martinotundo@gmail.

com:
Chemistry teaching materials. Martin Otundo is an educationalist and a PhD student at JKUAT Kenya

AIR AND COMBUSTION

A. THE ATMOSPHERE

1. The atmosphere is made up of air. Air is a mixture of colourless , odourless

gases which is felt as wind(air in motion).All living things breath in air for
respiration . Plants use air for respiration and photosynthesis.

2. The main gases present in the atmosphere/air:

Gas Approximate % composition by volume

Nitrogen 78.0
Oxygen 21.0
Carbon(IV)oxide 0.03
Noble gases 1.0
Water vapour Vary from region

3. The following experiments below shows the presence and composition of

the gases in air/atmosphere

(a) To find the composition of air supporting combustion using a candle stick

Procedure
Measure the length of and empty gas jar M1. Place a candle stick on a petri dish.
Float it on water in basin/trough. Cover it with the gas jar. Mark the level of the
water in the gas jar M2. Remove the gas jar. Light the candle sick. Carefully cover
it with the gas jar. Observe for two minutes. Mark the new level of the water M3.
Set up of apparatus

Novels,Updated ICT,KASNEB,College,High School,Primary Softcopy Notes +254721246744 (whatsapp)

1
 Sample observations
Candle continues to burn then extinguished/goes off
Level of water in the gas jar rises after igniting the candle
Length of empty gas jar = M1= 14cm
Length of gas jar without water before igniting candle = M2= 10 cm
Length of gas jar with water before igniting candle = M1 - M2= 14- 10 = 4 cm
Length of gas jar with water after igniting candle = M3 = 8 cm
Length of gas jar without water after igniting candle = M1 - M3 = 10 -8 = 2 cm
Explanation
Candle burns in air. In a closed system(vessel),the candle continues to burn using
the part of air that support burning/combustion. This is called the active part of
air.The candle goes off/extinguished when all the active part of air is used up.The
level of the water rises to occupy the space /volume occupied by the used active
part of air. The experiment is better when very dilute sodium/potassium
hydroxide is used instead of water . Dilute Potassium/ sodium hydroxide absorb
Carbon(IV)oxide gas that come out from burning/combustion of candle stick.
From the experiment above the % composition of the:
(i) active part of air can be calculated:
M2 - M3 x 100% => 10- 8 x 100% = 20%
M2 10cm
(ii) inactive part of air can be calculated:
100% -20% = 80% // M3 => 8 x 100% = 80%
 M2 10cm

(b) To find the composition of active part of air using heated copper turnings.

Procedure
Clamp a completely packed/filled open ended glass tube with copper turnings.
Seal the ends with glass/cotton wool.
Label two graduated syringes as “A” and “B” Push out air from syringe “A”. Pull in
air into syringe “B”.
Attach both syringe “A” and “B” on opposite ends of the glass tube.
Determine and record the volume of air in syringe “B” V1.
Heat the glass tube strongly for about three minutes.
Push all the air slowly from syringe “B” to syringe “A” as heating continues. Push
all the air slowly from syringe “A” back to syringe “B” and repeatedly back and
forth.
After about ten minutes, determine the new volume of air in syringe “B” V2

Set up of apparatus

Sample observations

Colour change from brown to black

Volume of air in syringe “B” before heating V1 = 158.0cm3
Volume of air in syringe “B” after heating V2 = 127.2cm3
Volume of air in syringe “B” used by copper V1 - V2 = 30.8cm3

Sample questions
1.What is the purpose of
(i) glass/cotton wool
To prevent/stop copper turnings from being blown into the syringe/out of the
glass tube

(ii) passing air through the glass tube repeatedly

To ensure all the active part of air is used up

(iii) passing air through the glass tube slowly

To allow enough time of contact beteewn the active part of and the heated
copper turnings.

2. State and explain the observations made in the glass tube.

Colour change from brown to black
Brown copper metal reacts with the active part of air/oxygen to form black
copper(II)oxide.
Chemical equation
Copper + Oxygen -> Copper(II)oxide
2Cu(s) + O2(g) -> 2CuO(s)

The reaction reduces the amount/volume of oxygen in syringe “B” leaving the
inactive part of air. Copper only react with oxygen when heated.

3. Calculate the % of
(i)active part of air
% active part of air = V1 - V2 x 100% => 30.8cm3 x 100% = 19.493%
V1 158.0cm3
(ii) inactive part of air
Method 1
% inactive part of air = V2 x 100% =>127.2cm3 x 100% = 80.506%
V1 158.0cm3

Method 2
% inactive part of air = 100% -% active part of air
=> 100 % - 19.493 % = 80.507%

4. The % of active part of air is theoretically higher than the above while % of
inactive part of air is theoretically lower than the above. Explain.
Not all the active part of air reacted with copper

5. State the main gases that constitute:

(a)active part of air.
 Oxygen
(b) inactive part of air
Nitrogen, carbon(IV)oxide and noble gases

6. If the copper turnings are replaced with magnesium shavings the % of active
part of air obtained is extraordinary very high. Explain.
Magnesium is more reactive than copper. The reaction is highly exothermic. It
generates enough heat for magnesium to react with both oxygen and nitrogen in
the air.
A white solid/ash mixture of Magnesium oxide and Magnesium nitride is formed.
This considerably reduces the volume of air left after the experiment.
Chemical equation
Magnesium + Oxygen -> magnesium (II)oxide
2Mg(s) + O2(g) -> 2MgO(s)
Magnesium + Nitrogen -> magnesium (II)nitride
3Mg(s) + N2(g) -> Mg3N2 (s)

(c) To find the composition of active part of air using alkaline pyrogallol.

