Subjects
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
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32
•
2 Dec 2025
•
holly
@holly123
Chemistry can seem overwhelming, but it's really about understanding how... Show more
Every element is made up of atoms containing protons , neutrons (no charge), and electrons . Isotopes are atoms of the same element with different numbers of neutrons - they're like identical twins with slightly different weights.
The periodic table is your roadmap to understanding elements. Groups (vertical columns) contain elements with similar properties, whilst periods (horizontal rows) show how properties change across elements. Group 1 metals (alkali metals) are soft, reactive metals that get more explosive as you go down the group - they react with water to produce hydrogen gas and alkalis.
Chemical bonding happens in three main ways. Ionic bonding occurs between metals and non-metals where electrons transfer completely, creating charged ions that attract each other strongly. Covalent bonding happens between non-metals that share electrons to form molecules like water. Metallic bonding involves a "sea" of free electrons that can move around, which is why metals conduct electricity and heat so well.
Quick tip: Remember that Group 7 (halogens) are coloured, toxic non-metals that get less reactive going down, whilst Group 0 (noble gases) are completely unreactive because they have full outer electron shells.
Chemical formulae tell you exactly what's in a compound - H₂O means two hydrogen atoms and one oxygen atom. State symbols show whether something is solid (s), liquid (l), gas (g), or dissolved in water (aq). These details matter because they affect how substances behave in reactions.
Conservation of mass is a fundamental rule - atoms can't just disappear during reactions, so the mass before equals the mass after. You'll use relative atomic mass (Ar) and relative formula mass (Mr) to calculate this. Moles are just a way of counting atoms - one mole contains 6.02 × 10²³ particles, and you calculate moles using: moles = mass ÷ Mr.
Acids produce H⁺ ions and turn litmus red, whilst bases neutralise acids and alkalis (soluble bases) produce OH⁻ ions, turning litmus blue. The key reaction is: acid + base → salt + water. Limiting reactants determine how much product you can make - like running out of bread when making sandwiches.
Exam focus: Master separation techniques - filtration for insoluble solids, crystallisation for soluble solids, distillation for liquids, and chromatography for coloured mixtures.
The reactivity series ranks metals by how easily they form positive ions - think of it as a league table of chemical aggression. More reactive metals displace less reactive ones from compounds, like magnesium kicking copper out of copper sulfate. This is called a displacement reaction.
Metal extraction depends on reactivity. Less reactive metals like iron can be extracted using carbon reduction - carbon steals oxygen from metal oxides. More reactive metals need electrolysis, where electricity breaks down compounds into elements. During electrolysis, the anode (positive electrode) attracts negative ions, whilst the cathode (negative electrode) attracts positive ions.
Strong acids like HCl completely split into ions in water, whilst weak acids only partially ionise in a reversible reaction. This affects their pH and reaction rates. Oxidation means losing electrons or gaining oxygen, whilst reduction is the opposite - gaining electrons or losing oxygen.
Memory trick: For electrolysis, remember "PANIC" - Positive Anode, Negative Is Cathode. Reduction happens at the cathode, oxidation at the anode.
Electrolysis splits ionic compounds when they're molten or dissolved. In lead bromide electrolysis, lead ions (Pb²⁺) gain electrons at the cathode to form lead metal, whilst bromide ions (Br⁻) lose electrons at the anode to form bromine gas. With aqueous solutions, water complicates things - hydrogen gas forms at the cathode if the metal is more reactive than hydrogen.
Energy changes in reactions follow simple rules. Exothermic reactions release energy to surroundings (like combustion and neutralisation), making things feel warmer. Endothermic reactions absorb energy, making surroundings cooler - photosynthesis is a classic example.
Bond energy calculations help predict energy changes. Breaking bonds requires energy input, whilst forming bonds releases energy. If more energy is needed to break bonds than is released forming new ones, the reaction is endothermic. If more energy is released than needed, it's exothermic.
Calculation shortcut: Energy change = Energy needed to break bonds - Energy released forming bonds. Negative values mean exothermic, positive values mean endothermic.
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.
