Chemistry - Water

Question 1

How can natural water supplies be harmful? (dissolved ions)

Answer 1

As water runs through and over the ground, it dissolves many chemicals from rocks which then become ions in the water. The most common ions found in natural water are sodium, calcium, magnesium, sulfate, chloride and hydrogen carbonate.

Question 2

How can natural water supplies be harmful? (dissolved gases)

Answer 2

Gases from our atmosphere can dissolve in water. The most important dissolved gases in natural water are oxygen (required for aquatic organisms for respiration) and carbon dioxide (required by plants for photosynthesis).

Question 3

How can natural water supplies be harmful?(microorganisms)

Answer 3

Natural water supplies contain vast numbers of bacteria and other microscopic organisms. Some of these microorganisms can cause disease and so it is potentially dangerous to drink untreated water from rivers and lakes. The harmful microorganisms can be removed effectively during the water treatment process.

Question 4

How can natural water supplies be harmful?(pollutants)

Answer 4

As water travels over or through the land to rivers and lakes, it can dissolve chemical fertilizers and pesticides from farms, as well as animal waste. These form the majority of the pollutants found in our natural water supplies and can be harmful to our health in high concentrations. Sometimes pollutants are deliberately ‘dumped’ into water supplies, although it is illegal to do this in the UK.

Question 5

What are some ways to reduce water consumption because of the cost of purifying water?

Answer 5

take short showers instead of longer showers or baths
re-use bath or sink water to water plants
make sure washing machines/dishwashers are full before use
make sure taps are turned off when not in use
fix leaky taps
install a toilet with a ‘short flush’ setting
insulate water pipes to reduce the chance of pipes bursting in the winter months
install a water meter to monitor water use

Question 6

What are some methods of abstracting water from natural sources?

Answer 6

pumping water from underground
building dams and creating reservoirs
taking water from rivers/lakes
collecting rainwater
desalination (removal of salt from seawater)

Question 7

What is sedimentation?

Answer 7

Sedimentation – the water is stored in a large tank or a reservoir. This slows down the flow of the water and allows large, insoluble particles to settle to the bottom. The cleaner water at the top can then be extracted for the next stage.

Question 8

What is filtration?

Answer 8

Filtration – the water is sprayed onto specially-prepared layers of sand and gravel. As it trickles through, smaller insoluble particles are removed. The filter beds are cleaned periodically by pumping clean water backwards through the filter.

Question 9

What is chlorination?

Answer 9

Chlorination – chlorine gas is injected into the water to sterilise it. The chlorine is poisonous and so kills microorganisms.

Question 10

What is demineralisation?

Answer 10

The dissolving of the enamel is known as demineralisation.

Question 11

Why do people think that fluoride should not be added to water?

Answer 11

-High fluoride levels can cause dental fluorosis. High fluoride intake can also cause health problems such as stomach cancer and infertility.
-Around 90 per cent of toothpastes and mouthwashes already contain high concentrations of fluoride and so some people say that fluoridated water is not needed.
-Some people see it as a form of mass medication that is being given without our consent. However, chlorine is universally accepted as a necessary addition to our water for public safety.

Question 12

How does fluoride help prevent tooth decay?

Answer 12

It alters the structure of the developing enamel of young children so that it is resistant to reacting with acid.
Low levels of fluoride increase the rate of remineralisation.
It reduces the ability of plaque bacteria to form acid.

Question 13

What is desalination?

Answer 13

Desalination is the removal of salt from seawater. This produces clean drinking water and is particularly useful in countries that have coastlines but no readily available fresh water sources, such as rivers and streams.

Question 14

What is reverse osmosis?

Answer 14

Where seawater is forced through a membrane at high pressure. The membrane allows water molecules to pass through but prevents any other chemicals dissolved in the water from passing through.

Question 15

What is thermal desalination?

Answer 15

the salt water is heated or the water is allowed to evaporate
the water vapour is collected rather than being lost
the water vapour is condensed to form pure water/fresh water
the salt is left behind and can be used for other purposes

Question 16

What are the disadvantages of desalination?

Answer 16

-Both desalination processes use a lot more energy than traditional water treatment methods.
-The increased energy demand means that desalination is more expensive than traditional water treatment methods. This means some poorer countries that would benefit from desalination cannot afford it.
-The increased energy demand means that desalination produces more greenhouse gases than traditional water treatment methods, which produce very little in comparison.
-The very salty water produced by membrane desalination is a pollutant and must be disposed of carefully.
-Desalination plants may be a very long distance from some populated areas and large lengths of water pipes must be built to get the water to the people who need it.

Question 17

Why is desalination most commonly used in the Middle East?

Answer 17

rainfall is low
many of the countries have coastlines
many of the countries have access to cheap oil for energy
many of the countries are quite wealthy

Question 18

What are miscible and immiscible liquids?