Procedure
Measure about 2cm3 of dilute sodium hydroxide into a graduated gas jar. Record
the volume of the graduated cylinder V1.
Place about two spatula end full of pyrogallol/1,2,3-trihydroxobenzene into the
gas jar. Immediately place a cover slip firmly on the mouth of the gas jar. Swirl
thoroughly for about two minutes.
Invert the gas jar in a trough/basin containing water. Measure the volume of air in
the gas jar V2

Sample observations
Colour of pyrogallol/1,2,3-trihydroxobenzene change to brown.
Level of water in gas jar rises when inverted in basin/trough.
Volume of gas jar /air in gas jar V1= 800cm3
Volume of gas jar /air in gas jar after shaking with alkaline pyrogallol/1,2,3-
trihydroxobenzene V2= 640 cm3

Sample questions
1. Which gas is absorbed by alkaline pyrogallol/1,2,3-trihydroxobenzene
Oxygen
2. Calculate the
(i) % of active part of air
V1-V2 x 100% => (800cm3 - 640 cm3) x 100% = 20%
V1 800cm3

(ii) % of inactive part of air

V2 x 100% => 640 cm3 x 100% = 80%
V1 800cm3

(d) To establish the presence of carbon(IV)oxide in air using lime water

Pass tap water slowly into an empty flask as in the set up below

Sample observation questions

1.What is the purpose of paper cover?
To ensure no air enters into the lime water.

2. What happens when water enters the flask?

It forces the air from the flask into the lime water.

3. What is observed when the air is bubbled in the lime water

A white precipitate is formed. The white precipitate dissolves on prolonged
bubbling of air.

4. (a) Identify the compound that form:

(i)lime water
 Calcium hydroxide / Ca(OH)2
(ii)white precipitate
Calcium carbonate/ CaCO 3
(iii)when the white precipitate dissolves
Calcium hydrogen carbonate/ CaHCO 3

(b) Write the chemical equation for the reaction that tale place when:
(i) white precipitate is formed
Calcium hydroxide + carbon(IV)oxide -> Calcium carbonate + water
Ca(OH)2(aq) + CO2 (g) -> CaCO3(s) + H2O(l)

(ii) white precipitate dissolves

Calcium carbonate + water+ carbon(IV)oxide -> Calcium hydrogen carbonate
CaCO3(s) + H2O(l) + CO2 (g) -> CaHCO3(aq)

5. State the chemical test for the presence of carbon (IV)oxide gas based on 4(a)
and (b)above:
Carbon(IV)oxide forms a white precipitate with lime water that dissolves in
excess of the gas.

6. State the composition of carbon(IV)oxide gas by volume in the air.

About 0.03% by volume

B.OXYGEN.
a) Occurrence.
1. Fifty 50% of the earths crust consist of Oxygen combined with other elements
e.g.oxides of metals
2. About 70% of the earth is water made up of Hydrogen and Oxygen.
3. About 20% by volume of the atmospheric gases is Oxygen that form the active
part of air.

b) School laboratory preparation.

Oxygen was first prepared in 1772 by Karl Scheele and later in 1774 by Joseph
Priestly.It was Antony Lavoisier who gave it the name “Oxygen”
Procedure

Method 1: Using Hydrogen peroxide

Half fill a trough/basin with tap water. Place a bee hive shelf/stand into the water.
Completely fill the a gas jar with water and invert in onto the bee hive shelf/stand.
Clamp a round bottomed flask and set up the apparatus as below.
Collect several gas jars of Oxygen covering each sample.

Sample observation questions

1. What is observed when the hydrogen peroxide is added into the flask
Rapid effervescence/bubbling/fizzing

2. Describe the colour and smell of the gas

Colourless and odourless.

3.(a)Name the method of gas collection used.

-Over water
-Upward delivery
-Down ward displacement of water

(b) What property of Oxygen make it to be collected using the method above
-Slightly soluble in water

4. What is the purpose of manganese(IV)oxide?

Manganese(IV)oxide is catalyst.
A catalyst is a substance that speeds up the rate of a chemical reaction but
remain chemically unchanged at the end of the reaction.
Hydrogen peroxide decomposes slowly to form water and Oxygen gas.
A little Manganese(IV)oxide speeds up the rate of decomposition by reducing the
time taken for a given volume of Oxygen to be produced.

5. Write the equation for the reaction.

 Hydrogen peroxide -> Water + Oxygen
2H2O2 (aq) -> 2H2O (l) + O2 (g)

6. Lower a glowing splint slowly into a gas jar containing Oxygen gas. State what
is observed.
The glowing splint relights/rekindles
Oxygen relights/rekindles a glowing splint. This is the confirmatory test for
the presence of Oxygen gas

Method 1: Using Sodium peroxide

Half fill a trough/basin with tap water. Add four drops of phenolphthalein
indicator.
Place a bee hive shelf/stand into the water.
Completely fill a gas jar with water and invert in onto the bee hive shelf/stand.
Clamp a round bottomed flask and set up the apparatus as below.

Collect several gas jars of Oxygen covering each sample.

Sample observation questions

1. What is observed when water is added

(i) into the flask containing sodium peroxide
Rapid effervescence/bubbling/fizzing
 (ii) phenolphththalein
Remains colourless /Phenolphthalein indicator is colourless in neutral solution

2. Describe the colour and smell of the gas

Colourless and odourless.

3.(a)Name the method of gas collection used.

-Over water.Oxygen is slightly soluble in water.

4. Test the gas by lowering a glowing splint slowly into a gas jar containingthe
prepared sample.
The glowing splint relights/rekindles. This confirms the presence of Oxygen
gas
5. Write the equation for the reaction.
Sodium peroxide + Water -> Sodium hydroxide + Oxygen
2Na2O2 (aq) + 2H2O (l) -> 4NaOH(aq) + O2 (g)

1. Test the gas by lowering a glowing splint slowly into a gas jar containing the
prepared sample.
The glowing splint relights/rekindles.
This confirms the presence of Oxygen gas
2. Write the equation for the reaction.
Potassium Chlorate(V) -> Potassium Chloride + Oxygen
2KClO3 (aq) -> 2KCl(aq) + 3O2 (g)

3. What is the purpose of manganese(IV)oxide?

Manganese(IV)oxide is catalyst.
A catalyst is a substance that speeds up the rate of a chemical reaction but
remain chemically unchanged at the end of the reaction.
Potassium Chlorate(V) decomposes slowly to form potassium chloride and
Oxygen gas.
A little Manganese(IV)oxide speeds up the rate of decomposition by reducing the
time taken for a given volume of Oxygen to be produced.

