Hardness of Water

Question 1

Hard water makes [ ] and [ ]

Answer 1

Scale and Scum

Question 2

What do you get when you mix soft water with soap?

Answer 2

Lather

Question 3

What do you get when you mix hard water with soap?

Answer 3

Scum.

Need lots of soap to create a lather

Question 4

Why is water hard?

Answer 4

It has dissolved calcium or magnesium ions in it.

Question 5

Is scum soluble?

Answer 5

No! It’s insoluble

Question 6

What happens when you heat hard water?

Answer 6

Forms a scale (mostly calcium carbonate) on the inside of pipes, boilers and kettles.

Question 7

What is the problem with scale in pipes?

Answer 7

Reduces the efficiency. Heating systems may need replaced - Costs money
Scale can eventually block pipes

Question 8

Scale is a thermal [ ] therefore…

Answer 8

Insulator

Kettles take longer to boil - less efficient

Question 9

How is hard water formed?

Answer 9

Rain falling on some types of rocks can dissolve compounds like magnesium sulphate (soluble) and Calcium sulphate (a bit soluble)

Question 10

What are Ca 2+ ions good for?

Answer 10

Teeth and bones

Question 11

How could the minerals in water effect the risk of having heart disease?

Answer 11

People who live in hard water areas are a Lower risk

Question 12

What are the 2 kinds of hardness?

Answer 12

Temporary and Permanent

Question 13

What is temporary hardness caused by?

Answer 13

The hydrogencarbonate ion HCO₃- , in Ca(HCO₃)₂

Question 14

What is permanent hardness caused by?

Answer 14

Dissolved calcium sulfate

Question 15

How do you remove temporary hardness? Why does this work?

Answer 15

Removed by boiling.

When heated, the calcium hydrogencarbonate decomposes to form calcium carbonate which is insoluble (limescale)

Question 16

Write the word and symbol equation for the heating of temporary hard water?

Answer 16

calcium hydrogencarbonate ->calcium carbonate + water + carbon dioxide
Ca(HCO₃)₂ (aq) -> CaCO₃ (s) + H₂O (l) + CO₂ (g)

Question 17

Which type of water can be softened by adding washing soda? How?

Answer 17

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

Question 18

Write down the symbol equation for calcium ion hard water being softened with washing soda

Answer 18

Ca²⁺ (aq) + CO₃²⁻ (aq) -> CaCO₃ (s)

Question 19

Describe the usage of an iron exchange column

Answer 19

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.

Question 20

Write the symbol equation for an ion exchange column

Answer 20

Na₂Resin (s) + Ca²⁺ (aq) -> CaResin (aq) + 2Na+ (aq)

Resin is a huge insoluble resin molecule

Question 21

What can you use to compare the hardness of water samples?

Answer 21

Titration

Question 22

What is a lasting lather?

Answer 22

A lather where the bubbles cover the surface for at least 30 seconds

Question 23

Describe the method for working out the hardness of water using titration (8) (sorry!)

Answer 23

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.

Question 24

Why do you test both water and boiled water when using titration to work out the hardness of water?

Answer 24

Shows you whether the water is temporarily hard or permanent.