Hardness in water Hard water does not easily
- Slides: 22
Hardness in water
• Hard water does not easily form a lather with soap • Caused by calcium (Ca 2+) or magnesium (Mg 2+) ions • Soft water easily forms a lather with soap (doesn’t contain these ions)
• Soap molecules are sodium or potassium salts of long-chain carboxylic acids called stearates • When put in water the sodium or potassium dissolves in the water and separates from the stearate • The stearate ions then “clean”
• If there are Ca 2+ or Mg 2+ ions present the stearate ions will attach to them and appear as a “scum” • Because the stearate ions are removed from the water they cannot clean
• If enough soap is added all the Ca 2+ and Mg 2+ ions will be used up and the soap will eventually form a lather • But this is costly and wasteful • Modern detergents act differently because they contain water softening agents
Temporary hardness • Rainwater is slightly acidic as it dissolves carbon dioxide in air to form carbonic acid CO 2(aq) + H 2 O(l) → H 2 CO 3(aq) • When this falls on limestone (insoluble calcium carbonate) it reacts to form soluble calcium hydrogencarbonate
H 2 CO 3(aq) + Ca. CO 3(s) → Ca(HCO 3)2(aq) Carbonic acid calcium carbonate calcium hydrogencarbonate • Calcium hydrogencarbonate is soluble in water so causes the presence of Ca 2+ ions • This type of hardness is called temporary hardness
Removing temporary hardness • Temporary hardness is hardness that can be removed by boiling • Can be removed by distillation or boiling • Distillation evaporates water, separating it from anything dissolved in it, then condenses it from a vapour back to a liquid
• If calcium hydrogencarbonate is boiled it decomposes into a carbonate, water and carbon dioxide Ca(HCO 3)2(aq) + heat → Ca. CO 3(s) + CO 2(g) + H 2 O(l) • So the calcium carbonate is removed and left as a deposit called lime scale
Limescale • Lime scale can cause problems when deposited • Can build up on surfaces like kettles, pipes and boilers causing blockages • Test for lime scale: Add hydrochloric acid to lime scale Bubble the gas formed in limewater If turns milky then carbon dioxide has been formed Proves carbonate was present
Permanent hardness • Caused by Ca 2+ or Mg 2+ ions that come from a source other than hydrogencarbonates, e. g. , chlorides and sulfates • This happens when rainwater passes over other forms of rocks, e. g. , gypsum
• Permanent hardness is hardness that can be removed by methods other than boiling • Can be removed by distillation, ion exchange and deionisation
Ion exchange • Involves swapping ions that cause hardness for ions that do not • Usually cation exchange – swapping hard ions for positive soft ions, e. g. , sodium (Na+)
• Sodium sits on a resin and the hard water passes through it, swapping the sodium ions for the calcium/magnesium ions Na 2 R(s) + Ca 2+(aq) → Ca. R(s) + 2 Na+(aq)
Deionisation • More advanced ion-exchange resins remove all ions from the water • Resin used is a mixture • Cation exchanger – swaps metal ions (Ca+/Mg+) in water for hydrogen ions (H+) • Anion exchanger – swaps negative ions for hydroxide ions (OH-)
• These ions then react together to form water H+ + OH- → H 2 O • Deionised water is not as pure as distilled water as deioniser only removes dissolved ions, not suspended solids and dissolved gases
- Gravimetric analysis of calcium and hard water lab answers
- Discuss how permanent hardness of water can be removed
- How to calculate total hardness
- How to calculate total hardness
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- Removal of water hardness
- Pseudo hardness
- Units of hardness of water
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