(c) Uses of Oxygen

1. Oxygen is put in cylinders for use where natural supply is not sufficiently
enough. This is mainly in:
(i) Mountain climbing/Mountaineering-at high altitudes, the concentration of
air/oxygen is low. Mountain climbers must therefore carry their own supply of
oxygen for breathing.
(ii) Deep sea diving-Deep sea divers carry their own supply of Oxygen.
(iii) Saving life in hospitals for patients with breathing problems and during
anaethesia.
2. A mixture of oxygen and some other gases produces a flame that is very hot.
(i) Oxy-acetyline/ethyne flame is produced when Ethyne/acetylene gas is
burnt in pure oxygen. The flame has a temperature of about 3000oC.It is
used for welding /cutting metals.
(ii) Oxy-hydrogen flame is produced when Hydrogen is burn in pure
oxygen. The flame has a temperature of about 2000 oC.It is used also for
welding
/cutting metals.
3. Oxy-hydrogen mixture is used as rocket fuel
4. A mixture of charcoal , petrol and liquid Oxygen is an explosive.

(d) Chemical properties of Oxygen /combustion.

Oxygen is a very reactive non metal. Many elements react with oxygen through
burning to form a group of compounds called Oxides.
Burning/combustion is the reaction of Oxygen with an element/substances.
Reaction in which a substance is added oxygen is called Oxidation reaction.
Burning/combustion is an example of an oxidation reaction.
Most non metals burns in Oxygen/air to form an Oxide which in solution /
dissolved in water is acidic in nature. They turn blue litmus red.e.g.
Carbon(IV)oxide/CO2 , Nitrogen(IV)oxide/ NO 2 , Sulphur(IV)oxide/ SO 2

Some non metals burns in Oxygen/air to form an Oxide which in solution /

dissolved in water is neutral in nature. They don’t turn blue or red litmus. e.g.
Carbon(II)oxide/CO, Water/ H2O.
All metals burns in Oxygen/air to form an Oxide which in solution/dissolved in
water is basic/alkaline in nature. They turn red litmus blue.e.g.
Magnesium oxide/MgO, Sodium Oxide/ Na2O ,Copper(II)oxide/CuO
Elements/substances burn faster in pure Oxygen than in air.
Air contains the inactive part of air that slows the rate of burning of
substances/elements.

(i) Reaction of metals with Oxygen/air

The following experiments show the reaction of metals with Oxygen and air.

I. Burning Magnesium
Procedure
(a) Cut a 2cm length piece of magnesium ribbon. Using a pair of tongs introduce
it to a Bunsen flame. Remove it when it catches fire. Observe.
Place the products in a beaker containing about 5cm3 of water. Test the
solution/mixture using litmus papers
(b) Cut another 2cm length piece of magnesium ribbon. Using a pair of tongs
introduce it to a Bunsen flame. When it catches fire, lower it slowly into a gas jar
containing Oxygen.
Place about 5cm3 of water into the gas jar. Test the solution/mixture using
litmus papers. Test the solution/mixture using litmus papers

Observations
(a)In air
Magnesium burns with a bright blindening flame in air forming white solid/ash
/powder. Effervescence/bubbles/ fizzing Pungent smell of urine. Blue litmus
paper remains blue. Red litmus paper turns blue
(b) In pure Oxygen
Magnesium burns faster with a very bright blindening flame pure oxygen forming
white solid/ash /powder. No effervescence/bubbles/ fizzing. No pungent smell of
urine. Blue litmus paper remains blue. Red litmus paper turns blue
Explanation
Magnesium burns in air producing enough heat energy to react with both Oxygen
and Nitrogen to form Magnesium Oxide and Magnesium nitride. Both Magnesium
Oxide and Magnesium nitride are white solid/ash /powder.

Chemical equations
Magnesium + Oxygen -> Magnesium Oxide
2Mg(s) + O2(g) -> 2MgO(s)
Magnesium + Nitrogen -> Magnesium Nitride
3Mg(s) + N2(g) -> Mg3N 2 (s)

Magnesium Oxide dissolves in water to form a basic/alkaline solution of

Magnesium hydroxide
Chemical equations
Magnesium Oxide + Water -> Magnesium hydroxide
2Mg(s) + O2(l) -> 2MgO(s)

Magnesium Nitride dissolves in water to form a basic/alkaline solution of

Magnesium hydroxide and producing Ammonia gas. Ammonia is also an
alkaline/basic gas that has a pungent smell of urine.
Chemical equations
Magnesium Nitride + Water -> Magnesium hydroxide + Ammonia gas
Mg3N2 (s) + 6H2O (l) -> 3Mg (OH)2 (aq) + 2NH3(g)

II. Burning Sodium

Procedure
(a) Carefully cut a very small piece of sodium . Using a deflagrating spoon
introduce it to a Bunsen flame. Remove it when it catches fire. Observe.
Place the products in a beaker containing about 20cm3 of water. Test the
solution/mixture using litmus papers
(b) Carefully cut another very small piece of sodium. Using a deflagrating spoon
introduce it to a Bunsen flame. When it catches fire, lower it slowly into a gas jar
containing Oxygen.
Place about 20 cm3 of water into the gas jar. Test the solution/mixture using
litmus papers. Test the solution/mixture using litmus papers

Observations
(a)In air
Sodium burns with a yellow flame in air forming a black solid. Blue litmus paper
remains blue. Red litmus paper turns blue
(b) In pure Oxygen
Sodium burns faster with a golden yellow flame in pure oxygen forming a yellow
solid. Effervescence/bubbles/ fizzing. Gas produced relights glowing splint.Blue
litmus paper remains blue. Red litmus paper turns blue.

