Hardness of Water Flashcards
Hard water makes [ ] and [ ]
Scale and Scum
What do you get when you mix soft water with soap?
Lather
What do you get when you mix hard water with soap?
Scum.
Need lots of soap to create a lather
Why is water hard?
It has dissolved calcium or magnesium ions in it.
Is scum soluble?
No! It’s insoluble
What happens when you heat hard water?
Forms a scale (mostly calcium carbonate) on the inside of pipes, boilers and kettles.
What is the problem with scale in pipes?
Reduces the efficiency. Heating systems may need replaced - Costs money
Scale can eventually block pipes
Scale is a thermal [ ] therefore…
Insulator
Kettles take longer to boil - less efficient
How is hard water formed?
Rain falling on some types of rocks can dissolve compounds like magnesium sulphate (soluble) and Calcium sulphate (a bit soluble)
What are Ca 2+ ions good for?
Teeth and bones
How could the minerals in water effect the risk of having heart disease?
People who live in hard water areas are a Lower risk
What are the 2 kinds of hardness?
Temporary and Permanent
What is temporary hardness caused by?
The hydrogencarbonate ion HCO₃- , in Ca(HCO₃)₂
What is permanent hardness caused by?
Dissolved calcium sulfate
How do you remove temporary hardness? Why does this work?
Removed by boiling.
When heated, the calcium hydrogencarbonate decomposes to form calcium carbonate which is insoluble (limescale)
Write the word and symbol equation for the heating of temporary hard water?
calcium hydrogencarbonate ->calcium carbonate + water + carbon dioxide
Ca(HCO₃)₂ (aq) -> CaCO₃ (s) + H₂O (l) + CO₂ (g)
Which type of water can be softened by adding washing soda? How?
Both temporary and hard
Washing soda = sodium carbonate Na₂CO₃
The added carbonate ions react with the sodium and magnesium ions to make an insoluble precipitate of calcium carbonate and magnesium carbonate
Write down the symbol equation for calcium ion hard water being softened with washing soda
Ca²⁺ (aq) + CO₃²⁻ (aq) -> CaCO₃ (s)
Describe the usage of an iron exchange column
Both types of hardness can be removed
The columns have lots of sodium ions (or hydrogen ions) and ‘exchange’ them for calcium or magnesium ions in the water that run through the column.
Write the symbol equation for an ion exchange column
Na₂Resin (s) + Ca²⁺ (aq) -> CaResin (aq) + 2Na+ (aq)
Resin is a huge insoluble resin molecule
What can you use to compare the hardness of water samples?
Titration
What is a lasting lather?
A lather where the bubbles cover the surface for at least 30 seconds
Describe the method for working out the hardness of water using titration (8) (sorry!)
1) Fill a burette with 50cm³ of soap solution
2) Add 50cm³ of the 1st water sample into a flask
3) Use the burette to add 1cm³ of soap into the flask
4) Put a Bung in the flask and shake for 10 secs
5) repeat steps 3 and 4 until a good lather has formed
6) Record how much soap was needed
7) Repeat steps 1 to 6 with the other samples
8) Boil fresh samples of each type of water for 10 mins and repeat the experiment.
Why do you test both water and boiled water when using titration to work out the hardness of water?
Shows you whether the water is temporarily hard or permanent.
