Back to: O level chemistry notes full Uganda syllabus
These elements are referred to as inert elements because they are non reactive. The elements include; helium,neon,argon,krypton,xenon and radon. Characteristics of these elements include:
- Their outer most energy levels are completely filled with electrons making them very stable elements and chemically non reactive. However in spite of their stable electronic configurations, a few compounds have been formed from xenon(e.g. Xenon(II)fluoride,XeF2) and krypton(E.g. krypton(II)fluoride, KrF2)
- The noble gases exist as monatomic gases.
- They have very low melting and boiling points due to negligible forces of attraction between the atoms.
- Noble gases are colorless
Though nobles gases are non reactive, they are used for a variety of purposes including the following:
- Argon is filled in light bulbs to prevent the tungsten fillament from reacting with air.
- Argon provides an inert environment for gas-liquid chromatography, for risky welding jobs and for some chemical reactions.
- Neon is used extensively in advertising signs and in lasers.
- Helium is used to inflate air ships, weather balloons and aeroplane tyres.
- A mixture of oxygen and helium is used by divers instead of air.
Sample questions on Acids, Bases, Indicators and Chemical Families
Acids, bases and indicators
- Write short notes on: acids, bases, alkalis, indicators and basicity. Illustrate with examples and equations.
- Outline three properties typical of acids and three properties typical of alkalis. (illustrate with equations). Differentiate between: a strong acid and a weak acid; an alkali and a base.
- Describe how you can use a named material from the environment to make an indicator in the laboratory.
- What is meant by neutralization reaction? Illustrate by use of equations.
Chemical families - Describe with equations, the reactions of
a) Alkaline metals with air, water, acids and chlorine.
b) Halogen with water and alkalis.
c) Alkaline earth metals with water. - Discuss the displacement reaction of halogens.
- Mention three examples of noble gases. Outline four uses of noble gases.
WATER AND HYDROGEN
WATER
Water is a chemical compound consisting of oxygen and hydrogen. Its chemical formula is H2O.
Sources of water
- Rain water
The water originates from rain and it is relatively pure with no dissolved minerals. However, it contains suspended materials and dissolved gases such as carbon dioxide and sulphur dioxide. Rain water is good for washing as it forms lather easily with soap and does not contain dissolved salts. It is also good for drinking since it has a flat taste. - Wells and spring water
The water comes from underground and is obtained by drilling the ground inform of a bore hole or, the water just springs out of the ground. It is actually rain water that has drained into the ground and collected into a pool of underground water. It has less suspended materials but contain dissolved mineral salts making it good for drinking. It may not be good for washing because of the dissolved mineral salts that delay the formation of lather. - River and lake water
The water is obtained from rivers and lakes. It contains both suspended materials and dissolved minerals. As water flows in to rivers and lakes, it gathers floating materials and pollutants such as sewage and industrial wastes, so it is not good for washing and drinking. - Sea and ocean water
These provide large sources of water. The water contains a lot of suspended materials and dissolved substances. The mineral salts accumulate settle at the bottom of the water and the water may become saturated with the salt. This is why the water tastes salty. - Ice caps
The water is obtained from melting ice and it is relatively pure. The ice formed at the peaks of mountains melts and the water runs down forming spings.
Uses of water - Biological use
All biological processes in living organisms require water as a medium for the reactions. For example, photosynthesis, excretion, transport of materials and transmission of impulses. - Domestic use
Drinking, cooking, washing, bathing, mopping, construction, recreation e.t.c. - Farm use
Irrigation, for dipping animals, for drinking by animals, for fish farming e.t.c. - Industrial use
Water is used as a solvent in many industries, for cleaning, for cooling purposes especially in power stations, used in steam engines, generating hydro electricity, transportation using boats, ships e.t.c, for recreation purposes and construction.
Water cycle
Water cycle is the process by which water circulates round the earth. The driving force for the cycle is the sun.
