AIR AND COMBUSTION

State the composition of air.
Determine experimentally the percentage of oxygen in air by volume.
Describe fractional distillation of air.
Define combustion.
Investigate the conditions for rusting and state the composition of rust.
State the methods of preventing rusting.
Prepare and investigate the properties of oxygen.
Experimentally compare the rates of combustion of elements in air and in oxygen.
State the nature of the products of burning elements in air and in oxygen.
State the pollution effects of combustion.
State the applications of the reactivity series.

(24 Lessons)

Air sustains life on earth. Living organisms need air for respiration. Plants need air for photosynthesis. Air is required for combustion of fuels to give energy.

Composition of air

Is air a mixture or a compound?

Air is a mixture of gases such as oxygen, nitrogen, carbon (IV) oxide and water vapour.

Air is considered a mixture and not a compound because the various components of air can be separated by physical means (fractional distillation of liquefied air).

Approximate percentage composition of air

*Component*	*Percentage composition*
Nitrogen	78.1
Oxygen	20.9
Carbon (IV) oxide	0.03
Noble gases	0.97
Water vapour	Variable
Dust	Variable

Experiments to determine the percentage of oxygen in air by volume.

The active part of air is oxygen. It occupies about 20% of air. Several experiments such as burning of candle, heating of copper(II) oxide (combustion), smoldering of phosphorus and rusting, all of which use oxygen, can be done to determine the active part of air.

(a) Burning of candle in air

Combustion or burning is a process in which a substance combines with oxygen with the production of heat.

The part of air that supports combustion is active air. The active part is oxygen, which forms about 20% of dry air by volume.

⚗Practically Speaking🔊 📌

The active part of air can be determined experimentally using a burning candle as follows:

Put dilute sodium hydroxide solution in a trough. Place a small candle on a cork and float it on the solution. Cover it with a gas jar. Mark on the gas jar, the level of the solution. Measure the height of the air column and record it.

Remove the jar and light the candle. Gently cover the burning candle with the gas jar. After the candle has gone off leave the apparatus to cool to room temperature. Mark on the gas jar the final level of the solution. Measure and record the height of the air column once more.

Remove the gas jar and measure the change in height. Record all your observations.

Observations and Discussion

The candle burns for a while then it goes off.

As the candle burns, it uses up the active part of air in the fixed amount of air enclosed.

The level of dilute sodium hydroxide solution in the gas jar rises after the candle goes off.

As the candle burns, it uses up the active part of air in the fixed amount of air enclosed in the gas jar. This leaves a partial vacuum in the jar. Greater atmospheric pressure acting on the surface of the sodium hydroxide forces the solution up into the jar.

The following are sample results for a similar experiment.

Height of air column before burning = 16.0cm

Height of air column after burning = 12.9cm

The percentage of air used up during the experiment can be determined as follows:

Height of air used during burning = 3.1cm

Percentage of air by volume used up = × 100

= × 100

= 19.375%

Side notes

Dilute sodium hydroxide is preferably used instead of water to absorb carbon (IV) oxide that was initially in the gas jar and that which is produced during combustion.
The experimental result is not the same as the theoretical value of the percentage of oxygen in air by volume. This is due to experimental errors, which may result from:

The sodium hydroxide solution may not absorb all the carbon (IV) oxide gas.
The candle may go off before all the oxygen is used up due to the build-up of carbon (IV) oxide levels.

Heating causes expansion of gases therefore the apparatus should be allowed to cool before the final reading is taken.

Conclusion

The active part of air is oxygen, which forms about 20% of dry air by volume. The part of air that remains in the gas jar does not support combustion. The component of air that is inactive is mainly nitrogen.

(b) Heating copper in a fixed volume of air

When copper is heated in air, it reacts with oxygen in air to form black copper (II) oxide.

⚗Practically Speaking🔊 📌

Pack copper turnings in a long hard glass tube about 6cm long. Connect the tube with two glass syringes with one syringe containing a specific volume of air while the other is empty.

Heat the copper turnings until they are red hot. Slowly pass the air from syringe A through the hot turnings to syringe B and back. Repeat this process while heating the copper turnings until the new volume of air in syringe A is constant. Allow the glass tube to cool and record the volume of the gas in syringe A.

Observations and Discussion

In the combustion tube, at the end of the experiment, the red-brown copper turnings turn black.

