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The Earth’s water

It is estimated that there are 1,380 million km3 of water on the Earth.

However, only 3% of the Earth’s water is fresh. The remaining 97% is saltwater, and forms the seas and oceans.


The water cycle


Events in the water cycle


Why is water so important?

Water is extremely important to life on Earth and is also used by humans in a wide range of activities.

transport coolants

washing

solvents

fishing leisure


How is water treated?

Water for our homes is treated to make it safe for drinking by the following system:

supplied to homes, offices and factories

chlorine added filter beds

settlement tank

chemicals added which combine with dirt to form a precipitate

water from reservoir


Stages in water treatment


What is desalination?

In some countries, there is not enough rainwater to collect in reservoirs. Instead, the process of desalination is used to make freshwater from seawater.

Can you think of any advantages and disadvantages of desalination?

The simplest method of desalination is to distil the water, leaving the salt behind.


Why is fluoride added to water?

In some countries, fluoride is added to drinking water in the form of hexafluorosilicic acid (H2SiF6). In the UK, about 10% of homes receive water with added fluoride.

Fluoride has been proven to strengthen tooth enamel, which helps to prevent tooth decay.

Scientists first made a link between fluoride and teeth in the 1930s. What evidence do you think they looked at?

Fluoride occurs naturally in some water supplies, and is also naturally present in tea.

Most toothpastes also contain added fluoride.


Should fluoride be added to our water?


Voting: fluoride in drinking water


Dissolving words

If a substance CAN be dissolved it is called __________.

If a substance CANNOT be dissolved it is called __________.

solute

solvent

solution

dissolving

There are many words to do with solubility – make sure you don’t confuse them!

Can you fill in the missing words in the sentences below?


the amount of a substance that will dissolve in 1 dm3 of water at 25°C (1 dm3 = 1000 cm3 = 1 litre)

What is solubility?

For example, the solubility of sodium chloride is 360 g/dm3.

Q2) What mass of sodium chloride would dissolve in 2 dm3 of water at 25 °C?

Q3) What mass of sodium chloride would dissolve in 500 cm3 of water at 25 °C?

Q1) What would happen if you added more than 360 g of sodium chloride to 1 dm3 of water?

Solubility is defined as:


Investigating solubility

An experiment was carried out to determine the solubility of a number of solid chlorides. Here are the results:

How can you use the results to calculate the solubility in g/dm3 of each solid?

200 cm3CuCl2

chloride volume of water

MgCl2

KCl

NaCl

ZnCl2

mass of chloride that dissolved

151 g

80 g

30 g

72 g

618 g

150 cm3

100 cm3

200 cm3

150 cm3


Calculating solubility


Which chloride is the most soluble?


Solubility and temperature

The solubility of a substance depends on the temperature of the solvent. This table summarises the effect of temperature on the solubility of four different solids:

1670

potassium chloride

temperature

0 °C 20 °C 40 °C 60 °C 80 °C

sodium chloride

sodium nitrate

potassium nitrate

solid:

139 316 613 1060

280 342 401 458 513

357 359 364 371 380

730 876 1020 1220 1480

solubility (g/dm3)

How would you carry out this experiment?


Solubility and temperature graph


Saturation and crystallization

Solutions become supersaturated when they contain more solute than they would normally be able to dissolve.

The solute will stay in solution until a ‘seed’ crystal is added. This causes it to crystallize out of the solution very quickly.

When it does this, it gives out heat energy.

This can happen when a saturated solution is cooled, or another change in conditions occurs that causes solubility to decrease.


Solubility of gases

Many gases are soluble in water. For example, fish can breathe because of the oxygen dissolved in water.

Fizzy drinks contain carbon dioxide that has been dissolved into them under pressure.

When the pressure inside the container is released, the carbon dioxide gas comes out of the solution, producing fizzy bubbles.


