Mr. Robert Zammit M.Sc.

Unit 4 – Water - Lesson 1 of 4 Page 1 of 5

Unit 4: Lesson 1 of 4

Water: Properties, Tests & Pollution.

General Properties of Water.

Water, H2O, is the most common molecule on Earth. It makes up 71% of the planet’s surface and from 55% to 78% of the human body. It is found as a liquid and solid (ice) in the oceans and as a gas in the atmosphere. However, natural water is rarely pure.

• Water is a colourless, tasteless, odourless liquid at room temperature and pressure.

• It can be produced in the lab by the combustion of hydrogen gas. The water vapour formed is condensed on a cold surface and collected.

2H2(g) + O2(g) → 2H2O(l) • Water can dissolve so many substances that it is often called the

universal solvent. • Water can be purified by repeated distillations. This water is called

distilled water. • Pure water has a freezing point of 0°C and boiling point of 100°C.

Impurities in water tend to DECREASE the melting point and INCREASE the boiling point.

• Pure water is an insulator of electricity since water molecules have no net charge. However, as solutes are added conductivity increases.

• Physical properties such as melting point, boiling point, density and conductivity are used as a measure of water purity.

• The chemical test for the presence of water is that it turns white anhydrous copper(II) sulfate blue. (It also turns blue anhydrous cobalt(II) chloride pink.)

CuSO4(s) + 5H2O(g) ⇌ CuSO4.5H2O(s)

Mr. Robert Zammit M.Sc.

Unit 4 – Water - Lesson 1 of 4 Page 2 of 5

Potable water.

Water is essential for life. Water that is safe for humans to drink is called potable water. Potable water is not pure water because it almost always contains dissolved impurities.

For water to be potable, it must have sufficiently low levels of dissolved salts and microbes. This is because dissolved salts can sometimes be harmful for humans and microbes can cause illnesses.

The methods used to make water potable depend on where you live. Starting with fresh water is easier than sea water, as removing the large amount of sodium chloride present in sea water requires a lot of energy.

In many countries, rain provides enough fresh water to meet the needs of the population. Rainwater collects in rivers, lakes and in rocks underground. This water contains low levels of dissolved substances. Most potable water in such countries is produced from naturally occurring fresh water by:

• passing the water through filter beds to remove insoluble particles • sterilising the water to kill microbes.

The methods used for sterilisation include chlorine, ozone and ultraviolet light.

Reverse Osmosis.

Tap water in Malta is a mixture of desalinated sea water and ground water. Since Malta is a small country with no rivers, lakes, or reservoirs, we get most of our water from the sea and just take salt out of it. This is done by a process called Reverse Osmosis.

Exercise 1:

1. Give three physical properties of water. 2. How can you show that air contains water vapour? Include a chemical equation. 3. Name 2 physical tests that can be done to show that a sample of water is pure. 4. Name and draw a diagram for the separation technique used to:

a. Obtain pure water from a suspension of zinc carbonate in water. b. Obtain pure water from an oil/water mixture. c. Obtain pure water from a solution of copper(II) sulfate. d. Obtain pure water from an alcohol/water mixture.

Mr. Robert Zammit M.Sc.

Unit 4 – Water - Lesson 1 of 4 Page 3 of 5

a) A semi-permeable membrane is a material that has tiny holes that allow water molecules to pass through but not solute particles.

b) When a concentrated solution and a dilute solution are separated using a semi-permeable membrane, water particles flow from the concentrated solution to the less concentrated one. In such way, two solutions of equal concentration are obtained. This process occurs naturally and is called osmosis.

c) Osmosis can be reversed by applying pressure on the concentrated solution, thus ‘forcing’ water molecules through the semi-permeable membrane – REVERSE OSMOSIS.

d) Reverse osmosis is the passage of water molecules from a concentrated solution to a less concentrated one caused by applying pressure.

e) Reverse osmosis plants also use filtration to remove residues and disinfection using chlorine (or some of its compounds) prior to reverse osmosis.

f) The process is a very high energy consuming process.

Water Pollution.