The water from the water bodies like seas, lakes, rivers e.t.c. evaporates into the atmosphere due to the sun heat. The vapor cools in the atmosphere and then condenses to form clouds which fall as rain. The rain then flows back into the water bodies.
N.B. Water vapor in the atmosphere may also result from transpiration processes in plants and respiration processes of all living organisms and burning of starchy food materials forming carbon dioxide and water which are expelled in to the atmosphere.
Water pollution
This is the discharge of unwanted materials/ substances into water bodies. The un wanted materials are generally called pollutants and are harmful to living organisms. Examples include; acids, bases, insecticides, fertilizers, sewage, solid particles, oil e.t.c.
Sources of water pollutants
- Atmospheric gases like carbondioxide, sulphur dioxide, nitrogen dioxide and hydrogen sulphide. These gases dissolve in the rain water making it acidic.
- Dissolved maerials from rocks and soil.
- Industrial discharge (effluent)
- Fertilizers washed from farm lands by running water.
- Insecticides and herbicides resulting from spraying from homes and farms.
- Soaps, detergents and other chemicals from homes, salons e.t.c.
Water treatment
This is the removal of harmful substances from water making it suitable for use (domestic use. In Uganda, the treatment of water is done by Uganda National Water and Sewage Co operation (UNWSC).
Processes involved in water treatment are: - Addition of chemicals
Chemicals are added to water to remove harmful substances and neutralize acids and bases. The chemicals as well remove hardness of water, precipitate metals and cause suspended solid particles to settle. - Filtration
Filtration is carried out to remove the solid particles. Water is made to pass through wire mesh screens to remove large and floating objects and then through sand and gravel to filter out smaller solid particles. The last part of the filter bed consists of sand with some microbes grown to remove some bacteria that might be harmful. - Chlorination and fluoridation
Chlorine or fluorine is added to kill the harmful organisms such as bacteria. Only a small concentration of chlorine is used because it is harmful. However, in swimming pools, high concentration of chlorine is used as the water may contain high bacteria concentration is not meant for drinking. In some countries, ozone is used instead of chlorine.
Sewage
Sewage is running water containing wastes from toilets, bathrooms, sinks, factories and streets of towns.
Sewage treatment
The reason for treating sewage is to remove harmful pollutants before allowing the sewage to join water bodies. The following are the processes involved in sewage treatment:
- Separation of solid and liquid wastes through sieving and sedimentation.
- Addition of certain bacteria to the liquid wastes to convert harmful materials to harmless substances.
- Addition of chlorine to kill the harmful bacteria
- The liquid waste (effluent) from the treatment plant is the discharged into the water body. The solid waste from the treatment of sewage is the called the sludge and has a number of uses.
Uses of sludge
- It is used as fertilizer because it is rich in nitrogen and phosphorus.
- It is used as a raw material to produce bio gas.
- It is used for road surfacing since it forms hard solids.
- It is used in land reclamation to fill quarries.
Properties of water
Physical properties
- Pure water is a clear, colorless and tasteless liquid.
- It boils at 100˚C and melts at 0˚C.
- It has a density of 1g/cm3 at 4˚C.
- Pure water is neutral to litmus i.e. has no effect on litmus.
- Water expands on freezing.
- It is a universal solvent.
Test for water
Water is chemically tested for using anhydrous copper (II) sulphate of cobalt chloride paper.
When water is added to anhydrous copper(II)sulphate, it changes color from white to blue. Or when water is added to cobalt chloride paper, it changes color from blue to pink.
a) Potassium
When a piece of potassium is placed into a trough containing water, it reacts very vigorously, catches fire and burns with a purple flame; it darts on the water surface producing a hissing sound and white fumes of hydrogen gas. It explodes with a pop sound as it disappears. The resultant solution turns red litmus paper to blue.
b) Sodium
Sodium reacts vigorously with water. When a piece of sodium is placed in cold water, it melts into a silvery ball and darts on the surface of water as it produces a hissing sound with evolution of a colorless hydrogen gas. The sodium gradually becomes smaller and smaller as it reacts with water to produce an alkaline solution of sodium hydroxide.