Explanation
(a) Sodium burns in air forming black Sodium Oxide

Chemical equations
->
->
Sodium Oxide dissolves in water to form a basic/alkaline solution of Sodium
hydroxide
Chemical equations
Sodium Oxide + Water -> Sodium hydroxide
Na2O(s) + H2O (l) -> 2NaOH(aq)

(b) Sodium burns in pure oxygen forming yellow Sodium peroxide

Chemical equations
Sodium + Oxygen -> Sodium peroxide
2Na(s) + O2(g) -> Na2O2 (s)
Sodium peroxide dissolves in water to form a basic/alkaline solution of Sodium
hydroxide. Oxygen is produced.
Chemical equations
Sodium Oxide + Water -> Sodium hydroxide + Oxygen
2Na2O2 (s) + 2H2O (l) -> 4NaOH(aq) + O2 (l)

III. Burning Calcium

Procedure
(a) Using a pair of tongs hold the piece of calcium on a Bunsen flame.
Observe.
Place the products in a beaker containing about 2cm3 of water. Test the
solution/mixture using litmus papers
(b) Using a pair of tongs hold another piece of calcium on a Bunsen flame.
Quickly lower it into a gas jar containing Oxygen gas .Observe.
Place about 2cm3 of water. Swirl.
Test the solution/mixture using litmus papers
Observations
(a) In air
Calcium burns with difficulty producing a faint red flame in air forming a white
solid. Blue litmus paper remains blue. Red litmus paper turns blue
(b) In pure Oxygen
Calcium burns with difficulty producing a less faint red flame Oxygen forming a
white solid. Blue litmus paper remains blue. Red litmus paper turns blue
Explanation
(a)Calcium burns in air forming white calcium Oxide. Calcium Oxide coat/cover
the calcium preventing further burning.
Chemical equations
Calcium + Oxygen/air -> calcium Oxide
2Ca(s) + O2(g) -> 2CaO(s)
Small amount of Calcium Oxide dissolves in water to form a basic/alkaline
solution of Calcium hydroxide. The common name of Calcium hydroxide is lime
water.
Chemical equations
Calcium Oxide + Water -> Calcium hydroxide
CaO(s) + H2O (l) -> Ca(OH) 2 (aq)

IV. Burning Iron

Procedure
(a) Using a pair of tongs hold the piece of Iron wool/steel wire on a Bunsen flame.
Observe.
Place the products in a beaker containing about 2cm3 of water. Test the
solution/mixture using litmus papers
(b) Using a pair of tongs hold another piece of Iron wool/steel wire on a Bunsen
flame.
Quickly lower it into a gas jar containing Oxygen gas .Observe.
Place about 2cm3 of water. Swirl. Test the solution/mixture using litmus papers
Observations
(a)In air
Iron wool/steel wire burns producing a Orange flame in air forming a brown solid.
Blue litmus paper remains blue. Red litmus paper turns faint blue
(b) In pure Oxygen
Iron wool/steel wire burns producing a golden Orange flame in Oxygen forming a
Brown solid. Blue litmus paper remains blue. Red litmus paper turns faint blue
Explanation
(a)Iron burns in air forming brown Iron(III) Oxide
Chemical equations
Iron + Oxygen/air -> Iron(III) Oxide
4Fe(s) + 3O2(g) -> 2Fe2O3(s)
Very small amount of Iron(III)Oxide dissolves in water to form a weakly
basic/alkaline brown solution of Iron(III) hydroxide.
Chemical equations
Calcium Oxide + Water -> Iron(III) hydroxide
Fe2O3(s) + 3H2O (l) -> 2Fe(OH) 3 (s)

V. Burning Copper

Procedure
(a) Using a pair of tongs hold the piece of copper turnings/shavings on a Bunsen
flame.
Observe.
Place the products in a beaker containing about 2cm3 of water. Test the
solution/mixture using litmus papers
(b) Using a pair of tongs hold another piece of Copper turnings/shavings on a
Bunsen flame. Quickly lower it into a gas jar containing Oxygen gas .Observe.
Place about 2cm3 of water. Swirl. Test the solution/mixture using litmus papers
Observations
(a) In air
Copper turnings/shavings burns with difficulty producing a green flame in air
forming a black solid. Blue litmus paper remains blue. Red litmus paper turns
faint blue
(b) In pure Oxygen
Copper turnings/shavings burns less difficulty producing a green flame in Oxygen
forming a Brown solid. Blue litmus paper remains blue. Red litmus paper turns
faint blue
Explanation
(a) Copper burns in air forming black Copper(II) Oxide
Chemical equations
Copper + Oxygen/air -> Copper(II) Oxide
2 Cu(s) + O2(g) -> 2CuO(s)
Very small amount of Copper(II)Oxide dissolves in water to form a weakly
basic/alkaline blue solution of Copper(II) hydroxide.
Chemical equations
Copper(II) Oxide + Water -> Copper(II) hydroxide
CuO(s) + H2O (l) -> Cu(OH) 2 (s)

(i) Reaction of non metals with Oxygen/air

The following experiments show the reaction of non metals with Oxygen and air.
I. Burning Carbon
Procedure
(a) Using a pair of tongs hold a dry piece of charcoal on a Bunsen flame.
Observe.
Place the products in a beaker containing about 2cm3 of water. Test the
solution/mixture using litmus papers
(b) Using a pair of tongs hold another piece of dry charcoal on a Bunsen flame.
Quickly lower it into a gas jar containing Oxygen gas .Observe.
Place about 2cm3 of water. Swirl. Test the solution/mixture using litmus papers
Observations

-Carbon chars then burns with a blue flame

-Colourless and odourless gas produced
-Solution formed turn blue litmus paper faint red.
Red litmus paper remains red.

Explanation
Carbon burns in air and faster in Oxygen with a blue non-sooty/non-smoky flame
forming Carbon (IV) oxide gas.
Carbon burns in limited supply of air with a blue non-sooty/non-smoky flame
forming Carbon (IV) oxide gas.
Carbon (IV) oxide gas dissolve in water to form weak acidic solution of Carbonic
(IV)acid.
Chemical Equation
Carbon + Oxygen -> Carbon(IV)oxide
(excess air/oxygen)
C(s) + O2(g) -> CO2(g) (in excess air)

Carbon + Oxygen -> Carbon(II)oxide

(limited air/oxygen)
 2C(s) + O2(g) -> 2CO(g) (in limited air)

Carbon(IV)oxide + Water -> Carbonic(IV)acid

CO2(g) + H2O (l) -> H2CO3 (aq) (very weak acid)

II. Burning Sulphur

Procedure

(a) Using a deflagrating spoon place sulphur powder on a Bunsen flame.

Observe.
Place the products in a beaker containing about 3cm3 of water. Test the
solution/mixture using litmus papers
(b) Using a deflagrating spoon place sulphur powder on a Bunsen flame. Slowly
lower it into a gas jar containing Oxygen gas. Observe.
Place about 5cm3 of water. Swirl. Test the solution/mixture using litmus papers.
Observations
-Sulphur burns with a blue flame
-Gas produced that has pungent choking smell
-Solution formed turn blue litmus paper faint red.
Red litmus paper remains red.

