3.2 Water

Question 1

What does hard water contain?

Answer 1

high concentrations of calcium and magnesium ions

Question 2

How is hard water formed?

Answer 2

• calcium and magnesium ions dissolve when streams and rivers flow over rocks containing these compounds e.g.gypsum (calcium sulfate)
• limestone (calcium carbonate) also causes hardness in water which is nearly insoluble in water but when rain falls carbon dioxide dissolves in it which makes rainfall slightly acidic - water in streams and rivers also becomes slightly acidic
• calcium carbonate reacts with weakly acidic solution, calcium hydrogencarbonate formed is soluble in water, therefore calcium ions can get into the water, making it hard
• CaCO3(s) + H2O(l) + CO2(aq) –> Ca2+(aq) + 2HCO3-(aq)
• dissolved ions are carried into reservoirs and into domestic water supply

Question 3

Why does hard water waste soap?

Answer 3

• can be expensive because we need to use much more soap
• before it can do its job, must react with calcium and magnesium ions in the water (forming insoluble salts that appear as scum)
• can only lather after reacting with all of the ions
• sodium stearate (soap) + Ca2+ ions –> calcium stearate precipitate (scum) + Na+ ions (soluble in water)

Question 4

How is limescale formed from hard water?

Answer 4

• insoluble solid can form if we heat one type of hard water e.g. washing machines, pipes, kettles etc (other hot water systems)
• pipes can eventually block up and other heating appliances can make them much less efficient…
• … scale is a very poor conductor of electricity so it takes longer to boil the water which uses more energy making it more expensive

Question 5

What are some advantages of hard water?

Answer 5

• calcium ions in drinking water help in the development of strong bones and teeth
• there is also evidence which suggests that hard water helps to reduce heart disease