N.B. Sodium burns with a yellow flame if its movement on water is restricted.
c) Calcium
Calcium reacts steadily with water. When a small piece of calcium is dropped is water, it sinks to the bottom producing streams of bubbles of a colorless gas (hydrogen gas) and an alkaline solution that turns red litmus paper blue. The water turns milky as the calcium hydroxide is slightly soluble in water.
d) Magnesium
Magnesium sinks to the bottom and reacts very slowly with cold water to produce very few bubbles of hydrogen gas on the surface of the metal and an alkaline solution of magnesium hydroxide.
However, hot magnesium reacts rapidly with steam, burning with a bright white flame leaving white ash of magnesium oxide. The hydrogen gas produced burns with a blue flame in air.
Note
- The magnesium metal used is first cleaned to remove the oxide coating due to corrosion.
- The cotton wool soaked in water is heated to release steam that reacts with the heated magnesium.
- The boiling tube is first heated to drive out all air (oxygen).
e) Iron and zinc
Zinc and iron do not react with cold water but react with steam when heated to red hot producing their respective oxides and hydrogen gas. Zinc reacts faster than iron.
Aluminium does not react with water or steam due to the formation of an insoluble oxide layer which prevents contact between water and the metal inhibiting any further reaction.
All the other metals below iron do not react with either cold water or even steam.
- Reaction with oxides
Water reacts with metallic oxides (basic oxides) to form alkaline solutions. E.g.
HARDNESS OF WATER
Water can be classified as hard or soft.
Soft water is one which forms lather readily with soap. Examples of soft water include: rain water, distilled water, deionised water.
Hard water
This is water that does not form lather readily with soap. Examples of hard water include: tap water, sea water, borehole, well water, spring water.
Causes of hardness of water
Hardness of water is caused by the presence of the calcium ion (Ca+) and magnesium ion (Mg+). These ions reach water when the soluble salts of calcium and magnesium are dissolved by rain water and carried to water bodies. The compounds that cause hardness of water include; calcium hydrogen carbonate (Ca(HCO3)2), magnesium
hydrogen carbonate(Mg(HCO3)2),calcium chloride (CaCl2), magnesium chloride (MgCl2), Calcium sulphate (CaSO4), Magnesium sulphate (MgSO4).
Effects of magnesium and calcium ions on soap
Soap is a sodium salt of a long chain carboxylic acid. It is known as sodium stearate (Na-st). Soap reacts with magnesium or calcium ions to form dirty white solids called scum. So, when hard water is used for washing, a lot of soap is used because part of the soap initially reacts with calcium and magnesium ions to form scum.
Types of hard water
There are two types of hard water i.e. permanent and temporary hard water.
Temporary hard water
This is a type of hard water whose hardness can be removed by boiling. I.e. the water can be made soft by boiling.
Causes: it is caused by the presence of dissolved calcium hydrogen carbonate or magnesium hydrogen carbonate.
Permanent hardness
Is a type of hard water whose hardness cannot be removed by boiling. I.e. the hard water cannot become soft on boiling.
Causes: it is caused by calcium sulphate, calcium chloride, magnesium sulphate and magnesium chloride.
Distinguishing between permanent hard water, temporary hard water and soft water
Procedure
- Place equal volumes of each unboiled water samples in three different conical flasks.
- Run soap solution from the burette into each sample while shaking the conical flask until when lather forms.
- Note the volume of soap solution required to form lather with each of the unboiled water samples.
Results
The soft water required the least volume of soap solution to form lather compared to the samples of hard water.
Repeat the above procedure with boiled water sample.
Results
Soft water required the same volume of soap solution to form lather as the unboiled soft water.