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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
holly
@holly123
Chemistry can seem overwhelming, but it's really about understanding how atoms behave and interact with each other. This guide covers the essential concepts you'll need for Paper 1, from atomic structure and bonding to chemical reactions and energy changes.
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Every element is made up of atoms containing protons , neutrons (no charge), and electrons . Isotopes are atoms of the same element with different numbers of neutrons - they're like identical twins with slightly different weights.
The periodic table is your roadmap to understanding elements. Groups (vertical columns) contain elements with similar properties, whilst periods (horizontal rows) show how properties change across elements. Group 1 metals (alkali metals) are soft, reactive metals that get more explosive as you go down the group - they react with water to produce hydrogen gas and alkalis.
Chemical bonding happens in three main ways. Ionic bonding occurs between metals and non-metals where electrons transfer completely, creating charged ions that attract each other strongly. Covalent bonding happens between non-metals that share electrons to form molecules like water. Metallic bonding involves a "sea" of free electrons that can move around, which is why metals conduct electricity and heat so well.
Quick tip: Remember that Group 7 (halogens) are coloured, toxic non-metals that get less reactive going down, whilst Group 0 (noble gases) are completely unreactive because they have full outer electron shells.
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Chemical formulae tell you exactly what's in a compound - H₂O means two hydrogen atoms and one oxygen atom. State symbols show whether something is solid (s), liquid (l), gas (g), or dissolved in water (aq). These details matter because they affect how substances behave in reactions.
Conservation of mass is a fundamental rule - atoms can't just disappear during reactions, so the mass before equals the mass after. You'll use relative atomic mass (Ar) and relative formula mass (Mr) to calculate this. Moles are just a way of counting atoms - one mole contains 6.02 × 10²³ particles, and you calculate moles using: moles = mass ÷ Mr.
Acids produce H⁺ ions and turn litmus red, whilst bases neutralise acids and alkalis (soluble bases) produce OH⁻ ions, turning litmus blue. The key reaction is: acid + base → salt + water. Limiting reactants determine how much product you can make - like running out of bread when making sandwiches.
Exam focus: Master separation techniques - filtration for insoluble solids, crystallisation for soluble solids, distillation for liquids, and chromatography for coloured mixtures.
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
The reactivity series ranks metals by how easily they form positive ions - think of it as a league table of chemical aggression. More reactive metals displace less reactive ones from compounds, like magnesium kicking copper out of copper sulfate. This is called a displacement reaction.
Metal extraction depends on reactivity. Less reactive metals like iron can be extracted using carbon reduction - carbon steals oxygen from metal oxides. More reactive metals need electrolysis, where electricity breaks down compounds into elements. During electrolysis, the anode (positive electrode) attracts negative ions, whilst the cathode (negative electrode) attracts positive ions.
Strong acids like HCl completely split into ions in water, whilst weak acids only partially ionise in a reversible reaction. This affects their pH and reaction rates. Oxidation means losing electrons or gaining oxygen, whilst reduction is the opposite - gaining electrons or losing oxygen.
Memory trick: For electrolysis, remember "PANIC" - Positive Anode, Negative Is Cathode. Reduction happens at the cathode, oxidation at the anode.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Electrolysis splits ionic compounds when they're molten or dissolved. In lead bromide electrolysis, lead ions (Pb²⁺) gain electrons at the cathode to form lead metal, whilst bromide ions (Br⁻) lose electrons at the anode to form bromine gas. With aqueous solutions, water complicates things - hydrogen gas forms at the cathode if the metal is more reactive than hydrogen.
Energy changes in reactions follow simple rules. Exothermic reactions release energy to surroundings (like combustion and neutralisation), making things feel warmer. Endothermic reactions absorb energy, making surroundings cooler - photosynthesis is a classic example.
Bond energy calculations help predict energy changes. Breaking bonds requires energy input, whilst forming bonds releases energy. If more energy is needed to break bonds than is released forming new ones, the reaction is endothermic. If more energy is released than needed, it's exothermic.
Calculation shortcut: Energy change = Energy needed to break bonds - Energy released forming bonds. Negative values mean exothermic, positive values mean endothermic.
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.
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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
Sociology