What chemical compounds might be present in drinking water?
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# Summary of Section 4

- Water hardness is often due to the presence of dissolved calcium and magnesium salts in the water, such as Ca(HCO3)2.
- Aluminium oxide and hydroxide are unusual in being amphoteric. This means that they dissolve in and neutralise both alkali and acidic solutions. For example: Al(OH)3(s) + 3H+(aq) = Al3+(aq) + 3H2O(l) Al(OH)3(s) + OH−(aq) = [Al(OH)4]−(aq)

- Aluminium sulfate is used to remove fine particles from water, wherein the following reaction occurs:

  Al3+(aq) + 3HCO3 −(aq) = Al(OH)3(s) + 3CO2(g)

  as the aluminium hydroxide precipitates it encourages the fine particles to settle out.

- In the body, aluminium(III) ions are toxic by interfering with the binding of magnesium(II) to ATP during its hydrolysis.
- Thallium(I) is poisonous by replacing potassium ions, which have a similar ionic radius, in the body and disrupting normal function.
- Although thallium is in Group 13 its common oxidation state is +1 due to the inert pair effect. The inert pair effect is also observed in Groups 14, 15 and 16.
- Activated carbon is a common adsorbent material in water filters that removes anionic, cationic and organic pollutants.

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