Temporary hard water after boiling required less volume of soap solution to form lather than the unboiled one. This is because; the water was made soft by boiling and thus formed lather readily.
Permanent hard water even after boiling required the same volume of soap solution as the unboiled to form lather. This is because even after boiling, the water still remained hard and took time to form lather.
Methods of softening water
Removal of only temporary hardness of water
- Boiling (Physical method)
Boiling decomposes calcium and magnesium hydrogen carbonates to their respective carbonates, water and carbondioxide gas. The calcium and magnesium ions are removed from the water as insoluble calcium and magnesium carbonates. The calcium and magnesium carbonates may be deposited as solids at the bottom of the boiler and are referred to as boiler scales of kettle fur.
Disadvantages of boiler scale/fur
- The fur produced is an insulator hence a lot of heat is required for boiling and this wastes a lot of fuel or electricity.
- The fur produced may also block pipes carrying boiled water and possibly causing it to burst.
By chemical means the fur can be removed from he kettle by adding mineral acids like hydrochloric acid and sulphuric acid.
- Addition of calcium hydroxide (lime water) (chemical method)
Calcium hydroxide reacts with the soluble magnesium or calcium hydrogen carbonates to form insoluble magnesium and calcium carbonates.
The disadvantage of this method is that when excess lime water is used, the water becomes hard again.
It has an advantage in that it is cheaper than boiling as calcium hydroxide is readily available.
- Addition of aqueous ammonia (ammonium hydroxide solution) (chemical method)
During the process, the soluble hydrogen carbonates are converted into insoluble carbonates. This enables magnesium and calcium ions to be removed from the water.
Methods for softening both temporary and permanent hardness
- Distillation (physical method)
During distillation, the soft water is collected as the distillate and the dissolved calcium and magnesium ions remain in the distillation flask as residue.
The advantage of this method is that the water obtained is actually pure. However, it can only be used in small scale because it is expensive. - Addition of sodium carbonate solution (washing soda) (chemical method)
Sodium carbonate converts magnesium or calcium ions into insoluble carbonates.
- Ion exchange method (permutit) (chemical method)
The calcium and magnesium ions in hard water can be exchanged with sodium ions using a suitable ion exchange material. This makes the water to become soft. The common ion exchange materials are zeolites and permutit. Zeolites are natural occurring forms of sodium aluminium silicate and permutit is the artificial form. Zeolite/permutit can be represented by the formula Na2Y.
The method works on the principle of exchange of ions. During the process, calcium and magnesium ions are removed from water and they appear as precipitates of aluminium silicate.
The advantage here is that, the permutit can be recovered and used several times.
Advantages of hard water
- Calcium compounds in water are important in our diet for strong teeth and bones.
- Magnesium in hard water is important for photosynthesis since it is used for the formation of chlorophyll.
- Calcium in hard water is responsible for the formation of strong shells of many animals e.g. snails and egg shells.
- Hard water tastes better and it is used in the brewing industry.
- Hard water reduces lead poisoning and it is the only type of water transported by lead pipes.
- Disadvantages of hard water
- It wastes a lot f soap, as more of the soap reacts the calcium and magnesium ions to form scum. There for lather does not form readily.
- When hard water ids used for washing, it leaves dirty marks on cloth due to formation of scum.
- Hard water forms kettle fur and boiler scales which are insulators, there fore a lot of fuel or electricity is used during boiling of water.
- The boiler scales may block the pipes carrying boiling water hence causing the pipe to burst.
Exercise
- Explain why soap does not form lather readily at first with hard water and then eventually forms the lather?
- Explain what is meant by hardness of water. What are the causes of temporary and permanent hardness of water?. How do these compounds that cause hardness of water reach the water?
- A white precipitate is formed when
i) Carbondioxide is blown into lime water
ii) Temporary hard water is boiled
iii) Washing soda is added to water containing dissolved magnesium chloride.