Hardness Of Water

Question 1

When mixed with soap, hard water will:

Answer 1

form scum from dissolved calcium and magnesium ions in the water reacting with soap.

Question 2

When mixed with soap, soft water will:

Answer 2

form a lather.

Question 3

What happens if you heat hard water?

Answer 3

It will form scale (calcium carbonate).

Question 4

Why is heated hard water forming scale a problem for a kettle?

Answer 4

• it won’t heat it properly, causing more energy to be supplied to the kettle and increase costs because the heating filament is covered in limescale
• goes into water and causes bad taste

Question 5

Name two positives of hard water.

Answer 5

• calcium carbonate is good for healthy bones and teeth

- studies show that people in hard water areas are less likely to develop heart disease

Question 6

How do you remove temporary hardness?

Answer 6

hydrogencarbonate can be removed by boiling-

calcium hydrogencarbonate = calcium carbonate + water + carbon dioxide

Question 7

How do you remove permanent hardness?

Answer 7

you can’t boil it to get rid of the hardness as it’s calcium sulphate, not calcium hydrogencarbonate. to remove both, add sodium carbonate and use in an ion exchange column-

Na2 resin + Ca2+ => Ca resin + 2Na+ (it’s a displacement reaction)

Question 8

How are stalactites formed?

Answer 8

• falling rain dissolves carbon dioxide from the air
• the rain makes carbonic acid : H2O + CO2 H2CO2
• carbonic acid falls on limestone rocks
• the limestone reacts with the carbonic acid
• CaCO3 + H2CO3 Ca(HCO3)2
• rainwater containing calcium hydrogen carbonate drips into a cave
• as water evaporates, calcium carbonate forms again Ca(HCO3)2 H2O +CO2
• over thousands of years, the calcium carbonate forms a stalactite

Question 9

How does limescale form in a boiler?

Answer 9

• falling rain dissolves carbon dioxide from the air
• the rain makes carbonic acid : H2O + CO2 H2CO2
• carbonic acid + limestoneor chalk = calcium
• hydrogen carbonate CaCO3 +H2CO3 Ca(HCO3)2
• tap water containing calcium hydrogen carbonate is heated in the boiler
• calcium hydrogen carbonate decomposes upon heating and forms limescale

Question 10

What is scum?

Answer 10

dissolved Mg2+ / Ca2+ in water

Question 11

What is scale?

Answer 11

permanent hard water that has been boiled.

Question 12

Name a method to make hard water oft (not boiling or ion exchange column).

Answer 12

Pour soda into the water- it causes a reaction and removes the ions that makes it hard.

Question 13

Why should hot water pipes and central heating boilers be checked regularly in hard water areas?

Answer 13

to prevent them clogging up with scale/scum.

Question 14

Name a positive of adding fluorine to water.

Answer 14

lowers tooth decay.

Question 15

Name a negative of adding fluorine to water.

Answer 15

people believe they could cause cancers or bone problems.

Question 16

Name a positive of adding chlorine to water.

Answer 16

removes microbes.

Question 17

Name a negative of adding chlorine to water.

Answer 17

toxic by-products.

Question 18

What happens when Lithium is added to water?

Answer 18

quick fizzing, turns universal indicator purple as it’s an alkali metal.

Question 19

What happens when Sodium is added to water?

Answer 19

fizzes, the outer shape forming an almost perfect sphere.

Question 20

What happens when Potassium is added to water?

Answer 20

It lights up, giving off a bright pink light.