Unit 3 - Water

Question 1

What is hard water?

Answer 1

Water which doesn’t easily form a lather with soap

Question 2

Which ions cause hardness of water?

Answer 2

Calcium (Ca²⁺) and Magnesium (Mg²⁺)

Question 3

How is scum formed?

Answer 3

When calcium/magnesium ions combine with stearate ions from soap to form insoluble calium/magnesium sterate which makes the water cloudy (this is known as scum)

Question 4

How is scale formed?

Answer 4

When hard water is heated

Question 5

HOW do pipes, kettles etc become effected by scale

Answer 5

• They can become blocked
• Kettles become less efficient because the scale causes the heating to take longer (scale is a thermal insulator)

Question 6

Give advantages of hard water?

Answer 6

• Calcium is good for bones and teeth
• Calcium ions can cause less risk of developing heart disease

Question 7

What are the two types of hardness?

Answer 7

Tempory and permamnent

Question 8

What is temporary hardness? And what is it caused by?

Answer 8

This can be removed by boiling, and is caused by the hydrogencarbonate ion

Question 9

What is permanent hardness? And what is it caused by?

Answer 9

It cannot be removed by boiling, it is caused dissolved calcium sulfate

Question 10

How is temporary hardness removed? (what happens)

Answer 10

It is removed by boiling.

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

This solid is ‘limescale’

Question 11

How can both types of hardness be softened?

Answer 11

By adding washing soda (sodium carbonate) to it

→ The added carbonate ions react with the hard ions to make an insoluble ppte

Question 12

How can both types of hardness be removed?

Answer 12

By running the water through an ion exchange column

The hard water is passed through a column containing an ion-exchange resin

This resin takes the calcium/magnesium ions (which cause hardness) and replaces them with sodium ions (which don’t cause hardness)

Question 13

How can you compare the hardness of water samples?

Answer 13

Titration

Question 14

LEARN TITRATION

Answer 14

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Question 15

Where does most of our drinking water come from?

Answer 15

Reservoirs

Question 16

Explain how water is treated in the treatment works

Answer 16

1. Water passes through a mesh screen and remove large solids eg twiga
2. Chemicals are added to make solids and microbes stick together and fall to the bottom
3. The water is filtered through gravel beds to remove all the solids
4. Water is chlorinated to kill of any microbes left

Question 17

How else can people at home filter their water?

Answer 17

By buying filters that contain carbon/silver to remove substances from their tap water

Question 18

How do carbon/silver filters work?

Answer 18

The carbon removed the chlorine taste and the silver kills bugs

Question 19

How can totally pure water be produced? What is it?

Answer 19

By distillation

The water is boiled to make steam and the steam is condensed

Question 20

Why isn’t distillation used for tap water? Where is it used?

Answer 20

It is too expensive to pay for the heat energy

Instead it is used in chemistry labs

Question 21

Why is flouride added to water in some parts of the country?

Answer 21

It helps to reduce tooth decay

Question 22

Why is chlorine added to water?

Answer 22

To prevent disease

Question 23

Why is it controversial to use chlorine in water?

Answer 23

Some studies have linked adding chlorine to water with an increase in certain cancers

→ Chlorine can react with other natural substances in water to produce toxic by-products which some people think can cause cancer

Question 24

Why is it controversial to add flourine to drinking water?

Answer 24

Because a high dose of flourine can cause cancer and bone problems in humans

→ There is also a concern about whether to ‘mass medicate’, as people can choose to use flouride toothpaste but not drink flouride water