Explanation
Sulphur burns in air and faster in Oxygen with a blue non-sooty/non-smoky flame
forming Sulphur (IV) oxide gas.
Sulphur (IV) oxide gas dissolve in water to form weak acidic solution of Sulphuric
(IV)acid.
Chemical Equation
Sulphur + Oxygen -> Sulphur(IV)oxide

S(s) + O2(g) -> SO2(g) (in excess air)

Sulphur(IV)oxide + Water -> Sulphuric(IV)acid

SO2(g) + H2O (l) -> H2SO3 (aq) (very weak acid)

III. Burning Phosphorus

Procedure

(a) Remove a small piece of phosphorus from water and using a deflagrating
spoon (with a lid cover)place it on a Bunsen flame.
Observe.
Carefully put the burning phosphorus to cover gas jar containing about 3cm3 of
water. Test the solution/mixture using litmus papers
(b) Remove another small piece of phosphorus from water and using a
deflagrating spoon (with a lid cover) place it on a Bunsen flame.
Slowly lower it into a gas jar containing Oxygen gas with about 5 cm3 of water.
Observe.
Swirl. Test the solution/mixture using litmus papers.
Observations
-Phosphorus catches fire before heating on Bunsen flame
-Dense white fumes of a gas produced that has pungent choking poisonous
smell
-Solution formed turn blue litmus paper faint red.
Red litmus paper remains red.
Explanation
Phosphorus is stored in water.On exposure to air it instantaneously fumes then
catch fire to burn in air and faster in Oxygen with a yellow flame producing dense
white acidic fumes of Phosphorus(V) oxide gas.
Phosphoric(V) oxide gas dissolve in water to form weak acidic solution of
Phosphoric (V)acid.
Chemical Equation
Phosphorus + Oxygen -> Phosphorous(V)oxide

4P(s) + 5O2(g) -> 2P2O5(s)

Phosphorous(V)oxide + Water -> Phosphoric(V)acid

P2O5(s) + 3H2O (l) -> 2H3PO 4 (aq) (very weak acid)

(e) Reactivity series/competition for combined Oxygen.

The reactivity series is a list of elements/metals according to their affinity for
oxygen.
Some metals have higher affinity for Oxygen than others.
A metal/element with higher affinity for oxygen is placed higher/on top of the one
less affinity.

The complete reactivity series of metals/elements

Element/Metal Most
 Carbon
Zinc Zn
Iron
Tin
Lead
Hydrogen
Copper Cu
Mercury Hg
Silver
Gold Au
Platinum Pt Least reactive

Metals compete for combined Oxygen. A metal/element with higher affinity for
oxygen removes Oxygen from a metal lower in the reactivity series/less affinity
for Oxygen.
When a metal/element gains/acquire Oxygen, the process is called Oxidation.
When a metal/element donate/lose Oxygen, the process is called Reduction.
An element/metal/compound that undergo Oxidation is called Reducing agent.
An element/metal/compound that undergo Reduction is called Oxidizing agent.
A reaction in which both Oxidation and Reduction take place is called a Redox
reaction.
Redox reaction between Magnesium and copper(II)Oxide
Procedure
Place about 2g of copper (II)oxide in a crucible with a lid. Place another 2g of
Magnesium powder into the crucible. Mix thoroughly.
Cover the crucible with lid. Heat strongly for five minutes.
Allow the mixture to cool. Open the lid. Observe.
Observation
Colour change from black to brown. White solid power formed.
Explanation
Magnesium is higher in the reactivity series than Copper. It has therefore higher
affinity for Oxygen than copper.
When a mixture of copper(II)oxide and Magnesium is heated, Magnesium
reduces copper(II)oxide to brown copper metal and itself oxidized to Magnesium
oxide. Magnesium is the reducing agent because it undergoes oxidation process.
Copper(II)oxide is the oxidizing agent because it undergo redox reduction
process.
The mixture should be cooled before opening the lid to prevent hot brown copper
from being reoxidized back to black copper(II)oxide.
The reaction of Magnesium and Copper(II)oxide is a reaction
Chemical equation

Reduction process

Oxidation

1. Copper (II)oxide + Magnesium -> Magnesium oxide + Copper

(black) (white ash/solid) (brown)
CuO(s) + Mg(s) -> MgO(s) + Cu(s)
(Oxidizing Agent) (Reducing Agent)

2. Zinc (II)oxide + Magnesium -> Magnesium oxide + Zinc

(yellow when hot) (white ash/solid) (grey)
ZnO(s) + Mg(s) -> MgO(s) + Zn(s)
(Oxidizing agent) (Reducing agent)

3. Zinc (II)oxide + Carbon -> Carbon(IV) oxide gas + Zinc

(yellow when hot) (colourless gas) (grey)
ZnO(s) + C(s) -> CO2(g) + Zn(s)
(Oxidizing agent) (Reducing agent)

The reactivity series is used during extraction of metals from their ore.An ore is a
rock containing mineral element which can be extracted for commercial purposes.
Most metallic ores occur naturally as:
(i) oxides combined with Oxygen
(ii)sulphides combined with Sulphur
(iii)carbonates combined with carbon and Oxygen.

Metallic ores that naturally occur as metallic sulphides are first roasted in air to
form the corresponding oxide. Sulphur(IV)oxide gas is produced. e.g.

