Calcium Bicarbonate Formula — Ca(HCO₃)₂ Explained

Chemical name: Calcium bicarbonate / Calcium hydrogen carbonate Molecular formula: Ca(HCO₃)₂ Molar mass: 162.11 g/mol

• Ca = 40.08
• 2 × H = 2 × 1.008 = 2.016
• 2 × C = 2 × 12.011 = 24.022
• 6 × O = 6 × 15.999 = 95.994 Total = 162.11 g/mol

Ca(HCO₃)₂ is an ionic compound. The calcium ion Ca²⁺ is balanced by two bicarbonate (hydrogen carbonate) ions HCO₃⁻. It does not exist in solid form under normal conditions — attempts to isolate it as a solid cause it to decompose.

Calcium bicarbonate forms when carbon dioxide dissolved in water reacts with calcium carbonate (limestone):

CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂

This is how natural water becomes temporarily hard. Rainwater absorbs CO₂ from the atmosphere and soil, forming weak carbonic acid (H₂CO₃). This acid then dissolves limestone: CaCO₃ + H₂CO₃ → Ca(HCO₃)₂

Alternatively: CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂

The reverse reaction (decomposition) occurs upon heating.

Temporary hardness of water is caused by dissolved calcium bicarbonate Ca(HCO₃)₂ and magnesium bicarbonate Mg(HCO₃)₂. It is called 'temporary' because it can be removed by simply boiling the water:

Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂

On boiling, calcium bicarbonate decomposes back to insoluble calcium carbonate (which precipitates as scale/fur in kettles and boilers), water, and carbon dioxide gas. This removes the hardness.

Temporary hardness can also be removed by:

1. Boiling — most common method
2. Adding Ca(OH)₂ (lime softening, Clark's process): Ca(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃↓ + 2H₂O
3. Ion exchange process

Temporary hardness:

• Caused by: Ca(HCO₃)₂ and Mg(HCO₃)₂
• Can be removed by: boiling or adding lime (Clark's process)
• Salts involved: bicarbonates

Permanent hardness:

• Caused by: CaSO₄, MgSO₄, CaCl₂, MgCl₂
• Cannot be removed by boiling
• Removed by: washing soda (Na₂CO₃), ion exchange resin, or permutit process
• Salts involved: sulphates and chlorides

Total hardness = temporary hardness + permanent hardness Hardness is measured in ppm (parts per million) of CaCO₃ equivalent.

• Exists only in solution (aqueous); cannot be isolated as a pure solid
• Solutions are colourless and slightly alkaline due to HCO₃⁻ hydrolysis
• Decomposes on heating: Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g)
• Forms scale/limescale inside boilers, kettles, pipes and water heaters
• Scale reduces heat transfer efficiency in boilers (economic problem)
• Ca(HCO₃)₂ reacts with strong acids: Ca(HCO₃)₂ + 2HCl → CaCl₂ + 2H₂O + 2CO₂
• Contributes to speleothem formation (stalactites and stalagmites in caves)