Bottled Water
Why is bottled water so popular? Is it safer than tap water? Or is it just more convenient?
Water for bottled water comes from underground springs, wells and municipal water supplies. Municipal water must be purified by distillation before bottling. But, is spring and well water as pure as the distilled water?
Recall the water cycle:
- when precipitation falls to Earth, it is collected in aquifers
- on its trip into the Earth, certain substances dissolve in the water
- low concentrations of some substances give flavour to the water and promote health (Fe2+(aq), Fe3+(aq), Mg2+(aq))
- other substances are harmful (mercury, lead, cadmium and arsenic ions) or distasteful (sulfur compounds)
Hard Water
You may have noticed that soap does not lather well at home. If this is the case, you may have hard water, which is inconvenient, but not harmful to you.
Hard water is water that contains dissolved calcium, magnesium and iron ions.
When slightly acidic water flows through limestone, it dissolves the metal ions. When soap mixes with hard water, the soap reacts with the calcium and magnesium ions to form insoluble compounds.
This precipitate is scum and causes the bathtub ring.
If you have scale deposits in your kettle, your water contains HCO3- in the form of Ca(HCO3)2(aq). When the water is boiled:
Ca(HCO3)2(aq) → CaCO3(s) + H2O(l) + CO2(g)
This removes the calcium ions from the water, which helps to soften it, but calcium carbonate scale is left on the kettle's heating element, reducing its efficiency.
Softening Hard Water
Removing calcium and magnesium ions will soften hard water.
- One method is to add washing soda, sodium carbonate, to the water. Hard water ions precipitate as calcium carbonate and magnesium carbonate.
- Another method involves both sodium carbonate and sodium hexametaphosphate, which react with calcium and magnesium ions to form soluble complexes:
2Ca2+(aq) + P6O186- → [Ca(P6O18)]2-(aq)
- Installed home water softeners use ion exchange to soften water. There is a large tank filled with ion exchange resin (plastic) grains. The resin has a high concentration of Na+, attached to sulfonate ions, SO3- (see fig 3, p. 277). When water passes through, the Ca2+(aq) and Mg2+(aq) attach to the sulfonate group and the sodium ions dissolve in the water (the sodium ions will not form scum with soap). After a while, the resin is depleted of sodium ions and must be regenerated by washing with concentrated brine. The calcium and magnesium ions are replaced by the sodium and the hard water ions are flushed away.
- In some parts of Ontario, municipalities use the soda-lime process. Sodium carbonate (washing soda) and calcium hydroxide (slaked lime) are added to the water. The calcium hydroxide dissolves to produce calcium and hydroxide ions:
Ca(OH)2(s) → Ca2+(aq) + 2OH-(aq)
The hydroxide ions react with hydrogen carbonate ions in hard water to form carbonate ions:
2OH-(aq) + 2HCO3-(aq) → 2CO32-(aq) + 2H2O(l)
The newly formed carbonate ions and those from the washing soda, combine with calcium and magnesium ions in hard water, to form solid calcium carbonate and magnesium carbonate:
CO32-(aq) + Ca2+(aq) or Mg2+(aq) → CaCO3(s) or MgCO3(s)
Dissolved Oxygen
So far, we have only looked at solid solutes; now let’s focus on gaseous solutes.
Some oxygen gas dissolves in water from the air above the surface of the water. Additional oxygen is added through aeration, which occurs as water flows over rocks or dams.
Oxygen is also added through photosynthesis as aquatic plants use sunlight and carbon dioxide.
Oxygen content drops as oxygen consuming bacteria feed on dead plants/animals and human/industrial waste - excess pollution will cause an explosion in bacterial life and a large reduction in oxygen levels
Dissolved oxygen is important to fish and other animals (fish die unless the oxygen content in water is greater than 4 ppm). Thus, as pollution increases, oxygen content decreases and aquatic diversity decreases. This relationship is used as a basis for assessing water quality, called the Benthic macro-invertebrate analysis.
Homework:
Using the text and today's blog post, fill in the following.
