripolyphosphate ion.
The structural formula of the tripolyphosphate ion is shown in Figure 9 and a 3-D representation in Figure 10.
Figure 10: 3D representation of the tripolyphosphate ion
Make sure that you can relate the two representations of the phosphate ion to one another. In the 3D representation, phosphorus atoms are coloured orange and the oxygen red. You may find it helpful to make a model of the phosphate ion using a ball and stick modelling kit such as Molymod? to help you understand how this complexing
Neutralisation (3 of 3)
When low levels of sodium tripolyphosphate are used, insoluble calcium and magnesium salts of tripolyphosphate, e.g. Ca(PO) , are formed which precipitate out. 53102
2++2NaPO + 5Ca Ca(PO) + 10Na 5310(aq)(aq)53102(s)(aq)
3-At higher concentrations of sodium tripolyphosphate, a soluble 1:1 complex [CaPO] 310
is formed. This effect it is known as the threshold effect.
2+3-+NaPO + Ca [CaPO] + 5Na 5310(aq)(aq)310(aq)(aq)
It is therefore important that the concentration of sodium tripolyphosphate is sufficient to form the soluble complex rather than the insoluble salt.
In some countries, the use of phosphates is restricted because they can cause eutrophication in high concentrations. This is the overgrowth of plants in watercourses with consequent lack of oxygen in the water which kills fish. So formulations of the same brand may well be different in different areas.
The pH of the solution
Many cleaning products act best in somewhat alkaline solution.
This is because alkalis:
？ emulsify grease by reacting with insoluble fatty acids to form ionic salts which
are soluble. Figure 11 shows how insoluble fatty acids react with hydroxide ions
nd are broken down into soluble ions which can then form ionic salts. a
Figure 11: Breaking down insoluble fatty acids into soluble ions
？ protect the metal of washing machines and dishwashers from acid corrosion
？ help to reduce re-deposition of dirt that has been removed by coating particles of
it with negatively charged hydroxide ions – this means the dirt particles repel
each other and remain in suspension rather than clumping together to form large
aggregates which would tend to precipitate out onto clothes etc
Grease contains long chain fatty acids such as CH(CH)COOH.These are weak 3214
a) Write the equation for the reaction of CH(CH)COOH with a hydroxide ion, 3214-OH.
b) Explain why the product is more soluble than the original fatty acid.