Copper is a red-brown metal. When it is heated in air, it turns black. This is because it combines with oxygen to form black copper (II) oxide.

Below is a word equation for the reaction.

The sample readings below can be used to determine the percentage of air used up in this experiment.

Volume of air in syringe A before heating = 7.5cm³

Volume of air in syringe A after heating = 6.0cm³

Volume of air used up during heating = 1.5cm³

Percentage by volume of air used during the experiment. =

= 20%

About 20% by volume of air is used during combustion and the 80% of air left does not react with heated copper. The gas left in the syringe does not react with copper. It is mainly nitrogen.

Side notes

The glass wool plug is used to stop the copper turnings from being sucked into the syringes.
The air is passed slowly to allow enough time of contact between the reactants.
The air is passed repeatedly over heated copper to ensure that all oxygen in the syringes and tube is used up.
The possible sources of error in this experiment include:

(a) The air initially present in the tube is not accounted for.

(b) There is possible leakage of air.

(c) Rusting of iron fillings

⚗Practically Speaking🔊 📌

Wet a measuring cylinder and sprinkle some iron filings on the wet surface. Remove the excess iron filings. Invert the measuring cylinder in a trough of water. Read the volume of air column in the measuring cylinder. Leave the set up for 48 hours. Read and record the volume of the air column. Record all your observations.

Side notes

The measuring cylinder is made wet to ensure that the iron filings stick onto the wet surface.

Observation and Discussion

After 48 hours;

A brown coating is formed on the filings.

The brown coating is rust. Rust is a compound of iron and oxygen.

The level of water in the measuring cylinder rises while that in the trough drops.

During rusting, oxygen is used and therefore water rises up in the measuring cylinder to replace the volume of air used during rusting.

About 20% of air by volume is used up during rusting.

(d) Smouldering of phosphorous

⚗Practically Speaking🔊 📌

Invert an empty measuring cylinder in a trough of water. Record the volume of the air column. Cut a small piece of white phosphorous under water. Attach the piece of white phosphorous to the end of a piece of copper wire.

Side notes

Phosphorous is stored under water as it does not react with water.
In order to obtain accurate results, the white phosphorous should not be allowed to come in contact with the walls of the measuring cylinder because it stops smouldering.

Observations and Discussion

When both yellow and white phosphorus are exposed to air, they smoulder (burn slowly with smoke but no flame).

This is because phosphorus reacts spontaneously with oxygen to form a mixture of oxides.

The reaction can be represented by the following word equations.

Phosphorous + oxygen phosphorous (III) oxide

Phosphorous + oxygen phosphorous (V) oxide

After 24 hours, the water level inside the measuring cylinder will have risen to occupy the volume of oxygen used up.

The difference in volume can be used to calculate the percentage of oxygen by volume in air.

How to determine the presence of Carbon (IV) oxide and water in air

The presence of carbon(IV) oxide and water in air can be determined by using calcium hydroxide which forms a white precipitate when carbon(IV) oxide gas is bubbled through it and anhydrous copper (II) sulphate powder or anhydrous cobalt (II) chloride paper which turn from white to blue and blue to pink respectively when in contact with water.

⚗Practically Speaking🔊 📌

(a) Place 2cm³ of fresh calcium hydroxide solution (lime water) in a boiling tube. Pass water slowly from a tap into an aspirator. Record your observations.

(b) Pack the bottom of a U-tube with anhydrous calcium chloride. Pass air through the U-tube by means of an aspirator or a suction pump. Record your observations.

Side notes

Water is allowed to flow into aspirator A to drive out air and bubble it through the calcium hydroxide solution.
Water is allowed to flow out of aspirator B in order to create a suction force which draws air through the U-tube.

Observations and Discussion

When the stream of air is passed through calcium hydroxide solution, a white precipitate is formed. This indicates that carbon (IV) oxide gas is present in air.

When air is passed through the U-tube the white anhydrous calcium chloride absorbs water vapour from the air and becomes wet. It may form a colourless solution depending on the amount of moisture in the air.

Substances, which absorb moisture from the air to form a solution are called deliquescent substances. Other deliquescent substances are anhydrous iron (III) chloride, magnesium chloride and zinc chloride.

Fractional Distillation of Liquefied Air

Air is a mixture of many gases. It can be separated into its constituent gases by fractional distillation of liquefied air.

