Maths
Chemistry
Biology
English Language
History
Responding to change (a2 only)
Infection and response
Homeostasis and response
Energy transfers (a2 only)
Cell biology
Organisms respond to changes in their internal and external environments (a-level only)
Biological molecules
Organisation
Substance exchange
Bioenergetics
Genetic information & variation
Inheritance, variation and evolution
Genetics & ecosystems (a2 only)
Ecology
Cells
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Britain & the wider world: 1745 -1901
1l the quest for political stability: germany, 1871-1991
The cold war
Inter-war germany
Medieval period: 1066 -1509
2d religious conflict and the church in england, c1529-c1570
2o democracy and nazism: germany, 1918-1945
1f industrialisation and the people: britain, c1783-1885
1c the tudors: england, 1485-1603
2m wars and welfare: britain in transition, 1906-1957
World war two & the holocaust
2n revolution and dictatorship: russia, 1917-1953
2s the making of modern britain, 1951-2007
World war one
Britain: 1509 -1745
Show all topics
Chemistry
10 Dec 2025
109
3 pages
- revision help - @xrevisiontipsandtricksx
Chemistry can seem complex, but it's all about understanding how atoms bond together and behave in different situations.... Show more
Understanding how atoms stick together is absolutely crucial for chemistry success. There are three main types of structures you need to master, each with completely different properties.
Ionic compounds form when metals give electrons to non-metals, creating oppositely charged ions. These arrange themselves in a giant ionic lattice held together by strong electrostatic forces. That's why salt has such a high melting point - you need loads of energy to break those strong attractions between positive and negative ions.
Simple molecular covalent compounds are totally different. Here, atoms share electrons to form molecules, but the forces between separate molecules are really weak. Think of water or carbon dioxide - they have low melting points because you're only breaking those weak intermolecular forces, not the actual covalent bonds within molecules.
Giant covalent structures like diamond are the extreme version of covalent bonding. Every single atom is covalently bonded to its neighbours in one massive network. Breaking this apart requires enormous energy, which is why diamond is so incredibly hard and has such a high melting point.
Key insight The type of bonding determines everything about a substance's properties - melting point, electrical conductivity, and hardness all depend on how the atoms are held together.
The periodic table isn't just a random arrangement - it reveals amazing patterns that make chemistry predictable. Group 1 elements (alkali metals) get more reactive as you go down because their outer electrons are further from the nucleus and easier to lose.
Watch lithium fizz gently in water, sodium react more vigorously, and potassium create that spectacular lilac flame. Group 7 elements (halogens) show the opposite trend - reactivity decreases down the group because it becomes harder for larger atoms to attract that extra electron they need.
Group 0 elements are the ultimate loners of chemistry. These noble gases exist as single atoms because they've got complete outer electron shells and don't need to bond with anyone. Their boiling points increase down the group due to stronger intermolecular forces between larger atoms.
Dmitri Mendeleev was brilliant enough to arrange elements by atomic weight and leave gaps for undiscovered elements. When those elements were found later, they matched his predictions perfectly - showing the periodic table's power.
Understanding acids and alkalis is essential for so many reactions. Strong acids like hydrochloric acid completely break apart in water, releasing loads of H⁺ ions. Weak acids like vinegar only partially ionise, so fewer H⁺ ions are floating about in solution.
The crystallisation practical shows you how to make pure copper chloride crystals by reacting copper oxide with hydrochloric acid, then carefully controlling the cooling to grow perfect crystals.
Top tip Remember that pH is all about H⁺ ion concentration - more H⁺ ions means lower pH and stronger acidity.
The reactivity series is your roadmap for predicting chemical reactions. Potassium sits at the top as the most reactive metal, whilst copper lurks near the bottom. This order determines how we extract metals from their ores.
Electrolysis is essentially chemistry's way of using electricity to force reactions that wouldn't happen naturally. At the cathode, positive metal ions gain electrons and turn into pure metal. At the anode, negative ions lose electrons and often form gases like oxygen or chlorine.
