Softening the Water
Precipitate calcium ions to soften water.
What this lesson covers
Why it matters
Hard water leaves stubborn white stains on taps and makes soap fail to lather. Watch as a simple chemical swap traps the troublemaker ions in a solid precipitate.
The idea in plain words
Map the ion swap: Na₂CO₃ + CaSO₄ → CaCO₃ + Na₂SO₄. Notice how CaCO₃ is the solid waste product.
Predict first
What happens when you add sodium carbonate to hard water containing calcium sulphate?
The calcium ions swap partners with sodium, forming insoluble calcium carbonate which drops out as a white precipitate.
- The solution becomes clearer and stays liquid
- A white solid forms and settles out — correct
- Bubbles of gas rise to the surface
- The water turns blue
What you do
Add the washing soda to the hard water. Observe the formation of the white precipitate.
Check yourself
Which compound is the white precipitate formed in this reaction?
The reaction data states: 'ppt: white ppt of CaCO₃'. Calcium carbonate is insoluble in water.
Why does this process soften the water?
Hardness is caused by dissolved calcium ions. Precipitating them as CaCO₃ removes them from the water, thus softening it.
What type of reaction is Na₂CO₃ + CaSO₄ → CaCO₃ + Na₂SO₄?
The reaction type is explicitly defined as 'double_decomposition' where ions exchange partners.
- Sodium sulphate (Na₂SO₄)
- Calcium carbonate (CaCO₃) — correct
- Calcium sulphate (CaSO₄)
- Sodium carbonate (Na₂CO₃)
- It removes the calcium ions from the solution — correct
- It adds more sodium ions to make it salty
- It boils the water to kill bacteria
- It turns the calcium into gas
- Decomposition
- Double decomposition — correct
- Combination
- Displacement