The air is first purified by passing it through filters to remove dust.

The dust-free air is then passed through a solution of concentrated sodium hydroxide to remove carbon (IV) oxide gas.

The remaining part of air is then cooled to -25°C to remove water vapour, which solidifies out as ice.

The remaining part of air is then compressed to a pressure of 200 atmospheres and allowed to expand. Repeated compression and expansion of the air cools it to liquid at -200°C.

The liquid air consists of oxygen, nitrogen and noble gases. Since these gases have different boiling points, they can be separated by fractional distillation. Liquid oxygen boils at -183°C and nitrogen at -196°C.

Nitrogen distils out first because it has a lower boiling point.

The other gases, made of mainly argon, boil at -186°C. They form the second fraction.

The argon can be separated from oxygen by further distillation.

Rusting

Rusting is the corrosion of iron due to its reaction with atmospheric oxygen and moisture.

Rust is hydrated iron (III) oxide. The chemical formula for rust is Fe₂O₃.H₂O

Rust forms a brown coating on the surface of iron material. Because rust is porous, once an object starts to rust, the process continues until the object is completely destroyed.

Conditions necessary for rusting

For rusting to occur, moisture and air must be available.

⚗Practically Speaking🔊 📌

The following experiment can be used to identify the conditions necessary for rusting to occur.

Label five boiling tubes 1, 2, 3, 4 and 5 respectively. Put two clean nails in each of the boiling tubes. To the first tube add 10cm³ of tap water. To the second tube, add 10cm³ of boiling water followed by about 3cm³ of oil. To the third tube, push a piece of cotton wool half-way and place some anhydrous calcium chloride on it and cork the tube. To the fourth tube, add salted water. The fifth boiling tube contains nails only. Observe the nails after three days and record your observations.

Side notes

Water in the second tube is boiled to expel all dissolved gases. The layer of oil covering the boiled water prevents re-entry of air.
Anhydrous calcium chloride absorbs moisture from the air, thus air in tube 3 is dry.
It is necessary to cork tube 3 to prevent entry of water vapour from the atmosphere

Observations and Discussion

After three days;

It is observed that the nails in boiling tube 1 would have rusted after three days. The nails in tube 5 would have rusted to a smaller extent. The rusting in tube 4 is more intense. No rust is observed in tubes 2 and 3.

Tap water contains dissolved oxygen. The iron nails combine with oxygen in the presence of water to form hydrated iron oxide.

Iron + oxygen Iron (III) oxide

Iron (III) oxide + water hydrated iron (III) oxide (brown)

Rusting occurred in tube 1 because both water and oxygen were present. Some rusting occurred in tube 5 since there was some moisture in the air. Rusting was more intense in tube 4 due to presence of dissolved sodium chloride.

There was no oxygen in tube 2 and therefore the nails did not rust. The nails in tube 3 do not rust because there was no water in it.

Learning outcome

The presence of water and oxygen are thus necessary for iron to rust.

The factors that accelerate rusting are salts and acids.

Rusting occurs faster in salty or acidic surroundings. For example, cars rust faster in Mombasa than in Nairobi.

Rusting destroys machinery, equipment and roofs made of iron. Rusting can be very expensive. Prevention of rusting is therefore of great importance.

Methods of Preventing Rusting

The basis of rust prevention is to keep iron out of direct contact with water and oxygen.

The following methods are widely used to prevent rusting of iron.

1. Painting e.g. cars, roofs, marine vessels etc.
2. Coating with other metals. This can be done through galvanisation or electroplating.
3. Alloying: This involves the mixing of iron with one or more metals to produce a substance, which does not rust.
4. Oiling and greasing: This method is used in moving engine parts where other methods cannot be used due to friction.
5. Sacrificial protection: In this arrangement, a more reactive metal such as zinc or magnesium is attached to the iron structure. The more reactive metal corrodes instead of iron. The method is applied in ships, water and oil pipes

Oxygen

Oxygen exists freely in the atmosphere as a gas. Its chemical symbol is O. Two atoms of oxygen combine to form a molecule with a chemical formula of O₂. Oxygen is also found combined with other elements such as hydrogen in water and metals in metal oxides. It is the most active component in air.

