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1 Dec 2025
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Combined Science Chemistry Revision: Key Chemical Changes
Charlotte
@charlotte26
1 / 5
Reactivity Series
Ever wonder why some metals rust whilst others stay shiny? The reactivity series ranks metals from most to least reactive, helping you predict what happens in chemical reactions.
The series includes hydrogen and carbon even though they're non-metals, because they behave similarly to metals in reactions. Remember this brilliant mnemonic: "Polite Spotted Leopard Can Meet Carbon Affectionate Zebra In Hydrogen Tender Loving Care Single Girls Please" (Potassium, Sodium, Lithium, Calcium, Magnesium, Carbon, Aluminium, Zinc, Iron, Hydrogen, Tin, Lead, Copper, Silver, Gold, Platinum).
Here's the key rule: metals more reactive than carbon need electrolysis for extraction, whilst those less reactive than hydrogen can be extracted by reduction. Gold and platinum are so unreactive they're found pure in nature - that's why they're called native metals.
Quick Tip: The more reactive a metal, the harder it fights to stay combined with other elements!
pH Scale and Neutralisation
The pH scale (1-14) measures how acidic or alkaline a solution is, and it's simpler than you think. Picture it as a sliding scale where 7 is neutral, below 7 is acidic, and above 7 is alkaline.
Acidic solutions produce H+ ions (hydrogen ions) in water, whilst alkaline solutions produce OH- ions (hydroxide ions). You can measure pH using universal indicators or a pH probe - both common in your practicals.
Neutralisation reactions happen when acids meet bases (alkalis), following this simple equation: H+ + OH- → H₂O. The result? Water with a neutral pH of 7, which explains why antacids work against stomach acid.
Remember: More H+ ions = more acidic, more OH- ions = more alkaline!
Electrolysis Basics
Electrolysis uses electrical energy to break down compounds called electrolytes - think of it as using electricity to force chemical reactions. The setup includes electrodes (anode and cathode) connected to a power supply.
Here's the crucial memory trick: "Positive Anode, Negative Is Cathode" (PANIC). Elements move to the oppositely charged electrode during the process.
In aqueous solutions (dissolved in water), you've got extra hydroxide and hydrogen ions floating around. At the cathode: if the metal is more reactive than hydrogen, you get hydrogen gas instead of the metal. At the anode: halides are produced if present, otherwise you get oxygen.
Key Point: Electrolysis only works with ionic compounds - the ions need to be free to move!
Oxidation, Reduction and Redox
Oxidation and reduction always happen together in what's called a redox reaction - master these and you'll ace your equations. Oxidation means gaining oxygen OR losing electrons, whilst reduction means losing oxygen OR gaining electrons.
Use this lifesaver: "Oxidation Is Loss, Reduction Is Gain" (OILRIG) - referring to electrons. When you see a more reactive metal displacing a less reactive one from a compound, that's a displacement reaction.
Half equations show exactly what happens at each electrode during electrolysis. At the cathode, positive ions gain electrons , whilst at the anode, negative ions lose electrons.
Exam Tip: If you can identify what's being oxidised and reduced, you're halfway to writing the correct equation!
Electrolysis of Aluminium
Aluminium extraction perfectly demonstrates industrial electrolysis in action. It starts with bauxite (aluminium ore), which gets purified into aluminium oxide - a white powder that's your actual starting material.
During electrolysis, aluminium ions head to the negative cathode where they gain electrons and become aluminium atoms (reduction). Meanwhile, oxide ions travel to the positive anode, lose electrons, and form oxygen gas (oxidation).
This process requires massive amounts of electricity, which explains why aluminium recycling is so important environmentally. The whole setup demonstrates redox reactions happening simultaneously at different electrodes.
Real-world Connection: This expensive process is why recycling aluminium cans saves about 95% of the energy compared to making new aluminium!