Copper(I) sulphide + Oxygen -> Copper(I)Oxide + Sulphur(IV)oxide

Cu2S(s) + O2(g) -> 2Cu(s) + SO2(g)

Zinc(II) sulphide + Oxygen -> Zinc(II)Oxide + Sulphur(IV)oxide

ZnS(s) + O2(g) -> Zn(s) + SO2(g)
 Lead(II) sulphide + Oxygen -> Lead(II)Oxide + Sulphur(IV)oxide
PbS(s) + O2(g) -> Pb(s) + SO2(g)

Iron(II) sulphide + Oxygen -> Iron(II)Oxide + Sulphur(IV)oxide

FeS(s) + O2(g) -> Fe(s) + SO2(g)

Metallic ores that naturally occur as metallic carbonates are first heated in air.
They decompose/split to form the corresponding oxide and produce Carbon (IV)
oxide gas. e.g.
Copper (II)carbonate -> Copper(II)oxide + Carbon(IV)oxide
CuCO3(s) -> CuO(s) + CO2(g)

Zinc (II)carbonate -> Zinc(II)oxide + Carbon(IV)oxide

ZnCO3(s) -> ZnO(s) + CO2(g)

Lead (II)carbonate -> Lead(II)oxide + Carbon(IV)oxide

PbCO3(s) -> PbO(s) + CO2(g)

Iron(II)carbonate -> Iron(II)oxide + Carbon(IV)oxide

FeCO3(s) -> FeO(s) + CO2(g)
Metallic ores
TOPICAL QUESTIONS ON THE TOPIC BASED ON PAST KCSE PAPERS
1. Study the experiment set up represented by the diagram below and answer the question
that follows.

a) Explain what would be observed if red and blue litmus papers were dipped into the
water at the end of experiment. (2mks)
b) Write an expansion in terms of X and Y to show the (%) percentage of gas used by
the burning candle. (1mk)
2. The diagram below represents two iron nails with some parts wrapped tightly with zinc
and copper strips respectively.

What observations would be made at the exposed points A and B if the wrapped nails
are left in the open for several months? Explain. (3mks)
3. In an experiment, rods of metals P, Q and R were cleaned with a sand paper and placed
in a beaker containing water. Another set of rods was also cleaned and placed in a
beaker containing dilute acid. After placing the rods in the two liquids bubbles of gas
were seen around some of the rods as shown in the diagram below.

23
 a) Why was it necessary to clean the rods with sand paper before dipping them into
the liquids? (1mk)

b) Arrange the three metals in order of their reactivity starting with the most reactive.
(1mk)
4. When magnesium is burnt in air it reacts with oxygen and nitrogen gas giving a white ash.
Write two equations for the two reactions that take place. (2mks)
5. Oygen reacts with the elements phosphorous, sulphur and chlorine to form oxides in
which the elements is in its highest oxidation number. The table below gives the oxide
of sulphur and its highest oxidation number. Complete the table for phosphorous and
chlorine. (Atomic number p=15, s=16, Cl= 17) (2mks)

Elements Oxides Highest oxidation

number

S SO 3 +6

Cl

6. Write an equation for the reaction that takes place when carbon (II) Oxide gas is passed
over heated Lead (II) Oxide. (1mk)
7. The set up below was used to study some properties of air

State and explain two observation that would be made t the end of the experiment.
(3mks)
8. Give the formula of an oxide which reacts both dilute Hydrochloric acid and hot
concentrated sodium hydroxide.
9. In an experiment a certain volume of air was passed repeatedly from syringe over heated
excess zinc powder as shown in the diagram below.
 The experiment was repeated using excess magnesium powder. In which of the
experiments was the change in volume of air greatest? Give reasons. (3mks)
10. State and explain the change in mass that occurs when the following substances are
separately heated in open crucibles.
i) Copper metal
ii) Copper (II) Nitrate (3mks)
11. The diagram below shows an iron bar, which supports a bridge. The iron is connected to
Iron bar
a piece of magnesium metal.

Connecting wire

Soil

Magnesium metal

Explain why it is necessary to connect the piece of magnesium metal to the iron bar.
12. Explain why magnesium continue to burn in a gas jar full of Sdulphur (IV) Oxude while
burning splint would be extinguished.
13. The diagram below is a set up for the laboratory preparation of oxygen gas.
 a) Name solid R. (1mk)

b) Write an equation for the reaction that takes place in the flask. (1mk)
c) Give one commercial use of oxygen.
14. Nitrogen (II) Oxide and nitrogen (IV) Oxide are some of the gases released from car
exhaust pipes. State these gases affect the environment. (2mks)

15. The set up below was used to abtain a sample of iron.

Write two equations which occur in the combustion tube. (2mks)

16. The low chart below outlines some of the process involved during extraction of copper
from pyrites. Study it and answer the questions that follow.

a) i) Name gas K. (1mk)

ii) Write an equation for the reaction that takes place in 1 st roasting furnance.
(1mk)
iii) Write the formula of the cations present in the slag M (1mk)
iv) Identify gas P. (1mk)
v) What name is given to the reaction that takes placein chamber N? Give a
reason for your answer. (2mks)
b) Copper obtained from chamber N is not pure. Draw a labelled diagram to show
the set up you would use to refine the copper by electrolysis. (2mks)
 c) Given that the mass of copper obtained from the above extraction was 210 kg,
determine the percentage purity of the ore (copper pyrite) if 810 kg of it was fed
to 1st roasting furnance. Cu= 63.5, Fe= 56.0, S=32.0 (3mks)
d) Give two effects that this process could have on the environment. (2mks)
17. The table below gives the information about the major constituents of crude oil. Study it
and answer the questions that follow.

Constituents Boiling point in

0
C

Gases Below 40

Petrol 49-175

Kerosene 175-250

Diesel oil 259-350

Lubricating oil 350-400

Bitumen Above 400

i) Which one of the constituent of crude oil has molecules with the highest number
of carbon atoms? (2mks)
ii) Name the process you would use to separate a mixture of petrol and diesel and
explain how the separation takes place. (2mks)
iii) Explain why constituents of crude oil do not have sharp boiling points. (2mks)
iv) a) Name one gas that is likely to be a constituent of crude oil and
write its formula. (2mks)
b) What conditions could cause a poisonous gas to be formed when kerosene
is burnt. Explain. (2mks)
c) Give one use of bitumen. (1mk)
18. The diagram below shows a set up used by a student in an attempt to prepare collect
oxygen gas
 a) i) Complete the diagram by collecting the mistakes in it. (2mks)

ii) Identify solid w. (1mk)

b) A piece of phosphorous was burnt in excess air. The amount of hot water to
make a solution.

i) Write an equation for the burning of phosphorous in excess air.

