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3 Dec 2025
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Armani Johnson
@armaniohnson
Understanding acids and chemical reactions is crucial for GCSE chemistry... Show more
All acids ionise when dissolved in water, but here's the key point: acid strength is completely different from acid concentration. Don't get these mixed up in your exams!
Strong acids fully ionise in water, meaning every acid molecule breaks apart to release H+ ions. Weak acids only partially ionise - just some of their molecules split up.
Remember: You can have a concentrated weak acid or a dilute strong acid - strength and concentration are totally separate concepts!
Most acids you encounter are actually weak acids, like ethanoic acid in vinegar (CH₃COOH).
Weak acids are everywhere in chemistry, and most acids fall into this category. They only partially ionise, so not all molecules release H+ ions when dissolved.
Take ethanoic acid - the main ingredient in vinegar. Its chemical formula is CH₃COOH, and when it ionises: CH₃COOH ⇌ CH₃COO⁻ + H+.
Quick Tip: Notice the reversible arrow (⇌) - this shows that only some molecules ionise whilst others remain whole!
This partial ionisation is why weak acids are generally safer to handle than strong acids.
Electrolysis uses electricity to break down ionic compounds - but only when they're dissolved in water or molten. You need a direct current (DC) power source to make it work.
At the positive electrode (anode), you'll get chlorine, bromine, or iodine if they're present - otherwise oxygen from water. At the negative electrode (cathode), you get metals that are less reactive than hydrogen (like copper) - otherwise hydrogen gas.
Memory Aid: Think "PANIC" - Positive Anode, Negative Is Cathode!
Key reactions include: 2Cl⁻ → Cl₂ + 2e⁻ and Cu²⁺ + 2e⁻ → Cu.
Making copper sulfate crystals is a classic practical you'll likely do. Mix copper oxide with sulfuric acid, then heat until half the water evaporates.
The pH scale is essential knowledge: acids are pH 1-6, neutral is pH 7, and alkalis are pH 8-14. Remember this for your exams!
Practical Tip: Always leave your solution to crystallise slowly - rushing this step gives you smaller, less pure crystals!
The key is controlling evaporation carefully to get those perfect blue crystals.
The reactivity series runs from potassium (most reactive) down to gold (least reactive). You absolutely must memorise this order: K, Na, Li, Ca, Mg, C, Zn, Fe, H, Cu, Au.
Making salts involves neutralisation reactions. The main patterns are:
Exam Success: These reaction patterns come up constantly - learn them inside out!
Strong acids fully ionise, which is why they're called "strong" - it's all about complete ionisation.
Displacement reactions happen when more reactive metals push out less reactive ones. Metals react with water to produce metal hydroxides and hydrogen gas (watch for fizzing and floating).
When metals react with chlorine, you get metal chlorides - look for grey metals burning with green-yellow flames to form white solids. Ions are simply charged atoms.
Key Insight: In alloys, distorted layers can't slide past each other - that's why alloys are stronger than pure metals!
Group 7 elements form diatomic molecules like Cl₂. In graphite, each carbon has delocalised electrons allowing conductivity.
Here's a classic reaction you'll see everywhere: hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide. This follows the standard pattern perfectly.
The general rule is simple: acid + carbonate → salt + water + carbon dioxide. Every single time, without fail.
Exam Pointer: This reaction type appears in multiple choice questions constantly - nail the pattern now!
Watch for the fizzing when CO₂ gas is produced - it's a dead giveaway that carbonates are involved.
Neutralisation reactions are beautifully simple. Take hydrochloric acid + sodium hydroxide → sodium chloride + water. Clean and straightforward!
The pattern is always: acid + metal hydroxide → salt + water. No gases produced, just salt and water every time.
Remember: Metal hydroxides are bases - they neutralise acids completely in these reactions!
This is probably the most important reaction type in acid chemistry.
Metals are strong, dense, and excellent conductors with high melting points. Non-metals are often brittle insulators with lower density - completely opposite properties.
Ionic bonding happens when electrons transfer completely. Magnesium gives electrons to oxygen: Mg → Mg²⁺ and O → O²⁻, forming giant ionic lattices.
Key Concept: Electrostatic forces between oppositely charged ions act in all directions, creating high melting and boiling points!
These strong bonds throughout the lattice structure explain why ionic compounds are so stable.
Testing for hydrogen: use a lighted splint - hydrogen makes a distinctive squeaky pop sound. Testing for carbon dioxide: bubble it through limewater, which turns cloudy white.
The key acid reactions with metals and carbonates follow predictable patterns you've seen before. Sodium carbonate + hydrochloric acid gives sodium chloride, CO₂, and water.
Practical Success: Master these gas tests - they're guaranteed to appear in your practicals and exams!
These simple tests can identify gases instantly and are essential practical skills.
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.