• Fertilizers contain anions such as nitrates and phosphates which are nutrients for aquatic plants and algae. Excessive growth of these results in blockage of sunlight in the deeper parts of the water. As these plants and algae die and decompose, they use up dissolved oxygen. An excessive number of these organisms results in an excessive usage of dissolved oxygen which badly affects the breathing ability of aquatic organisms that survive on oxygen.

• Soaps, detergents and sewage may be toxic and also result in excessive growth of algae. Apart from

this, sewage might also contain bacteria and viruses which might be harmful to some aquatic organisms.

Mr. Robert Zammit M.Sc.

Unit 4 – Water - Lesson 1 of 4 Page 4 of 5

• Pesticides can be harmful to some organisms. They build up through a food chain. For example, if a particular fish gets contaminated with a chemical in a pesticide, the organisms who eat the fish will also consume this chemical each time this type of fish is eaten. • Oil spills do not only occur when an oil tanker has an accident. Underwater

drilling for oil results in a small loss of crude oil in the sea. Oil is a cause for pollution for the shore lines, shore and aquatic animals and as it prevents light and oxygen from entering the water beneath it.

• Heavy metals are metals with

relatively high density when compared to other metals. These are

poisonous, sometimes even in very low concentrations. Examples include

lead, chromium, copper and mercury. These are sometimes discharged

illegally by industries and accumulate

up a food chain. • Thermal pollution is due to the

discharge of extremely hot or cold water which results in a temperature change in a particular area. Organisms are very susceptible to small changes in temperature (especially cold-blooded organisms) and heat pollution might kill a number of aquatic organisms.

By the end of this lesson you should be able to:

Describe, in outline, methods of purifying water supplies from naturally occurring impurities in terms of filtration, distillation, reverse osmosis and chlorination.

Describe a chemical test for the presence of water. Recall that physical properties can be used to test for purity of water. State a chemical test for the presence of water and a physical test for pure water. Recall ways of how water can be polluted and understand the harmful effects of water pollution.

Exercise 2:

1. Briefly explain: a. How potable (drinking) water is produced in dry countries such as Malta. b. How potable water is obtained in countries where rainfall is common throughout the year.

2. List: a. THREE ways how water and/or sea water pollution can occur. b. ONE harmful effect for EACH pollution method chosen in (a) above.

Mr. Robert Zammit M.Sc.

Unit 4 – Water - Lesson 1 of 4 Page 5 of 5

Model answers to Exercise 1:

1. Water is: colourless; tasteless; odourless.

2. Expose white anhydrous copper(II) sulfate to air which gradually turns to blue hydrated crystals. CuSO4(s) + 5H2O(g) ⇌ CuSO4.5H2O(s)

3. Check its physical properties: boiling point; melting point; density; conductivity.

4. Diagrams for: a. Filtration b. Separating funnel c. Simple distillation d. Fractional distillation

Model answers to Exercise 2:

Question 1: a) Most drinking water in Malta is obtained by Reverse Osmosis. This involves the passage of water

molecules from a concentrated solution to a less concentrated one caused by applying pressure through a semi-permeable membrane. This way most of the unwanted solutes in water are removed. Prior to reverse osmosis, filtration to remove residues and disinfection using chlorine are carried out.

b) Drinking water in such countries is obtained from rainwater which collects in rivers, lakes and in rocks underground. This water contains low levels of dissolved substances. This water is passed through filter beds to remove insoluble particles and is sterilised to kill microbes. The methods used for sterilisation include chlorine, ozone and ultraviolet light.

Question 2:

a) Any three of: Fertilizers; Sewage, soaps & detergents; Pesticides; Oil spills; Heavy metals; Thermal pollution.

b) Effects: Fertilizers: growth of algae causes oxygen depletion in water and blockage of sunlight; Sewage, soaps & detergents: growth of algae; harmful bacteria/viruses; toxic; Pesticides: Toxic Oil spills: toxic; blockage of sunlight Heavy metals: poisonous/toxic Thermal pollution: changes in temperature may kill cold-blooded organisms.