Solubility of gases and temperature

What happens to the solubility of gases as the temperature increases?

carbon dioxide 3.35 1.69 0.76

nitrogen

temperature

0 °C 20 °C 50 °C

oxygen

gas:

0.029 0.019 0.012

0.069 0.043 0.027

solubility (g/dm3)

If you leave a glass of lemonade for a few days, it goes flat.

If you left one glass of lemonade in the fridge and another beside the radiator, which is likely to go flat first?


Solubility of oxygen

As the temperature of water increases, the solubility of oxygen decreases.

This can be a problem for fish living in rivers near factories.

In industry, water is often used to keep machinery cool. This produces warm waste water which then flows out into nearby rivers.

Although the water is clean, it can still affect aquatic animals. The warm water contains less of the dissolved oxygen that fish need to breathe.


Solubility of oxygen

A team of scientists investigated the temperature and levels of dissolved oxygen at different points along a river.

Upstream of the factory: “The water is cool and clean, with lots of dissolved oxygen, and plenty of small invertebrates and fish.”

Downstream of the factory: “The water is clean, but warm. We

found a few dead fish in the river.”

What can be done by the factory owners to solve the problem?


Carbon dioxide and the ocean

Carbon dioxide is very soluble in water, and scientists believe that about a third of the carbon dioxide produced by burning fossil fuels dissolves into the oceans.

CO2

sedimentation

carbonate organic carbon

circulation


Solubility of carbon dioxide

Some scientists have suggested that the oceans could act as a “carbon dioxide reservoir” to remove it from the atmosphere.

However, if global warming continues and the oceans get warmer, what will happen to the amount of carbon dioxide dissolved in the oceans?

The gas could be pumped down to the bottom of the ocean and “stored” there.

CO2 is a greenhouse gas.


Solubility: true or false?


What is hard water?


Using hard and soft water

Soft water forms a lather easily with soap.

Hard water needs more soap to form a lather, adding additional cost to cleaning processes.

How could you test water samples to see if they are hard or soft?

This is because dissolved chemicals in the hard water react with soap to form a scum.


What is hard water?

Hard water contains dissolved calcium and magnesium compounds such as:

magnesium hydrogen carbonate [Mg(HCO3)2]

calcium hydrogen carbonate [Ca(HCO3)2]

magnesium sulfate [MgSO4]

calcium sulfate [CaSO4]

Hard water can be temporary or permanent.

Permanent hard water contains magnesium sulfate and calcium sulfate.

Temporary hard water contains magnesium hydrogen carbonate and calcium hydrogen carbonate.


Hard water chemicals


Hard water and soap

Rainwater becomes slightly acidic as carbon dioxide from the air dissolves in it, forming carbonic acid.

Limestone contains calcium carbonate. As the slightly acidic rainwater trickles through rocks, the calcium carbonate reacts to form soluble calcium hydrogen carbonate:

+ + +carbonic

acid

H2CO3 (aq)

calciumcarbonate

CaCO3 (s)

calcium hydrogencarbonate

Ca(HCO3)2 (aq)

carbondioxide

CO2 (g)

water

H2O (l)+ + +

One of the chemicals in soap is sodium stearate.

The dissolved calcium and magnesium compounds in hard water react with sodium stearate to form a solid called calcium stearate (or ‘scum’).


How does hard water react with soap?

Soap ‘scum’ is formed by the reaction:

The soap will only form a lather when all the dissolved calcium hydrogen carbonate in the water has reacted.

sodiumstearate

2C17H35COONa (s) 2NaHCO3 (aq)(C17H35COO)2Ca (s)

calciumhydrogencarbonate

Ca(HCO3)2 (aq)

+ calciumstearate

sodiumhydrogencarbonate

+

+ +

Using hard water can cause problems:

More soap is needed to get a lather.

It can be difficult to clean the scum from bathtubs and sinks.

Hard water can be unsuitable for industrial processes like dying.


Limescale

When hard water is heated, the dissolved calcium hydrogen carbonate decomposes to form solid calcium carbonate.