Aluminium extraction perfectly demonstrates this process. Since aluminium is more reactive than carbon, we can't use the usual method of heating with carbon. Instead, we use electrolysis on molten aluminium oxide mixed with cryolite to lower the melting point and save energy.
When dealing with aqueous solutions, you need to consider which ions will actually react. At the cathode, if the metal is more reactive than hydrogen, you'll get hydrogen gas instead of the metal. At the anode, halide ions will form halogens, but if there aren't any halides present, you'll get oxygen from the water.
Energy changes in reactions follow predictable patterns. Exothermic reactions release energy because the bonds formed in products are stronger than those broken in reactants. Endothermic reactions absorb energy because you're breaking stronger bonds than you're making.
Remember Activation energy is always needed to get reactions started, even if they're exothermic overall - think of it as the energy hill you need to climb before you can roll down the other side.
The practical investigation of neutralisation reactions shows how temperature changes can measure energy release, though you need good insulation to get accurate results since heat easily escapes to surroundings.
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.
You can download the app from Google Play Store and Apple App Store.
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
1
Smart Tools NEW
Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Mock Exam ✓ Essay Outlines
Quotes from every main character
English LiteratureApp Store
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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 S
iOS 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 Klich
Android 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.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
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 S
iOS 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 Klich
Android 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.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
- revision help -
@xrevisiontipsandtricksx
Chemistry can seem complex, but it's all about understanding how atoms bond together and behave in different situations. This guide covers the essential patterns you'll see across the periodic table, from ionic and covalent bonding to acids and energy changes... Show more
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Understanding how atoms stick together is absolutely crucial for chemistry success. There are three main types of structures you need to master, each with completely different properties.
Ionic compounds form when metals give electrons to non-metals, creating oppositely charged ions. These arrange themselves in a giant ionic lattice held together by strong electrostatic forces. That's why salt has such a high melting point - you need loads of energy to break those strong attractions between positive and negative ions.
Simple molecular covalent compounds are totally different. Here, atoms share electrons to form molecules, but the forces between separate molecules are really weak. Think of water or carbon dioxide - they have low melting points because you're only breaking those weak intermolecular forces, not the actual covalent bonds within molecules.
Giant covalent structures like diamond are the extreme version of covalent bonding. Every single atom is covalently bonded to its neighbours in one massive network. Breaking this apart requires enormous energy, which is why diamond is so incredibly hard and has such a high melting point.
Key insight: The type of bonding determines everything about a substance's properties - melting point, electrical conductivity, and hardness all depend on how the atoms are held together.
The periodic table isn't just a random arrangement - it reveals amazing patterns that make chemistry predictable. Group 1 elements (alkali metals) get more reactive as you go down because their outer electrons are further from the nucleus and easier to lose.
Watch lithium fizz gently in water, sodium react more vigorously, and potassium create that spectacular lilac flame. Group 7 elements (halogens) show the opposite trend - reactivity decreases down the group because it becomes harder for larger atoms to attract that extra electron they need.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Group 0 elements are the ultimate loners of chemistry. These noble gases exist as single atoms because they've got complete outer electron shells and don't need to bond with anyone. Their boiling points increase down the group due to stronger intermolecular forces between larger atoms.
Dmitri Mendeleev was brilliant enough to arrange elements by atomic weight and leave gaps for undiscovered elements. When those elements were found later, they matched his predictions perfectly - showing the periodic table's power.
Understanding acids and alkalis is essential for so many reactions. Strong acids like hydrochloric acid completely break apart in water, releasing loads of H⁺ ions. Weak acids like vinegar only partially ionise, so fewer H⁺ ions are floating about in solution.
The crystallisation practical shows you how to make pure copper chloride crystals by reacting copper oxide with hydrochloric acid, then carefully controlling the cooling to grow perfect crystals.
