combustion

Air is made up of different gases. Different gases perform different functions. Some gases are used by plants and animals, and even for purposes such as combustion. Let’s dig in and find out more.

LEARNING OBJECTIVES

By the end of this topic, you are expected to;

• Understand the composition of air
• Understand the roles of the different gases in air
• Understand the chemical properties of the gases of air

THE ATMOSPHERE

The atmosphere is made up of air. Air can be defined as a mixture of odorless, colorless gases which is felt as wind. Every living thing breathes in air for respiration. Plants mainly use air for photosynthesis and respiration. The main gases found in the air or atmosphere are;

Gas	Approximate % composition by volume
Nitrogen	78.0
Oxygen	21.0
Carbon (IV) oxide	0.03
Noble gases	1.0
Water vapor	Vary from region

Combustion refers to a chemical process where a substance reacts quickly with oxygen and produces heat. In such a case, the substance is called fuel. The source of oxygen is called the oxidizer.

The following experiment can be used to demonstrate the presence and composition of gases in the atmosphere or air.

Title: To find the composition of air supporting combustion using a candle stick

Procedure:

• Measure the length of an empty gas jar (marked M1)
• Place a candle stick on a Petri dish
• Float it on water in a trough or a basin
• Cover it with the gas jar
• Measure the level of water in the gas jar (marked M2)
• Remove the gas jar
• Light the candle stick
• Carefully cover it with the gas jar
• Observe for some few minutes
• Mark the new level of water M3

                                                               

OBSERVATIONS

• The candle continues to burn and then goes off after some time
• The level of water in the gas jar rises after the candle is ignited

EXPLANATION

Candle burns in air. In a closed system, the candle continues to burn using the part of air that support combustion or burning. This is known as the active part of air. The candle is extinguished when all the active part of air is used up. The level of water rises in order to occupy the volume or space occupied the used active part of air.

From the experiment above, the % of composition of the gas that supports combustion can be found.

• The active part of air can be calculated: (M2 – M3) x 100
• Inactive part of air: 100% - active part of air

Oxygen is the gas that supports combustion.

OXYGEN

OCCURRENCE

50% of the earth's crust is made up of oxygen combined with other elements such as metals and oxides. Approximately 70% of the earth is water made up of oxygen and hydrogen. About 20% by volume of the atmospheric gases is oxygen that forms the active part of air.

LABORATORY PREPARATION OF OXYGEN

Oxygen can be prepared in the laboratory using the following experiment.

Title; laboratory preparation of oxygen using hydrogen peroxide

procedure;

• Half fill a trough with tap water
• Place a bee hive stand into the water
• Completely fill the gas jar with water and invert it onto the bee hive stand
• Clamp a round bottomed flask and set up the apparatus as shown below
• Collect several gas jars of oxygen covering the water

                                                             

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 to form oxygen gas and water. Manganese (IV) oxide speeds up the decomposition rate by reducing the time taken for a given volume of oxygen to be produced.

USES OF OXYGEN

• Oxygen is put in cylinders for use where natural supply is not sufficiently enough. This is mainly applied in; mountain climbing, deep sea diving and saving life in hospitals for patients with breathing problems.
• A mixture of oxygen and other gases produces a very hot flame.
• Oxy-hydrogen mixture is used as rocket fuel
• A mixture of petrol, charcoal and liquid oxygen is an explosive.