⚗Practically Speaking🔊 📌

Laboratory preparation of oxygen

In the laboratory, oxygen can be prepared by the decomposition of hydrogen peroxide using manganese(IV) oxide as a catalyst. Sodium peroxide may also be reacted with water to produce oxygen. When potassium permanganate (VII) is heated, oxygen is also produced.

Side notes

The first few bubbles of oxygen are not collected because the gas is mixed with air which was originally in the flask.
Oxygen is slightly soluble in water and so it can be collected over water.
Manganese (IV) oxide acts as a catalyst. A catalyst is a substance that alters the rate of a reaction. In the absence of manganese (IV) oxide, the hydrogen peroxide can be warmed to speed up the reaction.

Producing Oxygen: The Reactions

Hydrogen peroxide decomposes slowly to produce oxygen and water under normal conditions. On adding manganese (IV) oxide the rate of decomposition is speeded up. Manganese (IV) oxide is used as a catalyst in the decomposition of hydrogen peroxide.

Hydrogen peroxide Oxygen _(gas) + Water

Adding water to Solid Sodium Peroxide

Sodium peroxide + water → Sodium hydroxide + Oxygen

Heating Potassium Manganate (VII) (Potassium Permanganate)

Potassium Manganate (VII) Potassium + Manganese (IV) oxide + Oxygen

Test for Oxygen

Oxygen relights a glowing splint. This is the test for oxygen

Properties of oxygen

Oxygen is a colourless, odourless gas with a low boiling point of -183°C.

Burning of Substances in Air

The most familiar chemical reaction of air is burning. There are many substances, which burn in air. It has been established that oxygen is the active part of air, which supports burning.

Burning of metals in air

Many metals burn in air and in oxygen at different rates. They burn faster in oxygen than in air. Nitrogen is the component of air which slows down the rate of burning.

⚗Practically Speaking🔊 📌

Warm a piece of sodium in a deflagrating spoon until it begins to burn. Lower it into a gas jar of air as shown below Record your observations.

Allow the gas jar to cool, add some water to the product and shake the mixture. Test any gas given out with moist red and blue litmus paper. Test the solution in the gas jar using litmus papers and record your observations. Repeat the experiment using oxygen instead of air.

Repeat the whole procedure using calcium, magnesium, iron and copper in place of sodium.

Discourse

Sodium reacts most vigorously with oxygen while copper is the least reactive.

Metal	How it burns in air	Appearance of product	Name of product	Solubility of product in water	Nature of solution
Sodium	Burns with a yellow-orange flame	White solid	Sodium oxide and sodium nitride	Soluble, alkaline gas evolved	Alkaline
Calcium	Burns with a red flame in air enriched with oxygen.	White solid	Calcium oxide and calcium nitride	Soluble, alkaline gas evolved	Alkaline
Magnesium	Burns with a brilliant white flame.	White powder	Magnesium oxide and magnesium nitride	Soluble, alkaline gas evolved	Alkaline
Iron	Glows red with sparks	Red-brown solid	Iron (III) oxide	Insoluble	–
Copper	Burns with a blue flame in air enriched with oxygen, surface turns black	Black solid	Copper (II) oxide	Insoluble	–

Products of Burning Metals in Oxygen

When metals burn in oxygen they form metal oxides.

Sodium + Oxygen sodium oxide

Calcium + Oxygen calcium oxide

Magnesium + Oxygen Magnesium oxide

Iron + Oxygen Iron (III)oxide

Reactive metals such as sodium, calcium and magnesium react with nitrogen in the air to form nitrides.

Sodium + Nitrogen Sodium nitride

Calcium + Nitrogen Calcium nitride

Magnesium + Nitrogen Magnesium nitride

When the nitrides react with water, ammonia gas is given out.

Sodium nitride + Water Sodium hydroxide + Ammonia

Calcium nitride + Water Calcium hydroxide + Ammonia

Magnesium nitride + Water Magnesium hydroxide + Ammonia

The reactions in which elements combine with oxygen are referred to as oxidation. The substance to which the oxygen is added is said to have been oxidised

The metals can be arranged in order of their rates of reaction with oxygen from the most reactive to the least reactive. This arrangement is referred to as a reactivity series of metals.

Mercury, silver and gold are less reactive than copper and are not easily oxidised.

Reactivity Series of Metals

The following is part of the Reactivity Series for some metals.

