Chemistry is basically understanding how everything around you - from... Show more
Chemistry240 views·Updated Jun 2, 2026·5 pagesGCSE Edexcel Chemistry Combined Notes
J
Julianne Croft@julescroft_
1 / 5
1
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Hydrocarbons and Crude Oil
Think of crude oil as nature's ultimate chemistry set - it's a complex mixture of hydrocarbons (molecules containing only carbon and hydrogen) that literally powers modern life. These molecules come in chains and rings, and each type has its own special job.
Fractional distillation is how we separate crude oil into useful bits. Picture heating oil in a massive tower where different substances boil and condense at different temperatures - gases for heating rise to the top, petrol sits in the middle, and thick bitumen for roads stays at the bottom.
The alkanes are the main family here, following the pattern CnH2n+2. Methane (CH₄) is the simplest, then ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀). The longer the chain, the higher the boiling point and the thicker (more viscous) it becomes.
Quick Test Tip: Remember that shorter chains = more flammable = better fuels. That's why petrol (short chains) works brilliantly in car engines, whilst bitumen (long chains) is perfect for road surfaces.
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Reaction Rates and Catalysts
Rate of reaction measures how fast reactants turn into products - imagine timing how quickly an Alka-Seltzer dissolves. Chemical reactions only happen when particles crash into each other with enough energy (called activation energy).
Want to speed things up? You've got four main tricks: increase temperature (particles move faster and hit harder), increase pressure or concentration (more particles in the same space means more collisions), or increase surface area (more exposed area means more chances to react).
Catalysts are the ultimate chemistry cheat codes - they speed up reactions without getting used up themselves. They work by lowering the activation energy, giving particles an easier pathway to react. Enzymes are biological catalysts that work at body temperature and are incredibly selective.
Real-Life Connection: Enzymes in yeast convert sugar into alcohol and carbon dioxide - that's literally how bread rises and beer gets made!
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Combustion and Cracking
Complete combustion happens when hydrocarbons burn with plenty of oxygen, producing carbon dioxide and water whilst releasing loads of energy. It's the clean burn you want in your car engine.
Incomplete combustion occurs when there's not enough oxygen around. Instead of clean CO₂, you get nasty carbon monoxide (CO) - a colourless, odourless killer that stops your blood from carrying oxygen. You also get soot and unburnt fuel.
Cracking solves a major problem: crude oil gives us more long-chain hydrocarbons than we need, but we're desperate for shorter chains like petrol. So we heat long chains with a catalyst until they break into smaller, more useful molecules including alkenes (unsaturated hydrocarbons with double bonds).
Environmental Note: Hydrogen fuel only produces water when burnt - no CO₂ - but it's expensive and tricky to store safely.
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Halogens
The halogens are like a chemical family with serious personality differences. Chlorine is a yellow-green gas, bromine is a red-brown liquid, and iodine is a purple solid. As you go down the group, they get less reactive but have higher melting and boiling points.
Here's the key pattern: reactivity decreases down the group because the outer electrons get further from the nucleus and are easier to lose. This creates a pecking order for displacement reactions - more reactive halogens can kick out less reactive ones from their compounds.
Redox reactions happen when halogens swap places. The more reactive halogen gains electrons (gets reduced) whilst the less reactive one loses electrons (gets oxidised). Chlorine will bully both bromine and iodine out of their compounds.
Lab Test: Chlorine turns damp blue litmus paper red, then bleaches it white - that's your definitive test for this gas.
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Atomic Structure Basics
Every atom is built like a mini solar system with electrons orbiting the nucleus in shells. The first shell holds 2 electrons maximum, the second holds 8, and the third holds 8. Electrons always fill the lowest energy level first - they're basically lazy and want the easiest spot.
The atomic number tells you how many protons an atom has, whilst the mass number is protons plus neutrons. Most atoms have unfilled outer shells, which is why they're desperate to react and complete their outer shell.
Isotopes are like identical twins with different weights - same number of protons but different neutrons. Elements contain atoms with identical proton numbers, whilst compounds form when different elements chemically bond together.
Memory Trick: Think of electronic configuration like filling seats on a bus - passengers (electrons) fill the front rows (lower energy shells) before moving to the back.