(1mk)
ii) The solution obtained in (b) above was found to have a PH of 2.0. Give
reasons for this observation. (2mks)
c) Explain why cooking pots made of aluminium do not corrode easily when exposed
to air. (1mk)
d) The reaction between sulphure (IV) Oxide and oxygen to form Sulphur (VI) Oxide
per day (condition for the reaction a catalyst, 2 atmospheric pressure and
temperature between 400 0 5000 C)
2SO(aq) + O 2(g) 2SO 3(g)

Factory manufacturing sulphuric acid by contact process produces 350kg of sulphur

trioxide per day (conditions) for the reaction catalyst. 2 atmospheres pressure and
temperatures between 400 – 500 o C.
i) What is meant by an exothermic reaction? (1mk)
ii) How would the yield per day of sulphur trioxide be affected if temperatures
lower than 400oC are used? Explain. (1mk)
iii) All the sulphur (VI) Oxide produced was absorbed in concentrated
sulphuric acid to form oleum.
SO3(g) + H4 SO4(l) → H2 S2 O7(l)
Calculate the mass of oleum that was produced per day.

(S+ 32.0, O= 16: H 1.0) (3mks)

19. a) Fractional distillation of liquid air usually produces nitrogen and oxygen as the
major by-product.
i) Name one substance that is used to remove carbon (IV) Oxide from air
before it is changed into liquid. (1mk)
 ii) Describe how liquid Nitrogen gas is obtained from liquid air.

Boiling points; Nitrogen = -1960C; Oxygen = -1830C. (1mk)

b) Study the flow chart below and answer the questions that follows

i) Name element M. (1mk)

 ii) State and explain the change in mass that is likely to occur in tube N by the
end of the experiment. (2mks)
iii) Name two gases that come out through tube M. (1mk)

iv) Write an equation for the reaction in stem 7. (1mk)

v) Give one use of Ammonium –Nitrate. (1mk)
c) State and explain the observations that would be made if a sample of
sulphur is heated with concentrated Nitric acid. (Nitric (V) acid.
20. a) Candle wax is mainly a compound consisting of two elements. Name the two
elements
b) The up below was used to investigate the burning of candle. Study it and answer
the questions that follow.

i) What would happen to the burning candle if the pump were turned off? Give
reasons. (3mks)
ii) State and explain the change in mass that is likely to occur in tube N by the
end of the experiment. (2mks)

iii) Name another substance that would be used in place of calcium oxide.
(1mk)
21. Why is iron not used to make steam boilers? (1mk)
22. Study the arrangement below and answer the questions that follows.
 Explain what happens to the lime water after some time. (1mk)

23. When air is bubble through pure water (Ph 7.0). The PH drops to 6.0. Explain why.
(1mk)
24. A white compound was moistened with a little concentrated Hydrochloric acid and
placed over a flame. A yellow flame was observed. Identify the metallic ions in the
compound. (1mk)
25. Magnesium ribbon was burned in a gas jar of Nitrogen. A few drops of water were then
added to the jar. Write equation for the reactions in the jar. (2mks)
26. The diagram below shows an experiment to compare the heating effect of luminous and
non luminous flame.

a) What was observed at the bottom of each beaker at the end of the experiment?
(1mk)
b) Which sample of water boils first? Give a reason for your answer. (2mks)

c) Besides the amount of heat produced by the two flames, state other differences.
(2mks)
27. a) Study the equation below and answer the questions that follow.

CO 3-2(aq) + H2 O (l) → HCO (aq) + OH (aq)

Which substance is an oxidizing agent? Give reasons. (2mks)
b) Identify the reducing agent in the equation below
→ Fe3+(aq) + 2CL –(aq)
2+
Fe (aq) + Cl2(g)

28. A candle was burnt using the apparatus shown below. The initial volume of measuring
cylinder was 90cm3. The apparatus was allowed to cool and the volume of air in the
 measuring cylinder had dropped to 70cm 3.

a) Why was the volume recorded when the air was cooled? (1mk)
b) What was the purpose of sodium Hydroxide? (1mk)
c) Use the results given to calculate the percentage of oxygen in air. (2mks)
29. The graph below shows the changes that occur when a pure and an impure substance
are heated.

a) Which curve represents pure substance? Explain. (2mks)

b) Name one factor which affects the melting point of a solid and state effects.
(2mks)
ANSWERS
1. (a) The blue litmus paper would turn pink/ red. Red litmus paper remains red.
The carbon (IV) oxide produced when the candle burns dissolves in water to form
a solution of weak carbonic acid.
(b) x- y x 100%
x
2. Observation: At No rusting takes place
Explanation: Zinc is more reactive than iron. It reacts with oxygen in presence to iron
hence preventing it from rusting. It acts as a sacrificial metal
Observation at B
The nail is covered by reddish brown substance/coating/rust
Explanation: Copper is less reactive than iron. Iron combines first with oxygen in
presence of moisture and rust.

3. (a) To remove the layer of oxide on their surfaces which could inhibit the
reaction
(b) Q, R,P

4. 2Mg(s) + O¬ 2(g) → 2MgO(s)

3 Mg(s) + N 2(g) →Mg3 N2(s)

5. Oxide: Highest oxidation number

P2O5 (+5)
Cl2 O 7 (+7)

6. CO(g) + PbO(s) →Pb(s) + CO2 (g)

Observations
7. -Iron will be covered by a reddish brown substance/coating/rust
-Water in test tube rise and water in a beaker drops
Explanation:
Iron Combines with oxygen in a presence of moisture to form hydrated Iron (III) oxide /
rust water rises up to occupy the space which was occupied by oxygen in the tube.

8. Al2O3 (Aluminium Oxide)

9. Change was greatest with Magnesium. Both react with oxygen gas to form oxides, but
magnesium also reacts with nitrogen to form magnesium nitrate (Mg3N2)
10. (i) Mass increase: Oxygen combines with copper metal to form copper (II)
Oxide.
 (ii) Mass decrease: copper Nitrate decomposes to give gases that escape leaving
behind copper (II) oxide.

11. Magnesium is above iron in the reactivity series. It supply electrons to the iron bar hence
prevent it from rusting/ cathode protection.

12. Magnesium produces a lot of heat/ energy when burning. This splint sulphur (IV) oxide
into sulphur and Oxygen. Magnesium burns in the oxygen produced. Burning splint
produces less energy which is not enough to break sulphur (IV) oxide.

