Acid-base equilibria

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Why is soft water (usually) slightly alkaline and hard water slightly acidic?
CO₂ (from pond life's respiration) + H₂O react to form H₂CO₃
H₂CO₃ <=> HCO₃^- + H⁺ <=> CO₃^2- + 2H⁺

Igloo writes ..........
 
Actually it is hard water which is usually alkaline, whereas soft water is usually slightly acidic.
 
Soft water
Soft water is water which contains a very low concentration of metallic ions derived from rock, e.g. Ca²⁺ and Mg²⁺.
 
When carbon dioxide from the atmosphere dissolves in fairly pure water, the gas ionises to produce hydrogencarbonate ions and a very small number of carbonate ions, and, of course, the hydrogen ions which cause this type of water to be acidic:
CO₂(g) + H₂O(l) <=> HCO₃^-(aq) + H⁺(aq)
HCO₃^-(aq) + H₂O(l) <=> CO₃^2-(aq) + H⁺(aq)
Soft water has a pH in the region of 5.5 – 7.0
 
Hard water
When water comes into contact with carbonate mineral rock, such as limestone or chalk (CaCO₃) or magnesite (MgCO₃), the rock dissolves slightly producing Ca²⁺, Mg²⁺ and CO₃^2-ions:
CaCO₃(s) <=> Ca²⁺(aq) + CO₃^2-(aq) MgCO₃ (s) <=> Mg²⁺(aq) + CO₃^2-(aq)
These carbonate ions can interact with both hydrogen ions and water molecules:
CO₃^2-(aq) + 2H⁺(aq) <=> CO₂ (aq) + H₂O(l)
CO₃^2-(aq) + H₂O(l) <=> HCO₃^-(aq) + OH^-(aq)
 
and these hydrogencarbonate ions can ionise:
HCO₃^-(aq) <=> CO₂ (aq) + OH^-(aq)
 
These three processes not only remove hydrogen ions but also create hydroxide ions, which is why in hard water the pH rises to an alkaline value – typically in the region of about 8.5.
 
All these processes are reversible. They represent a complex set of equilibria involving H₂O, CaCO₃ (or MgCO₃), HCO₃^-, CO₃^2-, H⁺ and OH^-.
 
This is a simplified explanation!

Risk assessment
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updated: 24 January 2006