Topic I: Planet Earth - The Atmosphere
Hey there! Welcome to your study notes for the first part of "Planet Earth". Ever stopped to think about the air you're breathing right now? It's easy to forget it's even there, but this invisible mixture of gases is essential for all life on our planet. In this chapter, we're going to explore what our atmosphere is made of, how we can separate its most important gases, and how to prove we've got pure oxygen. It's the first step in understanding the amazing chemistry of the world around us. Let's get started!
1. The Composition of Air
First things first: air is not a single substance. It's a mixture of different gases. A mixture is where different elements or compounds are mixed together without chemically reacting. Think of it like a bowl of mixed nuts – you have almonds, walnuts, and cashews all in the same bowl, but they are still individual nuts. Air is just like that, but with gases.
The main gases in clean, dry air are:
• Nitrogen (N₂): about 78%
• Oxygen (O₂): about 21%
• Argon (Ar): about 0.9%
• Carbon Dioxide (CO₂): about 0.04%
There are also tiny amounts of other gases, called trace gases, like neon and helium, as well as varying amounts of water vapour.
Memory Aid!
To remember the order of the main gases from most to least abundant, think: "Nice Old Animals Chat". (Nitrogen, Oxygen, Argon, Carbon dioxide).
Key Takeaway
Air is a mixture, not a pure substance. Its main component is nitrogen (~78%), followed by oxygen (~21%).
2. Separating Gases from the Air
Since nitrogen and oxygen have so many important uses (e.g., oxygen in hospitals and welding, nitrogen in food packaging to keep it fresh), we need a way to separate them from the air. The industrial method to do this is called fractional distillation of liquid air.
Don't worry if that sounds complicated! The main idea is simple.
The Core Principle: Different Boiling Points
Fractional distillation works because the different gases in the air turn from a liquid back into a gas at different temperatures. This temperature is called the boiling point.
Analogy: Imagine you have a mix of water (boils at 100°C) and pure alcohol (boils at 78°C). If you gently heat the mix, the alcohol will boil and turn into vapour first. You can then collect this vapour and cool it down to get pure alcohol, leaving the water behind. We do the exact same thing with liquid air, just at extremely cold temperatures!
Step-by-Step: The Journey of Air
The whole process happens in two main stages:
Stage 1: Turn Air into a Liquid
1. Filter the air: First, the air is passed through filters to remove dust particles.
2. Remove Water and Carbon Dioxide: The air is then cooled to about -80°C. At this temperature, water vapour freezes into ice and carbon dioxide turns into a solid (dry ice). They must be removed.
3. Compress and Cool: The clean, dry air is compressed to a very high pressure, which makes it hot. It's then allowed to expand rapidly, which makes it extremely cold. This process is repeated until the air gets so cold (around -200°C) that it turns into a pale blue liquid.
Did you know?
We have to remove water and carbon dioxide because if we didn't, they would freeze into solids at the low temperatures required and block up the pipes in the machinery!
Stage 2: Separate the Liquid Air (Fractional Distillation)
The liquid air is pumped into the bottom of a tall tower called a fractionating column. This column is warm at the bottom and gets colder towards the top.
Here are the key boiling points to remember:
• Nitrogen: -196°C (the lowest boiling point)
• Argon: -186°C
• Oxygen: -183°C (the highest boiling point)
A quick tip for negative numbers: the "bigger" the negative number (like -196), the colder it is, so it's the lowest temperature.
Here’s what happens inside the column:
1. The liquid air is gently warmed at the bottom of the column.
2. Nitrogen, with the lowest boiling point (-196°C), boils first. It turns into a gas and rises all the way to the top of the cold column, where it is collected.
3. Oxygen, with the highest boiling point (-183°C), remains a liquid for longer. It flows down to the bottom of the column, where it is collected as a liquid.
4. Argon has a boiling point in between nitrogen and oxygen, so it rises to the middle of the column before turning back to liquid and being collected there.
Key Takeaway
Gases in the air are separated by fractional distillation. This works because each gas has a different boiling point. Nitrogen has the lowest boiling point and is collected at the top of the column. Oxygen has the highest boiling point and is collected at the bottom.
3. The Chemical Test for Oxygen
So, you've separated a gas and you think it's oxygen. How can you be sure? Chemists use a simple and reliable chemical test called the glowing splint test.
Procedure: The Glowing Splint Test
1. Collect a sample of the gas you want to test in a test tube.
2. Take a wooden splint and light it with a flame.
3. Gently blow out the flame. The tip of the splint should still be red and glowing. This is now a glowing splint.
4. Immediately place the glowing end of the splint into the mouth of the test tube containing the gas.
Positive Result for Oxygen
If the gas is oxygen, the glowing splint will relight and burst back into flame!
Why does this happen?
Oxygen itself does not burn, but it is required for things to burn (it supports combustion). The glowing splint is still very hot, and when it's put into a high concentration of pure oxygen, it has enough fuel (the wood) and oxygen to start burning again.
The word equation for the splint burning is:
Carbon (from the wood) + Oxygen → Carbon dioxide
Quick Review: Common Mistake Alert!
Make sure you use a glowing splint, not a burning splint. A burning splint will just burn more brightly in oxygen, which can be hard to judge. A glowing splint relighting is a much clearer and more definitive test!
Key Takeaway
The test for oxygen gas is to place a glowing splint into it. A positive test is when the splint relights.