Quotes from every main character
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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
Armani Johnson
@armaniohnson
Understanding acids and chemical reactions is crucial for GCSE chemistry success. These concepts cover everything from acid strength and electrolysis to making salts and the reactivity series - all essential knowledge you'll need for your exams.
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All acids ionise when dissolved in water, but here's the key point: acid strength is completely different from acid concentration. Don't get these mixed up in your exams!
Strong acids fully ionise in water, meaning every acid molecule breaks apart to release H+ ions. Weak acids only partially ionise - just some of their molecules split up.
Remember: You can have a concentrated weak acid or a dilute strong acid - strength and concentration are totally separate concepts!
Most acids you encounter are actually weak acids, like ethanoic acid in vinegar (CH₃COOH).
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Weak acids are everywhere in chemistry, and most acids fall into this category. They only partially ionise, so not all molecules release H+ ions when dissolved.
Take ethanoic acid - the main ingredient in vinegar. Its chemical formula is CH₃COOH, and when it ionises: CH₃COOH ⇌ CH₃COO⁻ + H+.
Quick Tip: Notice the reversible arrow (⇌) - this shows that only some molecules ionise whilst others remain whole!
This partial ionisation is why weak acids are generally safer to handle than strong acids.
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Electrolysis uses electricity to break down ionic compounds - but only when they're dissolved in water or molten. You need a direct current (DC) power source to make it work.
At the positive electrode (anode), you'll get chlorine, bromine, or iodine if they're present - otherwise oxygen from water. At the negative electrode (cathode), you get metals that are less reactive than hydrogen (like copper) - otherwise hydrogen gas.
Memory Aid: Think "PANIC" - Positive Anode, Negative Is Cathode!
Key reactions include: 2Cl⁻ → Cl₂ + 2e⁻ and Cu²⁺ + 2e⁻ → Cu.
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Improve your grades
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Making copper sulfate crystals is a classic practical you'll likely do. Mix copper oxide with sulfuric acid, then heat until half the water evaporates.
The pH scale is essential knowledge: acids are pH 1-6, neutral is pH 7, and alkalis are pH 8-14. Remember this for your exams!
Practical Tip: Always leave your solution to crystallise slowly - rushing this step gives you smaller, less pure crystals!
The key is controlling evaporation carefully to get those perfect blue crystals.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
The reactivity series runs from potassium (most reactive) down to gold (least reactive). You absolutely must memorise this order: K, Na, Li, Ca, Mg, C, Zn, Fe, H, Cu, Au.
Making salts involves neutralisation reactions. The main patterns are:
Exam Success: These reaction patterns come up constantly - learn them inside out!
Strong acids fully ionise, which is why they're called "strong" - it's all about complete ionisation.
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Displacement reactions happen when more reactive metals push out less reactive ones. Metals react with water to produce metal hydroxides and hydrogen gas (watch for fizzing and floating).
When metals react with chlorine, you get metal chlorides - look for grey metals burning with green-yellow flames to form white solids. Ions are simply charged atoms.
Key Insight: In alloys, distorted layers can't slide past each other - that's why alloys are stronger than pure metals!
Group 7 elements form diatomic molecules like Cl₂. In graphite, each carbon has delocalised electrons allowing conductivity.
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Here's a classic reaction you'll see everywhere: hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide. This follows the standard pattern perfectly.
The general rule is simple: acid + carbonate → salt + water + carbon dioxide. Every single time, without fail.
Exam Pointer: This reaction type appears in multiple choice questions constantly - nail the pattern now!
Watch for the fizzing when CO₂ gas is produced - it's a dead giveaway that carbonates are involved.
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Neutralisation reactions are beautifully simple. Take hydrochloric acid + sodium hydroxide → sodium chloride + water. Clean and straightforward!
The pattern is always: acid + metal hydroxide → salt + water. No gases produced, just salt and water every time.
Remember: Metal hydroxides are bases - they neutralise acids completely in these reactions!
This is probably the most important reaction type in acid chemistry.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Metals are strong, dense, and excellent conductors with high melting points. Non-metals are often brittle insulators with lower density - completely opposite properties.
Ionic bonding happens when electrons transfer completely. Magnesium gives electrons to oxygen: Mg → Mg²⁺ and O → O²⁻, forming giant ionic lattices.
Key Concept: Electrostatic forces between oppositely charged ions act in all directions, creating high melting and boiling points!
These strong bonds throughout the lattice structure explain why ionic compounds are so stable.
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Testing for hydrogen: use a lighted splint - hydrogen makes a distinctive squeaky pop sound. Testing for carbon dioxide: bubble it through limewater, which turns cloudy white.
The key acid reactions with metals and carbonates follow predictable patterns you've seen before. Sodium carbonate + hydrochloric acid gives sodium chloride, CO₂, and water.
Practical Success: Master these gas tests - they're guaranteed to appear in your practicals and exams!
These simple tests can identify gases instantly and are essential practical skills.
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