C3 - Hard Water Flashcards
What are the 2 things that hard water produces and what are their chemical compositions?
Scum (calcium and magnesium ions reacting with soap)
Scale (mostly calcium carbonate)
What is hard water?
Water that contains lots of calcium 2+ and magnesium 2+ ions
What causes hard water to produce scum?
When hard water is mixed with soap it takes longer to form a lather because the calcium and magnesium ions in the hard water react with the soap to form scum which is insoluble
What is the problem with scum being produced instead of a leather?
More soap is needed to make a lather so more money is spent on soap
What causes hard water to produce scale?
Hard water produces scale when it is heated
What are the 2 problems with scale?
The scale can block up pipes which might need to be replaced
Scale is a thermal insulator so scale in the kettle causes the kettle to take longer to boil reducing its efficiency
What causes water to be hard?
Rain falling on some types of rocks like limestone, chalk and gypsum can dissolve compounds like magnesium sulphate and calcium sulphate
What are the 2 advantages of hard water?
Ca2+ ions are good for healthy teeth and bones
People who live in hard water areas seem to be at less risk of heart disease
What are 2 types of hardness and what are they caused by?
Temporary caused by hydrogen carbonate ion (HCO^3-)
Permanent is caused by mainly calcium sulphate
Describe how only temporary hardness is removed and what results from this
I will use calcium as an example
When heated, calcium hydrogen carbonate decomposes to form calcium carbonate (scale), water and carbon dioxide
What are the 2 ways of removing both temporary and permanent hardness?
Adding washing soda (sodium carbonate)
Running it through ion exchange columns
Describe how adding washing soda removes hardness of water
The added carbonate ions react with the calcium and magnesium ions to form an in soluble precipitate of calcium carbonate and magnesium carbonate
The calcium and magnesium ions are no longer dissolved so they can’t make it hard
Ca2+ + (CO^3)2- –> CaCO^3
Describe how ion exchange columns remove hardness of water
The column has many sodium ions in a resin
When the hard water runs through it, the calcium and magnesium ions are exchanged for sodium ions
Na^2Resin + Ca2+ –> CaResin + 2Na+
Describe the method of using titration to compare the hardness of water samples
Fill a burette with 50cm^3 soap solution
Add 50cm^3 of the first water sample to a flask
Use the burette to add 1cm^3 to the flask
Put a bung in the flask and shake it for 10 seconds
Repeat the last 2 steps until a long lasting lather is formed
Record how much soap was needed to form a lather
Repeat the experiment for the different water samples
What will the results be for the hard water titration experiment?
The hard water samples will have a much higher amount of soap to form a lather
What 2 things does tap water have to be free of?
Poisonous salts (phosphates, nitrates)
Harmful microbes
Describe how water from reservoirs is treated so it can be tap water
Passes through a mesh screen to remove big bits like twigs
Chemicals are added to make solids and microbes stick together and fall to the bottom
The water is filtered through gravel beds to remove all solids
Water is chlorinated to kill any harmful microbes left
How do we obtain completely pure water?
Distillation
Why don’t we put distilled water into taps?
Too expensive
What 2 things are added to tap water?
Fluoride and chlorine
Why is fluoride added to tap water?
Helps reduce tooth decay
What is the problem with adding fluoride to water?
Can cause cancer and bone problems in high doses
Why is chlorine added to tap water?
To prevent disease from harmful microbes
What is the problem with adding chlorine to tap water?
Can react with other substances in the water to produce toxic by-products
What is the problem with adding chemicals to tap water in general?
We don’t get to choose whether we want the chemicals in our tap water