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Chemistry240 views·Updated Jun 2, 2026·5 pagesGCSE Edexcel Chemistry Combined Notes
J
Julianne Croft@julescroft_
Chemistry is basically understanding how everything around you - from the petrol in cars to the air you breathe - works at the molecular level. This content covers the building blocks of chemistry: hydrocarbons that fuel our world, chemical reactions... Show more
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Hydrocarbons and Crude Oil
Think of crude oil as nature's ultimate chemistry set - it's a complex mixture of hydrocarbons (molecules containing only carbon and hydrogen) that literally powers modern life. These molecules come in chains and rings, and each type has its own special job.
Fractional distillation is how we separate crude oil into useful bits. Picture heating oil in a massive tower where different substances boil and condense at different temperatures - gases for heating rise to the top, petrol sits in the middle, and thick bitumen for roads stays at the bottom.
The alkanes are the main family here, following the pattern CnH2n+2. Methane (CH₄) is the simplest, then ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀). The longer the chain, the higher the boiling point and the thicker (more viscous) it becomes.
Quick Test Tip: Remember that shorter chains = more flammable = better fuels. That's why petrol (short chains) works brilliantly in car engines, whilst bitumen (long chains) is perfect for road surfaces.
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Reaction Rates and Catalysts
Rate of reaction measures how fast reactants turn into products - imagine timing how quickly an Alka-Seltzer dissolves. Chemical reactions only happen when particles crash into each other with enough energy (called activation energy).
Want to speed things up? You've got four main tricks: increase temperature (particles move faster and hit harder), increase pressure or concentration (more particles in the same space means more collisions), or increase surface area (more exposed area means more chances to react).
Catalysts are the ultimate chemistry cheat codes - they speed up reactions without getting used up themselves. They work by lowering the activation energy, giving particles an easier pathway to react. Enzymes are biological catalysts that work at body temperature and are incredibly selective.
Real-Life Connection: Enzymes in yeast convert sugar into alcohol and carbon dioxide - that's literally how bread rises and beer gets made!
3
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Combustion and Cracking
Complete combustion happens when hydrocarbons burn with plenty of oxygen, producing carbon dioxide and water whilst releasing loads of energy. It's the clean burn you want in your car engine.
Incomplete combustion occurs when there's not enough oxygen around. Instead of clean CO₂, you get nasty carbon monoxide (CO) - a colourless, odourless killer that stops your blood from carrying oxygen. You also get soot and unburnt fuel.
Cracking solves a major problem: crude oil gives us more long-chain hydrocarbons than we need, but we're desperate for shorter chains like petrol. So we heat long chains with a catalyst until they break into smaller, more useful molecules including alkenes (unsaturated hydrocarbons with double bonds).
Environmental Note: Hydrogen fuel only produces water when burnt - no CO₂ - but it's expensive and tricky to store safely.
4
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Halogens
The halogens are like a chemical family with serious personality differences. Chlorine is a yellow-green gas, bromine is a red-brown liquid, and iodine is a purple solid. As you go down the group, they get less reactive but have higher melting and boiling points.
Here's the key pattern: reactivity decreases down the group because the outer electrons get further from the nucleus and are easier to lose. This creates a pecking order for displacement reactions - more reactive halogens can kick out less reactive ones from their compounds.
Redox reactions happen when halogens swap places. The more reactive halogen gains electrons (gets reduced) whilst the less reactive one loses electrons (gets oxidised). Chlorine will bully both bromine and iodine out of their compounds.
Lab Test: Chlorine turns damp blue litmus paper red, then bleaches it white - that's your definitive test for this gas.
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Atomic Structure Basics
Every atom is built like a mini solar system with electrons orbiting the nucleus in shells. The first shell holds 2 electrons maximum, the second holds 8, and the third holds 8. Electrons always fill the lowest energy level first - they're basically lazy and want the easiest spot.
The atomic number tells you how many protons an atom has, whilst the mass number is protons plus neutrons. Most atoms have unfilled outer shells, which is why they're desperate to react and complete their outer shell.
Isotopes are like identical twins with different weights - same number of protons but different neutrons. Elements contain atoms with identical proton numbers, whilst compounds form when different elements chemically bond together.
Memory Trick: Think of electronic configuration like filling seats on a bus - passengers (electrons) fill the front rows (lower energy shells) before moving to the back.
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI Companion is a student-focused AI tool that offers more than just answers. Built on millions of Knowunity resources, it provides relevant information, personalised study plans, quizzes, and content directly in the chat, adapting to your individual learning journey.
Where can I download the Knowunity app?
You can download the app from Google Play Store and Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
Most popular content: Hydrocarbons
9Most popular content in Chemistry
9Most popular content
9Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5App Store
4.7/5Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan SiOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha KlichAndroid user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
AnnaiOS user