Top tip: Remember that pH is all about H⁺ ion concentration - more H⁺ ions means lower pH and stronger acidity.
The reactivity series is your roadmap for predicting chemical reactions. Potassium sits at the top as the most reactive metal, whilst copper lurks near the bottom. This order determines how we extract metals from their ores.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Electrolysis is essentially chemistry's way of using electricity to force reactions that wouldn't happen naturally. At the cathode, positive metal ions gain electrons and turn into pure metal. At the anode, negative ions lose electrons and often form gases like oxygen or chlorine.
Aluminium extraction perfectly demonstrates this process. Since aluminium is more reactive than carbon, we can't use the usual method of heating with carbon. Instead, we use electrolysis on molten aluminium oxide mixed with cryolite to lower the melting point and save energy.
When dealing with aqueous solutions, you need to consider which ions will actually react. At the cathode, if the metal is more reactive than hydrogen, you'll get hydrogen gas instead of the metal. At the anode, halide ions will form halogens, but if there aren't any halides present, you'll get oxygen from the water.
Energy changes in reactions follow predictable patterns. Exothermic reactions release energy because the bonds formed in products are stronger than those broken in reactants. Endothermic reactions absorb energy because you're breaking stronger bonds than you're making.
Remember: Activation energy is always needed to get reactions started, even if they're exothermic overall - think of it as the energy hill you need to climb before you can roll down the other side.
The practical investigation of neutralisation reactions shows how temperature changes can measure energy release, though you need good insulation to get accurate results since heat easily escapes to surroundings.
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.
You can download the app from Google Play Store and Apple App Store.
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
1
Smart Tools NEW
Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Mock Exam ✓ Essay Outlines
Quotes from every main character
English LiteratureApp Store
Google 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 S
iOS 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 Klich
Android 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.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
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 S
iOS 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 Klich
Android 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.
Anna
iOS user
Best app on earth! no words because it’s too good
Thomas R
iOS user
Just amazing. Let's me revise 10x better, this app is a quick 10/10. I highly recommend it to anyone. I can watch and search for notes. I can save them in the subject folder. I can revise it any time when I come back. If you haven't tried this app, you're really missing out.
Basil
Android user
This app has made me feel so much more confident in my exam prep, not only through boosting my own self confidence through the features that allow you to connect with others and feel less alone, but also through the way the app itself is centred around making you feel better. It is easy to navigate, fun to use, and helpful to anyone struggling in absolutely any way.
David K
iOS user
The app's just great! All I have to do is enter the topic in the search bar and I get the response real fast. I don't have to watch 10 YouTube videos to understand something, so I'm saving my time. Highly recommended!
Sudenaz Ocak
Android user
In school I was really bad at maths but thanks to the app, I am doing better now. I am so grateful that you made the app.
Greenlight Bonnie
Android user
very reliable app to help and grow your ideas of Maths, English and other related topics in your works. please use this app if your struggling in areas, this app is key for that. wish I'd of done a review before. and it's also free so don't worry about that.
Rohan U
Android user
I know a lot of apps use fake accounts to boost their reviews but this app deserves it all. Originally I was getting 4 in my English exams and this time I got a grade 7. I didn’t even know about this app three days until the exam and it has helped A LOT. Please actually trust me and use it as I’m sure you too will see developments.
Xander S
iOS user
THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE THE SCHOOLGPT. IT ALSO IS LITREALLY LIKE CHATGPT BUT SMARTER!! HELPED ME WITH MY MASCARA PROBLEMS TOO!! AS WELL AS MY REAL SUBJECTS ! DUHHH 😍😁😲🤑💗✨🎀😮
Elisha
iOS user
This apps acc the goat. I find revision so boring but this app makes it so easy to organize it all and then you can ask the freeeee ai to test yourself so good and you can easily upload your own stuff. highly recommend as someone taking mocks now
Paul T
iOS user
Sociology