C3.2 Water Flashcards
What does soft water form with soap?
A lather
Why is more soap needed to form a lather with hard water?
Hard water reacts with soap to form scum which is insoluble.
What doesn’t form scum with hard water?
Soapless detergents
What are dissolved in hard water that make it hard?
Calcium (Ca2+) and magnesium (Mg2+) ions
How do compounds become dissolved in water to make it hard?
- Rain falling on some types of rocks e.g limestone and chalk can dissolve compounds like magnesium sulfate and calcium sulfate.
- The water picks up the ions, making it hard.
What are the benefits of drinking hard water (3)?
- Good for healthy teeth
- Good for healthy bones
- Reduces heart disease risk
What process can you use to measure the hardness of water?
Titration with a soap solution
What are the two kinds of hardness and what are they caused by?
- Temporary hardness, caused by the hydrogencarbonate ion in calcium hydrogencarbonate
- Permanent hardness, caused by dissolved calcium sulfate
Why is hard water a problem in pipes, boilers and kettles?
Hard water forms scale (mostly calcium carbonate) on their insides, reducing their efficiency meaning they may need to be replaced which costs money.
How is temporary hardness removed?
By boiling, when heated the calcium hydrogencarbonate decomposes to form calcium carbonate which is insoluble.
calcium hydrogencarbonate ====> calcium carbonate + water + carbon dioxide
Why does boiling permanently hard water not work?
Heating sulfate ions has no effect
How can both types of hardness be softened using washing soda?
- By adding washing soda (sodium carbonate).
- The added carbonate ions react with Mg2+ and Ca2+ ions to make an insoluble precipitate of calcium carbonate and magnesium carbonate.
- The ions are no longer dissolved in water so they can’t make it hard
How can both types of hardness be removed using ion exchange resins?
- By running water through ion exchange resins
- The columns have a high amount of sodium and hydrogen ions and ‘exchange’ them for calcium and magnesium ions in hard water running through them
What do water filters contain and why?
Silver - to prevent the growth of microbes
Carbon - to remove the taste of chlorine
What do some people in hard water areas buy?
Water softeners which contain ion exchange resins
How can completely pure water be made?
Using distillation - boiling water to make steam and condensing it
Why isn’t distillation used all the time to make pure water?
The process is too expensive as it uses a high amount of energy
Where and why is distilled water used?
In chemistry labs to make solutions without other ions contaminating them.
Explain how water is treated.
- The water passes through a mesh screen to remove large objects e.g twigs
- Chemicals are added to make solids and microbes stick together and fall to the bottom
- Water is then filtered through gravel beds to remove solids
- Water is then chlorinated to reduce the amount of harmful bacteria.
What two chemicals are added to water during treatment?
- Fluorine = helps reduce tooth decay
- Chlorine = added to prevent disease
What are the disadvantages to adding fluorine and chlorine to water?
- Chlorine can react with natural substances in water to form toxic (possibly cancerous) by-products
- Fluoride can cause cancer and bone problems in high amounts
What is the ethical problem with treating water by adding chlorine and fluorine?
“Mass medication”, people can’t choose whether their tap water has these chemicals added
Where does drinking water come from?
- Water flows into reservoirs from rivers and ground water.
- Companies choose to build reservoirs where there is a good supply of clean water
What must good quality water be free of and why?
- Microbes, they can cause water diseases (e.g cholera and dysentery)
- Poisonous salts (e.g nitrates and phosphates)