Potassium Most reactive

Sodium

Lithium

Calcium

Magnesium

Aluminium

Zinc

Iron

Lead

Copper

Mercury

Silver

Gold Least reactive

Burning of non-metallic elements burn in oxygen

Sulphur burns in oxygen to give a gaseous product which has a choking irritating smell. The product is sulphur (IV) oxide.

Sulphur + Oxygen Sulphur (IV) oxide

A solution of sulphur (IV) oxide in water is acidic and turns blue litmus paper red. The acid is called sulphuric (IV) acid (sulphurous acid).

Sulphur (IV) oxide + Water Sulphurous acid.

Oxides which dissolve in water to form acidic solutions are referred to as acidic oxides.

Non -metal	How it burns in oxygen	Name of product	Appearance of product	Effect of solution on litmus
Sulphur	Burns with a blue flame	Sulphur (IV) oxide	White fumes	Turns red
Carbon	Burns with a yellow flame	Carbon (IV) oxide	White fumes	Turns red
Phosphorus	Burns with a white flame	Phosphorus (V) oxide and Phosphorus (III) oxide	White fumes	Turns red

Summary of effect of burning non-metals in oxygen

The following equations represent the reactions of the non-metals with oxygen.

Carbon + Oxygen Carbon (IV) oxide

(excess)

Carbon + Oxygen Carbon (II) oxide

(limited supply)

Phosphorus+ Oxygen Phosphorus (V) oxide

(excess)

Phosphorus + Oxygen Phosphorus (III) oxide

(limited supply)

Some non-metallic elements form oxides which are neither acidic nor basic. These oxides are referred to as neutral oxides.

Carbon (II) oxide and water (hydrogen oxide) are examples of neutral oxides.

Change in mass when substances burn in air

When substances burn in air, they combine with oxygen to form oxides. If the product is a solid there is increase in mass.

When the product is gaseous there is decrease in mass. The decrease in mass is because the products, being gaseous escape into the air.

⚗Practically Speaking🔊 📌

Weigh about 1g clean magnesium ribbon in a crucible. Heat the crucible, occasionally lifting the lid to let air in. Do not allow any contents to escape from the crucible. When all the magnesium has burned, allow the crucible to cool. Weigh the cool crucible and its contents again.

Record your observations as follows:

Mass of crucible + Magnesium before burning = Xg

Mass of crucible + contents after burning = Yg

Change in Mass = (Y–X)g

Side note

When magnesium is burned in a closed crucible, most of the oxygen inside is consumed. It is therefore necessary to allow air in so that burning can continue.

Discourse

The mass of the product is more than the original mass of magnesium. This shows that as it burns, magnesium combines with air to form a new product.

Competition for combined oxygen

A more reactive metal removes combined oxygen from a metal oxide of a less reactive metal. More reactive metals displace less reactive metals from their oxides.

For example, Magnesium combines with oxygen more readily than copper. Therefore, magnesium removes combined oxygen in copper (II) oxide to form magnesium oxide. Copper is said to have been displaced by magnesium.

Copper on the other hand does not remove combined oxygen from the oxides of magnesium, lead, zinc and iron. This is due to the fact that copper reacts with oxygen less readily than these metals. It is the least reactive.

Removal of oxygen from a substance is called reduction. When a metal oxide loses oxygen, it is said to have been reduced. The metal, which gains oxygen is said to have been oxidised.

Zinc + Copper (II) oxide Zinc oxide + Copper

In the above equation, zinc is oxidised while copper oxide is reduced. Both reduction and oxidation take place simultaneously.

A reaction in which both reduction and oxidation occur simultaneously is called a REDOX reaction.

Summary of the competition for combined oxygen by elements
	Magnesium	Zinc	Lead	Iron	Copper
Magnesium oxide (white)	No reaction	No reaction	No reaction	No reaction	No reaction
Copper (II) oxide (black)	Magnesium oxide and copper formed	Zinc oxide and copper formed	Lead oxide and copper formed	Iron (II) oxide and copper formed	No reaction
Lead (II) oxide (yellow)	Magnesium oxide and lead formed	Zinc oxide and lead formed	No reaction	Iron (III) oxide and lead formed	No reaction
Zinc oxide (white)	Magnesium oxide and zinc formed	No reaction	No reaction	No reaction	No reaction
Iron (III) oxide	Magnesium oxide and iron formed	Zinc oxide and iron formed	No reaction	No reaction	No reaction

From the table, magnesium displaces four metals from their oxides. Zinc displaces three while lead and iron displaces two and one respectively. Copper displaces none. Therefore, magnesium is the most reactive while copper is the least reactive.