Answer 18

When two liquids can fully mix together they are called miscible liquids.
Liquids that do not mix and form layers are known as immiscible liquids.

Question 19

What is distillation?

Answer 19

A separation technique which involves a solution being heated so that the solvent evaporates before being cooled to form a pure liquid. Distillation can be used to separate two liquids that are fully mixed together to form a solution.

Question 20

What is a soluble substance?

Answer 20

If a substance is soluble it will dissolve in a given amount of liquid, called the ‘solvent’.

Question 21

What is an experiment to determine solubility?

Answer 21

Measure accurately 100 cm3 of water and add to a beaker
Add small amounts of the solute until no more can dissolve
Record the mass of an evaporating dish
Filter the mixture so the undissolved solid is left behind and the solution is in the evaporating dish
Remove the water by heating or evaporation
Weigh the evaporating dish with the solute in it and calculate the mass of the solute that was dissolved

Question 22

What is a solubility curve?

Answer 22

Every solute has a fixed solubility at room temperature. However, the solubility of a solute changes as the temperature of the water changes. The solubility of a substance in water at different temperatures can be displayed using a solubility curve. Solubility curves can be used to determine the mass of crystals formed when a solution is cooled.

Question 23

What is the effect of temperature on solubility?

Answer 23

In general, solids become more soluble as the temperature increases. This is why sugar dissolves better in hot water than in cold water.

Question 24

What does hard water contain?

Answer 24

Hard water contains dissolved magnesium ions (Mg2+) and calcium ions (Ca2+), which can get into the water when it comes into contact with limestone and other rocks that contain calcium compounds. This can happen, for example, when rainwater flows over rocks on its way to a reservoir.

Question 25

What is temporary hardness?

Answer 25

Temporary hardness is removed by boiling the water. The boiling of temporary hard water forms a layer of limescale. This may coat the heating element in kettles and irons, for example, making them less efficient. Limescale is unsightly and it clogs up hot water pipes and boilers.

Question 26

What is temporary hardness caused by?

Answer 26

Temporary hardness is caused by dissolved calcium hydrogencarbonate

Rainwater is naturally acidic because it contains dissolved carbon dioxide from the air. It reacts with calcium carbonate in rocks to form calcium hydrogencarbonate.

Question 27

What is permanent hardness?

Answer 27

Unlike temporary hardness, permanent hardness is not removed by boiling the water but by adding sodium carbonate/washing soda.

Question 28

What are the advantages of hard water?

Answer 28

Some people prefer the flavour.
Calcium strengthens children’s teeth and bones.
Helps to reduce heart disease.
Some brewers and bakers prefer hard water.

Question 29

What are the disadvantages of hard water?

Answer 29

Difficult to form a lather with soap.
Scum forms and wastes soap.
Limescale forms within appliances such as kettles, washing machines and dishwashers – this slows down heat transfer, making them less efficient.
Hot water pipes ‘fur up’ and can become completely blocked.

Question 30

How do you investigate hardness in water?

Answer 30

Put a known volume of water into a boiling tube.
Add soap solution, one drop at a time, and shake the mixture for 10 seconds.
Record the number of drops of soap needed to make a lather. The greater the number of drops needed to make a stable lather, the harder the water.

Question 31

How do you soften hard water?

Answer 31

Sodium carbonate is also known as washing soda. It can soften water that has temporary hardness and it can soften water that has permanent hardness.

It is an inexpensive method of removing hardness but leads to the formation of limescale, which can block narrow water pipes.

Question 32

What happens to the sodium carbonate in hard water?

Answer 32

Sodium carbonate is soluble in water and adds a large amount of carbonate ions to the water. These react with dissolved calcium ions, forming a precipitate of calcium carbonate. The calcium ions come from the hard water and the carbonate ions from the washing soda.

Question 33

What is ion exchange?

Answer 33

Ion-exchange columns can also soften water that has temporary hardness as well as water that has permanent hardness. The hard water passes through the column continuously, where the calcium and magnesium ions are removed. This technique is more convenient than using washing soda because you don’t need to change the column very often. However, the ion-exchange columns are expensive.

Question 34

What happens when temporary hard water is boiled (calcium hydrogencarbonate)?

Answer 34

When temporary hard water is boiled, the soluble calcium hydrogencarbonate decomposes (breaks down) to form calcium carbonate (which is insoluble), water and carbon dioxide:

Question 35

What is permenant hardness caused by?

Answer 35

Permanent hardness is caused by dissolved calcium sulfate, which does not decompose (break down) when heated.

Question 36

How does ion exchange soften water?

Answer 36

The resin within the column is made into small balls around 1-2 mm in diameter, which are packed into a tube or ‘column’.

The ion-exchange resin starts with sodium ions stuck to it. As the hard water passes through the column, sodium ions come off the resin and go into the water, while calcium ions come out of the water and stick to the resin. In effect, calcium ions that cause hardness are swapped for sodium ions that do not cause hardness.