Hardness of Water Flashcards
1
Q
What is water hardness caused by?
A
Rainwater dissolves some of the rock it flows over and calcium ions dissolve in it.
2
Q
How is water made slightly acidic?
A
By CO2 being dissolved in the water.
3
Q
How is temporary hardness formed?
A
Water + carbon dioxide + calcium carbonate —> calcium hydrogencarbonate
4
Q
How is permanent hardness formed?
A
Calcium sulphate dissolves in water.
5
Q
How can water hardness be measured?
A
- By how much soap is needed to produce a lather
- When soap is shaken in a water sample the calcium ions react in the water with the soap and turn it into scum
- Eventually as more soap is added all of the calcium ions would have reacted
- Anymore soap after that produces a stable lather
6
Q
How is temporary hardness removed?
A
- Boiling
- Calcium hydrogencarbonate decomposes easily in hot water to form calcium carbonate, water and carbon dioxide
- Ca(HCO3)2 —> CaCO3 + CO2 + H2O
- Heating removes soluble calcium ions from the water by changing them into insoluble calcium carbonate
7
Q
Why is permanent hardness not affected by boiling?
A
The calcium sulphate ions are too stable.
8
Q
How do ion-exchange resins remove permanent hardness?
A
- Water flows over beads of solid resin
- Resin traps calcium ions on its surface
- The calcium ions are exchanged for sodium ions
- Remove temporary and permanent
9
Q
What is washing soda and how does it remove temporary and permanent hardness?
A
- Sodium carbonate
- When washing soda dissolves it reacts with the calcium sulphate in the water to form insoluble calcium carbonate, locking up the calcium ions
- CaSO4 + Na2CO3 —> Na2SO4 + CaCO3