By considering the number of metals displaced, a reactivity series can be obtained.

Magnesium Highest ability

Zinc increasing ability to

Iron take away combined oxygen

Lead

Copper Least ability

Application

The extraction of metals from their ores uses the concept of reduction. The ores that contain the metal oxides are reduced by more reactive metals. For example, Aluminium is used to reduce iron (III) oxide by the thermite process.

Carbon, a non-metal can remove combined oxygen from some metal oxides such as iron (III) oxide and copper (II) oxide.

Carbon + Copper (II) oxide Carbon (IV) oxide + Copper

The ability of carbon to reduce some metal oxides is applied in the extraction of metals such as copper and zinc from their ores.

Atmospheric Pollution

A pollutant is a substance (contaminant) or form of energy which has harmful effects to the environment.

Human activities have changed the composition of air in some places. Gases such as carbon (IV) oxide, carbon (II) oxide, sulphur (IV) oxide and phosphorous (V) oxide, are examples of harmful substances emitted into the atmosphere mainly from the combustion of fossil fuels. These gases cause pollution of the atmosphere. For example, sulphur (IV) oxide dissolves in rain water and is converted to sulphurous acid, which forms “acid rain”. Acid rain destroys plants and aquatic life. It also corrodes iron sheets, zinc roofing and buildings.

Uses of Oxygen

1. Air enriched with oxygen is used in hospitals by patients with breathing difficulties.

2. When mixed with helium it is used by mountain climbers and deep-sea divers.

3. Oxygen is used to burn fuels such as those used for propelling rockets.

4. A mixture of oxygen and acetylene burns to produce a very hot flame used in welding and for cutting metals.

5. During steel making, oxygen is used to remove iron impurities.

6. Oxygen is used as one of the reactants in fuel cells.

Review Exercises

(a) Complete the following table to show the components of air and their relative percentage abundance.

Components	% Composition
Nitrogen Oxygen Carbon(IV)oxide Noble gases

(b) Name two noble gases that are likely to be found in air.

(a) Name three substances, other than oxygen and nitrogen, that are always present in the atmosphere.

(b) Outline an experiment that you could perform in the laboratory to identify one of the substances you listed in part (a).

Air is a mixture of several different gases. Identity the gas in air which

(a) puts off a burning split. __

(b) supports combustion. __

(d) condenses to form a colourless, odourless liquid at room temperature.

State the main uses of

(a) oxygen. (b) nitrogen

A burning candle was placed in a bell jar containing sodium hydroxide solution as follows.

(a) Draw another diagram to show what you would expect after 10 minutes.

(b) Explain the observations in 6 a) above.

(a) State two conditions necessary for rusting.

(b) How is the rusting process accelerated in our daily life?

Iron rubbish bins coated with a complete layer of zinc do not rust easily

(a) What name is given to the coating of iron with zinc?

(b) Explain how the method works.

The set-up below was used In an experiment to determine the conditions necessary for rusting.

In which set up will the iron nails rust? Explain.

A large piece of magnesium buried in the ground and connected to an underground iron pipe prevents the corrosion of the iron pipes. Explain.
How does oiling of iron tools prevent rusting?
Common salt is often spread on roads cold countries.

(a) What is the purpose of adding common salt on roads in cold countries?

(b) In such countries, cars rust faster. Explain.

When magnesium metal is burnt in air the product formed weighs more than the original magnesium. Explain.
When a substance combines with oxygen, the process is called (a)___________and the substance is said to be (b)__________
A mixture of copper(ii)oxide and zinc metal was heated strongly in a crucible.

(a) State and explain observations made at the end of the experiment.

(b) Write the equation for the reaction that takes place in the crucible.

Sodium metal on a deflagrating spoon was burnt in air as shown below

(a) What is the colour of the flame produced when sodium is burnt?

(b) Write a word equation for the reaction that takes place.

(c) The product formed in this process is dissolved in water. Blue and red litmus papers are dipped in the resultant solution. State the observations

The boiling points (at atmospheric pressure) of oxygen and nitrogen are -183°C and – 196°C respectively. Which of these two gases would you expect to be separated first?
(a) Given that oxygen gas is denser than air, draw a well labelled diagram to show how dry oxygen can be prepared and collected in the laboratory

(b) When elements burn in oxygen, they combine with it to form _

(d) Outline two uses of oxygen gas.