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Stefan S
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Samantha Klich
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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
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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
Combined Science Chemistry Revision: Key Chemical Changes
Charlotte
@charlotte26
Chemical changes are all about how substances react and transform, which is crucial for your GCSE chemistry exams. You'll master the reactivity series, pH scale, and electrolysis - three fundamental topics that show up repeatedly in your assessments.
Reactivity Series
Ever wonder why some metals rust whilst others stay shiny? The reactivity series ranks metals from most to least reactive, helping you predict what happens in chemical reactions.
The series includes hydrogen and carbon even though they're non-metals, because they behave similarly to metals in reactions. Remember this brilliant mnemonic: "Polite Spotted Leopard Can Meet Carbon Affectionate Zebra In Hydrogen Tender Loving Care Single Girls Please" (Potassium, Sodium, Lithium, Calcium, Magnesium, Carbon, Aluminium, Zinc, Iron, Hydrogen, Tin, Lead, Copper, Silver, Gold, Platinum).
Here's the key rule: metals more reactive than carbon need electrolysis for extraction, whilst those less reactive than hydrogen can be extracted by reduction. Gold and platinum are so unreactive they're found pure in nature - that's why they're called native metals.
Quick Tip: The more reactive a metal, the harder it fights to stay combined with other elements!
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pH Scale and Neutralisation
The pH scale (1-14) measures how acidic or alkaline a solution is, and it's simpler than you think. Picture it as a sliding scale where 7 is neutral, below 7 is acidic, and above 7 is alkaline.
Acidic solutions produce H+ ions (hydrogen ions) in water, whilst alkaline solutions produce OH- ions (hydroxide ions). You can measure pH using universal indicators or a pH probe - both common in your practicals.
Neutralisation reactions happen when acids meet bases (alkalis), following this simple equation: H+ + OH- → H₂O. The result? Water with a neutral pH of 7, which explains why antacids work against stomach acid.
Remember: More H+ ions = more acidic, more OH- ions = more alkaline!
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Electrolysis Basics
Electrolysis uses electrical energy to break down compounds called electrolytes - think of it as using electricity to force chemical reactions. The setup includes electrodes (anode and cathode) connected to a power supply.
Here's the crucial memory trick: "Positive Anode, Negative Is Cathode" (PANIC). Elements move to the oppositely charged electrode during the process.
In aqueous solutions (dissolved in water), you've got extra hydroxide and hydrogen ions floating around. At the cathode: if the metal is more reactive than hydrogen, you get hydrogen gas instead of the metal. At the anode: halides are produced if present, otherwise you get oxygen.
Key Point: Electrolysis only works with ionic compounds - the ions need to be free to move!
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Oxidation, Reduction and Redox
Oxidation and reduction always happen together in what's called a redox reaction - master these and you'll ace your equations. Oxidation means gaining oxygen OR losing electrons, whilst reduction means losing oxygen OR gaining electrons.
Use this lifesaver: "Oxidation Is Loss, Reduction Is Gain" (OILRIG) - referring to electrons. When you see a more reactive metal displacing a less reactive one from a compound, that's a displacement reaction.
Half equations show exactly what happens at each electrode during electrolysis. At the cathode, positive ions gain electrons , whilst at the anode, negative ions lose electrons.
Exam Tip: If you can identify what's being oxidised and reduced, you're halfway to writing the correct equation!
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Electrolysis of Aluminium
Aluminium extraction perfectly demonstrates industrial electrolysis in action. It starts with bauxite (aluminium ore), which gets purified into aluminium oxide - a white powder that's your actual starting material.
During electrolysis, aluminium ions head to the negative cathode where they gain electrons and become aluminium atoms (reduction). Meanwhile, oxide ions travel to the positive anode, lose electrons, and form oxygen gas (oxidation).
This process requires massive amounts of electricity, which explains why aluminium recycling is so important environmentally. The whole setup demonstrates redox reactions happening simultaneously at different electrodes.
Real-world Connection: This expensive process is why recycling aluminium cans saves about 95% of the energy compared to making new aluminium!
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?
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Most popular content: Reduction
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Students love us — and so will you.
4.9/5
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4.8/5
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