These deposits of calcium carbonate are called limescale.

Limescale can block pipes and coat the heating elements in kettles, washing machines and heaters.

+

+


Ca(HCO3)2 (aq)

calciumcarbonate

CaCO3 (s)

water

H2O (l)

carbon dioxide

CO2 (g)

+

+

Limescale is a poor heat conductor, and reduces the efficiency of appliances.


Removing limescale

Weak acids, such as ethanoic acid, can be used as descalers.

The acid reacts with the limescale to form soluble compounds, which are then washed away.

+ + +ethanoic

acid

2CH3COOH (aq)

calciumcarbonate

CaCO3 (s)

calciumethanoate

(CH3COO)2Ca (aq)

carbondioxide

CO2 (g)

water

H2O (l)+ + +

What would a person observing this reaction see?


What are the benefits of hard water?

Hard water can be good for health:

calcium is needed for healthy bones and teeth

magnesium is needed for effective metabolism.

Some studies have also shown that people living in hard water areas are less likely to suffer from heart disease.

The World Health Organisation states that there is not yet enough evidence to confirm a link between hard water and heart disease.

What evidence would you gather to look for a link between hard water and heart disease?


Softening temporary hard water

Temporary hard water contains calcium hydrogen carbonate.

This is relatively easy to remove, because it decomposes on heating to form solid calcium carbonate:

Knowing this, how would you remove the solid calcium carbonate from the water?

What would be left behind after removing temporary hardness using this method?

+

+


Ca(HCO3)2 (aq)

calciumcarbonate

CaCO3 (s)

water

H2O (l)

carbon dioxide

CO2 (g)

+

+


Softening hard water

Both temporary and permanent hard water can be softened by adding sodium carbonate (washing soda).

The sodium carbonate reacts with the calcium compounds in the water to form calcium carbonate and soluble sodium compounds, which do not contribute to hardness:

+ +calcium

hydrogencarbonate

Ca(HCO3)2 (aq)

sodiumcarbonate

Na2CO3 (s)

calciumcarbonate

CaCO3 (s)

sodiumhydrogencarbonate

2NaHCO3 (aq)+ +

What is the word and symbol equation for the reaction between calcium sulfate (which causes permanent hardness) and sodium carbonate?


Softening hard water


Hard water: summary


water

oil

soap molecule

What is soap?

If oil is added to water, the two liquids do not mix.

Because of this, grease stains can be difficult to remove during washing.

Soaps are compounds which act as emulsifiers.

This means that they help the oil to mix with the water.


How do soaps work?


How are soaps made?

Soaps are usually made from vegetable fats and oils. These consist of 3 fatty acid chains, held together by a glycerol molecule.

The reaction used to make soap from fats and oils is called saponification.


How are soaps made?


What are soapless detergents?

Soapless detergents can be made from crude oil, rather than vegetable oils or animal fats.

sodium octadecylsulfonate

hydrophobic end interacts with oil molecules

hydrophilic end interacts with water molecules

Soapless detergents are still made of long, hydrophobic carbon chains, but the hydrophilic end of the molecule is a sulfonate:


Soaps vs. soapless detergents

What are the advantages and disadvantages of soapless detergents over soaps?

Soaps form a scum with hard water, meaning that it is more difficult to produce a lather. Soapless detergents react with the substances in hard water to form soluble compounds, and so do not form a scum.

Soapless detergents can be made from the by-products of the oil refining process, so can be cheaper to produce than soaps.

Some soapless detergents are not biodegradable. This means that they stay in the water system causing froth in rivers and streams.


Biological and non-biological detergents

Biological detergents are soapless detergents that contain enzymes.

The enzymes are used to break down chemicals, e.g. in stains on clothes.

Non-biological detergents do not contain enzymes, but just rely on the action of the detergent to remove grease and stains from items.


Advantage or disadvantage?


Glossary


Anagrams


Multiple-choice quiz

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