The following set up was used to prepare and collect a sample of oxygen gas

(a) (i) Identify solid T.

(ii) State the role of solid T in this reaction.

(b) Write a word equation for the reaction that produces oxygen.

(d) Explain why it is important not to collect any gas for the first few seconds of the experiment.

The following table gives three elements that were burnt by students in dry oxygen gas. Complete the table to show how the elements burnt and the name of the product

Element	How it burnt	Name of product
Sulphur Magnesium Carbon

Oxygen can be prepared by dripping cold water on solid P.

(a) Name solid P.

(b) Write a word equation for the reaction that produces oxygen.

(a) Name two substances which when heated produce oxygen gas.

(b) Write the word equation for the reaction in (a) above.

Sodium metal was warmed until it began burning. The burning sodium was lowered in a gas jar of oxygen as shown below.

(a) State the expected observations.

(b) Some water was added to the product of this reaction and the resulting solution tested with blue and red litmus papers. State and explain the observations made.

(i) sodium with oxygen. _

(ii) product with water.

(d) What type of reaction takes place when sodium burns in oxygen?

Burning sulphur on a deflagrating spoon was lowered into a gas jar of oxygen gas.

(a) State the observations made.

(b) Write a word equation for the reaction that occurs. _

(c) A little water was used to dissolve the product of this experiment. What effect would the solution formed have on blue and red litmus papers? Explain using a word equation.

(a) Name the process used to obtain oxygen on large scale.

(b) During the process in 26 (a), dust particles are first removed from air How are the dust particles removed?

(i) Carbon (IV) oxide.

(ii) Water vapour.

(d) Describe how air free of dust particles, carbon (IV)oxide and water vapour is liquefied.

(e) Which component of liquid air is obtained first? Explain.

(f) Nitrogen, oxygen and argon are obtained from liquid air by fractional distillation. State the physical property that makes it possible to separate them.

(g) Arrange the gases in (f) in order of how they distil, starting with the first.

(h) State four uses of oxygen gas.

PAST KCSE QUESTIONS ON AIR AND COMBUSTION

2006 Q 2 P1

The diagram below represents a set-up that was used to show that part of air is used during burning.

(a) Given that phosphorus used was in excess, draw a diagram of the set-up at the end of the experiment (when there was no further observable change).

(b) Suggest one modification that should be made on the apparatus if the percentage of the air used is to be determined.

2007 Q 1a P1

State two factors that should be considered when choosing fuel for cooking.

2009 Q 21 P1

Give the name of the product formed when magnesium reacts with phosphorus. (1 mark)

2010 Q26 P1

A water trough, aqueous sodium hydroxide, burning candle, watch class and a graduated gas jar were used in an experimental set up to determine the percentage of active part of air. Draw a labelled diagram of the set up at the end of the experiment. (3 marks)

2012 Q1 P1

Charcoal is a fuel that is commonly used for cooking. When it burns it forms two oxides.

(a) Name the two oxides (2 marks)

(b) State one use of the two oxides (1 mark)

2012 Q24 P1

The following set up of three-tubes was used to investigate rusting of iron. Study it and answer the questions that follow.

(a) Give a reason why rusting did not occur in test-tube C. (1 mark)

(b) Aluminium is used to protect iron sheets from rusting. Explain two ways in which aluminium protects iron from rusting. (2 marks)

2013 Q1 P1

The set up below can be used to prepare oxygen gas. Study it and answer the questions that follow.

(a) Identify X (1 mark)

(b) What property of oxygen makes it possible for it to be collected as shown in the above set up? (1 mark)

2014 Q16 P1

A measuring cylinder fitted with moist steel wool was inverted in a trough of water as shown in the diagram below.

(a) State and explain the observations made on the:

(i) Moist steel wool after four days; (1 mark)

(ii) Water level in the measuring cylinder after four days. (1 mark)

(b) What would be the effect of using steel wool moistened with salty water? (1 mark)

2016 Q18 P1

A water trough, aqueous sodium hydroxide, burning candle, watch glass and a graduated jar were used in an experimental set up to determine the percentage of active part of air. Draw a labelled diagram of the set up at the end of the experiment. (3 marks)