13. (a) Manganese (IV) Oxide/ MnO2(s)

(b) 2 H2 O2 (aq) MnO 2 2 H2 ¬O(l) + O2

(c) - Respiratory aids from patients suffering from respiratory diseases /

during surgery.
- High mountain climbers and deep see divers
- Helps in combustion of rocket fuel
- Welding together with other gases such as hydrogen/ oxygen (hydrogen
flame) acetylene/ oxyacetylene flame.

14. Nitrogen (II) Oxide is oxidized by oxygen in air to form nitrogen (IV) oxide. This gas is
acidic when dissolved in water. May cause acidic rain. If inhaled by animals/ man may
corrode respiratory surfaces exposing them to disease causing agents.

15. 2C(s) + O2 (g) →2CO(g)

Fe2 O3 + 3CO(g) →2 Fe(s) + 3 Co2(g)

16. (i) SO2/ sulphur(IV) Oxide

(ii) 2CuFeS 2 + 402 (g) → 2FeO(s) + 3S)2 (g) + Cu2S(s)
(iii) Fe2+
(iv) Carbon (IV) Oxide or carbon (II) Oxide
(v) Reduction/ oxidation = Redox since Cu2O is reduced to Cu and CO oxidized to Co2
(b)

(c) Mole ratio of CU in CuFeS 2 = 1.1

 Moles of Cu produced = 210 = 3.3 moles
63.5

RFM of CuFeS2 = 63.5 + 56 + 64 = 183.5

Mass of Cu in CuFeS 2 = 3.3 x 183. 5 = 605. 6 kg
% purity = 605 x 100 = 74. 76%
810

(d) - Formation of acidic rain due to presence of sulphur (IV) oxide

- Sulphur (IV) oxide is poisonous
- Carbon (II) is poisonous
- Global warming due to presence of carbon (IV) oxide
- Dumping of wastes like slag prevents growth of vegetation
- Soil erosion due to the excavation of the ores

17. (i) Bitumen: It has the highest boiling point

(ii) Fractional distillation: they have different boiling points, petrol boils out first
(iii) Each component is a mixture of hydrocarbons/ impure or there is presence of
isomes in each component.
(iv) Methane → CH4 all alkane gases up to C = 4
(b) Burning in limited air will produce carbon (II) oxide which is poisonous
(c) - Manufacture of tar used in tarmac road/ surface of roads
- Amending leaking roofs.
18. (a) (i)

(ii) Sodium peroxide Na 2 O 2

(b) (i) 4P(s) + 5O2 (g) → 2P2 O5 (g)
(ii) Phosphorous (V) oxide dissolves in water to form an acid (Phosphoric acid)
(c) A firm oxide (aluminium Oxide) is formed on the surface of the metal. This oxide
protect aluminium from further attack
(d) (i) A reaction which proceeds by production of heat i.e heat is lost to
the surroundings.
 (ii) The yield be lowered: through by Le- Chateliers principle, the yield is
expected to increase. But lower temperatures will result into fewer
particles attaining activation energy.

(iii) RMM of SO 3 = 32 + 48 = 80
Moles of SO 3 used = 350 = 4.38 moles
80
Moles of H2S2O7 = 4.38 moles
RMM of H2 S2 O7 = 2 + 64 + 112 = 178
Mass of H 2 S2 O7 = 4.38 x 178 = 779.6 kg

19. (a) (i) Potassium Hydroxide or sodium hydroxide

(ii) Air allowed to expand and warm up. Nitrogen gas vaporizes first since it is
more volatile. On further heating- oxygen vaporizes.
(b) (i) Hydrogen gas
(ii) - For the complete oxidation of ammonia gas
To increase the yield of nitrogen (II) Oxide
To reduce the cost
(iii) Nitrogen gas
(iv) NH3(g) + HNO3(aq) →NH4 NO3(aq)
(c) Brown gas (Nitrogen (IV) Oxide gas) and an acidic gas (sulphur (IV) oxide) formed
Nitric acid reduced into nitrogen (IV) oxide, water and oxygen. Sulphur is oxidized
into sulphur (IV) oxide which dissolves in water forming sulphuric acid.

20. (a) Carbon and hydrogen

(b) (i) The candle will go off/ extinguished since carbon (IV) oxide and
water vapour accumulate around the candle carbon (IV) oxide does not
support burning.
OR The supply of oxygen will be supported and candle goes off
(ii) Mass increase
Water combines with calcium oxide to form calcium hydroxide solution.
This combine with carbon (IV) oxide to form calcium carbonate.
(iii) - Carbon (IV) oxide
- Carbon (II) oxide
(iv) Protect calcium from obtaining water from the atmosphere
(v) -Concentrated sulphuric acid
-Calcium chloride

21. Iron metal is corroded by rust in presence of water and oxygen

22. There will be formation of a white precipitate. Candle burns producing carbon (IV) oxide.
23. Air contains carbon (IV) Oxide which dissolve in water producing a weak carbonic acid
24. Na + ions
25. 3Mg(s) + N 2 (g) →Mg3 N2 (s)
Mg3 N2(s) + 6H 2 O(l) → 3Mg(OH)2(aq) + 2NH3

26. (a) Beaker A: No soot at the bottom

Beaker B: A lot of black soot at the pattern
(b) Sample A: Non luminous flame produces a lot of heat.
(c)
Luminous Non Luminous
- Produce a lot of - Produces less light
light - Not Sooty
- Very sooty - Short and steady
- Large and wary - Burns with roaring
- Burns quietly noise

27. (a) CO -2 is an oxidizing agent. It removes hydrogen from water (H2O) and
oxidizes it to OH.
(b) Fe2+ is a reducing agent. It adds electrons to Cl2 and reduces it to 2CL-

28. (a) To allow all oxygen to be used up and also to allow the gas to contract/
cater for any expansion of gases
(b) To absorb carbon (IV) oxide which was produced by the burning candle
(c) % of oxygen 90 – 70 x 100 = 22.2%
90
29. (a) Curve B: Pure substances has sharp/ fixed constant melting and boiling
points
(b) Impurities rises the boiling point pressure rises the boiling point i.e when pressure
is high b.p is very high.