C3.2 Water

Question 1

What does soft water form with soap?

Answer 1

A lather

Question 2

Why is more soap needed to form a lather with hard water?

Answer 2

Hard water reacts with soap to form scum which is insoluble.

Question 3

What doesn’t form scum with hard water?

Answer 3

Soapless detergents

Question 4

What are dissolved in hard water that make it hard?

Answer 4

Calcium (Ca2+) and magnesium (Mg2+) ions

Question 5

How do compounds become dissolved in water to make it hard?

Answer 5

• 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.

Question 6

What are the benefits of drinking hard water (3)?

Answer 6

• Good for healthy teeth
• Good for healthy bones
• Reduces heart disease risk

Question 7

What process can you use to measure the hardness of water?

Answer 7

Titration with a soap solution

Question 8

What are the two kinds of hardness and what are they caused by?

Answer 8

• Temporary hardness, caused by the hydrogencarbonate ion in calcium hydrogencarbonate
• Permanent hardness, caused by dissolved calcium sulfate

Question 9

Why is hard water a problem in pipes, boilers and kettles?

Answer 9

Hard water forms scale (mostly calcium carbonate) on their insides, reducing their efficiency meaning they may need to be replaced which costs money.

Question 10

How is temporary hardness removed?

Answer 10

By boiling, when heated the calcium hydrogencarbonate decomposes to form calcium carbonate which is insoluble.
calcium hydrogencarbonate ====> calcium carbonate + water + carbon dioxide

Question 11

Why does boiling permanently hard water not work?

Answer 11

Heating sulfate ions has no effect

Question 12

How can both types of hardness be softened using washing soda?

Answer 12

• 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

Question 13

How can both types of hardness be removed using ion exchange resins?

Answer 13

• 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

Question 14

What do water filters contain and why?

Answer 14

Silver - to prevent the growth of microbes

Carbon - to remove the taste of chlorine

Question 15

What do some people in hard water areas buy?

Answer 15

Water softeners which contain ion exchange resins

Question 16

How can completely pure water be made?

Answer 16

Using distillation - boiling water to make steam and condensing it

Question 17

Why isn’t distillation used all the time to make pure water?

Answer 17

The process is too expensive as it uses a high amount of energy

Question 18

Where and why is distilled water used?

Answer 18

In chemistry labs to make solutions without other ions contaminating them.

Question 19

Explain how water is treated.

Answer 19

• 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.

Question 20

What two chemicals are added to water during treatment?

Answer 20

• Fluorine = helps reduce tooth decay

- Chlorine = added to prevent disease

Question 21

What are the disadvantages to adding fluorine and chlorine to water?

Answer 21

• 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

Question 22

What is the ethical problem with treating water by adding chlorine and fluorine?

Answer 22

“Mass medication”, people can’t choose whether their tap water has these chemicals added

Question 23

Where does drinking water come from?

Answer 23

• Water flows into reservoirs from rivers and ground water.
• Companies choose to build reservoirs where there is a good supply of clean water

Question 24

What must good quality water be free of and why?

Answer 24

• Microbes, they can cause water diseases (e.g cholera and dysentery)
• Poisonous salts (e.g nitrates and phosphates)