Key Concepts

Habari Mwanafunzi! Let's Uncover the Secrets of Air and Fire!

Ever sat by a bonfire during the cold season, or watched your mum light a charcoal jiko to make that delicious chapati? Ever wondered what exactly is happening? Why does the fire need air? What is this "air" anyway? Well, get ready! Today, we are diving deep into the amazing chemistry of Air and Combustion. It's the science that's happening all around you, from the air you breathe to the fire that cooks your food. Let's begin!

What is This Stuff We Call Air?

First things first, air is not just one thing. It’s a mixture of several different gases, all jumbled up together. Think of it like a bowl of githeri – you have your maize, your beans, and maybe other things mixed in. Air is a "githeri of gases"!

The main ingredients in our air "githeri" are:

• Nitrogen (N₂): The biggest part, making up about 78% of the air. It's a bit of a "lazy" gas and doesn't like to react much. We call it the inactive part of air.
• Oxygen (O₂): This is the superstar! It's about 21% of the air. It's the gas we need to breathe (respiration) and the gas that things need to burn (combustion). It is the active part of air.
• Argon (Ar): About 0.9%, another "lazy" gas.
• Carbon Dioxide (CO₂): A tiny amount, about 0.04%, but very important for plants.
• Other gases and water vapour make up the rest.

Composition of Air (Approximate)
================================

Nitrogen (78%)      #########################
Oxygen (21%)        #######
Argon (0.9%)        #
CO2 & Others (0.1%) #

(Each '#' represents about 3% of the atmosphere)

Proving Oxygen is the "Active" Part

How do we know that it's the oxygen that's so important for burning? We can do a simple, classic experiment!

Imagine lighting a small candle, placing it in a basin of water, and then covering it with a gas jar. Watch what happens!

Image Suggestion: A clear, labeled scientific diagram for a school textbook. It shows a lit candle on a small block inside a trough of water. A gas jar is being lowered over the candle. An arrow shows the initial water level. A second diagram next to it shows the final state: the candle is out, and the water level inside the gas jar has risen significantly. The rise in water should be labeled as 'approximately 1/5th'.

    +-----------+
    |           | <-- Gas Jar
    |           |
    |   /\\      | <-- Candle goes out
    |  🕯️ (out)  |
    |-----------| <-- Water level rises
    |~~~~~~~~~~~|
+-------------------+
| ~~~~~~~~~~~~~~~ | <-- Basin of water
+-------------------+

Two things happen:

1. The candle flame slowly dies and goes out.
2. The water level inside the gas jar rises up.

Why? The burning candle used up all the active gas (oxygen) inside the jar. Once the oxygen was gone, the flame could not survive. The water then rises to fill the space that the consumed oxygen was occupying. If you measure carefully, you'll find the water rises by about one-fifth (1/5) of the volume of the jar. And what is 1/5 as a percentage? It's 20%, which is very close to the 21% of oxygen in the air!

Calculating the Percentage of Oxygen

Here's how we do the math for this experiment:

Step 1: Measure the initial volume of air in the gas jar.
   (Let's say it was 100 cm³)

Step 2: Measure the volume of air left after the candle goes out.
   (The volume will be smaller because oxygen was used up. Let's say it is 79 cm³)

Step 3: Calculate the volume of oxygen used.
   Volume of Oxygen = Initial Volume - Final Volume
   Volume of Oxygen = 100 cm³ - 79 cm³ = 21 cm³

Step 4: Calculate the percentage of oxygen.
   Percentage (%) = (Volume of Oxygen / Initial Volume) x 100
   Percentage (%) = (21 / 100) x 100 = 21%

Combustion: The Magic of Fire!

Combustion is just the scientific name for burning. It's a chemical reaction that releases heat and light. For combustion to happen, you absolutely need three things. We call this the Fire Triangle.

      /\\
     /  \\
    /HEAT\\
   /______\\
  /        \\
 /  FUEL    \\
/____________\\
     OXYGEN

• FUEL: This is the substance that will burn. Examples from home include kuni (firewood), makaa (charcoal), paraffin in a lamp, or LPG gas in a cooker.
• OXYGEN: This comes from the air. Without it, there is no fire.
• HEAT: This is needed to start the fire (e.g., a matchstick) and to keep it going.

Think about reviving a dying charcoal jiko. What do you do? You use a fan (kupepea jiko)! You aren't adding more fuel or heat, but you are forcing more air (oxygen) onto the charcoal, making the fire roar back to life. If you want to put the fire out, you can cover the jiko with a lid, which cuts off the oxygen supply, or pour water on it, which removes the heat.

Types of Flames: Not All Fires are the Same!

The type of flame you get depends on how much oxygen is available.

1. Complete Combustion

This happens when there is plenty of oxygen. The fuel burns completely and efficiently.

• Flame: Blue, non-sooty (doesn't produce black smoke).
• Heat: Produces a lot of heat.
• Products: Carbon Dioxide (CO₂) and Water (H₂O).
• Example: The flame from a gas cooker or a well-maintained Bunsen burner. This is the best flame for cooking because it's hot and doesn't make your sufurias black!

2. Incomplete Combustion

This happens when there is a limited supply of oxygen. The fuel does not burn completely.

• Flame: Yellow or orange, and very sooty (produces black smoke).
• Heat: Produces less heat.
• Products: Besides some Carbon Dioxide and Water, it produces poisonous Carbon Monoxide (CO) and Carbon (soot) - the black stuff that makes your pots dirty.
• Example: A smoky paraffin lamp or a jiko that is not getting enough air.

WARNING: Carbon Monoxide is a very dangerous, poisonous gas. It has no smell and can be deadly. This is why you should NEVER bring a charcoal jiko into a closed room to stay warm. Always ensure there is good ventilation!

Image Suggestion: A split-screen image. On the left, a clean blue flame from a modern gas stove burner, labeled "Complete Combustion (Plenty of Oxygen)". On the right, a smoky, yellow-orange flame from a traditional paraffin lamp (koroboi), labeled "Incomplete Combustion (Limited Oxygen)". A black, sooty sufuria is shown above the yellow flame.

Rusting: Slow and Silent Combustion

Did you know that rusting is a type of combustion? It's true! Rusting is just very, very slow oxidation (reaction with oxygen) of iron. It produces heat, but so slowly that you can't feel it, and of course, there is no flame.

For rusting to occur, two things are essential:

• Oxygen (from the air)
• Water (or moisture in the air)

You see it all the time in Kenya. An old jembe (hoe) left out in the shamba after the rains. The scratches on a mabati (iron sheet) roof. An old gate that hasn't been painted. That flaky, reddish-brown substance is rust, scientifically known as hydrated iron(III) oxide. It forms because the iron is exposed to both rain and air.

To prevent rusting, you simply need to keep either oxygen or water (or both) away from the iron. That's why we paint gates, oil moving parts, and galvanize iron sheets by coating them with zinc!

Let's Wrap It Up!

Wow, we've covered a lot! You now know that air is a vital mixture, with oxygen as its active component. You understand the Fire Triangle (Fuel, Oxygen, Heat) and can tell the difference between a clean, complete combustion flame and a dirty, incomplete one. You even know that the rust on a gate is a form of super-slow combustion!

Chemistry is not just in the lab; it's in your kitchen, on your roof, and in every single breath you take. Keep your eyes open and stay curious!