C3.2 Water

Question 1

What ions does hard water contain a lot of?

Answer 1

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

Question 2

How do Magnesium and Calcium ions get into water?

Answer 2

When the water comes in contact with particular types of rock (like limestone, chalk and gypsum)

Question 3

What happens when you use soft water with soap?

Answer 3

You get a nice lather

Question 4

What happens when you use hard water with soap?

Answer 4

You get scum - unless you’re using a soapless detergent

Question 5

Why does scum form when hard water is used with soap?

Answer 5

The magnesium and calcium react with the soap to make (insoluble) scum - to get a decent lather you need to use a lot of soap

Question 6

What are drawbacks of using hard water with soap?

Answer 6

A lot of soap is need to get a nice lather and soap isn’t free so you spend more money because you use more soap

Question 7

What are benefits of hard water?

Answer 7

The calcium ion help to maintain healthy teeth and bones and studies have found that people who drink hard water are at less risk of heart disease than people who drink soft water

Question 8

What are the different types of hard water? And what’s the difference?

Answer 8

Temporary and permanent hardness - Temporary hard water is made soft when boiled, permanent hard water remains hard after boiling

Question 9

What happens when temporary hard water is boiled?

Answer 9

Scale is formed (mainly calcium carbonate) - this can form in pipes, boilers and washing machines

Question 10

What happens when scale is formed on heating units?

Answer 10

Scale is a bit of a thermal insulator so when it covers a heating unit (e.g. in a kettle) the system becomes less efficient and may need to be replaced - which is expensive

Question 11

What ions does temporary hard water contain?

Answer 11

Temporary hard water contains hyrdrogencarbonate ions (HCO3-) forming calcium hydrogencarbonate (Ca(HCO3)2) and magnesium hydrogencarbonate Mg(HCO3)2*)

Question 12

Why is scale produced when temporary hard water is heated? And why can temporary hard water become soft?

Answer 12

The hydrogencarbonate ions decompose to form carbonate ions (CO3*2-) which then make insoluble precipitates: magnesium and calcium carbonate ions. This makes the Ca and Mg ions precipitate out of the water making it soft.

Question 13

Why can permanent hard water not become soft?

Answer 13

Because it contains salts which don’t decompose and form insoluble precipitates

Question 14

What methods could you use to soften both permanent and temporary hard water?

Answer 14

By adding sodium carbonate or by using an ion exchange column

Question 15

How does adding sodium carbonate make water soft?

Answer 15

The sodium carbonate (Na2CO3) reacts with the Ca2+ and Mg2+ ions to make insoluble precipitates: Magnesium and Calcium carbonate - the Ca2+ and Mg2+ ions are no longer dissolved in the water, making it soft

Question 16

What is the symbol equation of Mg2+ and Ca2+ reacting with sodium carbonate?

Answer 16

CO32+(aq) + Mg2+ ==> MgCO3(s)

CO32+(aq) + Ca2+ ==> CaCO3(s)

Question 17

How does passing water through an ion exchange column make it soft?

Answer 17

Insoluble resins in the column containing sodium or hydrogen ions exchange the Ca2+ and Mg2+ ions for the H+ and Na+ ions - the Ca2+ and Mg2+ ions are now trapped in the resin

Question 18

What is the symbol equation of water passing through an ion exchange column?

Answer 18

Na2*Resin(s) + Ca2+(aq) ==> CaResin(s) + 2Na+(aq)