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GCSE Chemistry AQA: Acid Reactions Notes
Mason Haynes
@masonhaynes_nyun
1 / 5
Understanding Acids and Alkalis
Ever wondered why lemon juice tastes sour or why soap feels slippery? It's all about acids and alkalis! Acids are compounds that release H⁺ ions when dissolved in water, whilst alkalis release OH⁻ ions.
The three main acids you'll encounter are sulphuric acid (H₂SO₄), nitric acid (HNO₃), and hydrochloric acid (HCl). These pop up constantly in reactions, so memorise their formulas now.
The pH scale runs from 1 to 14 and tells you how acidic or alkaline something is. Solutions with pH less than 7 are acidic, pH 7 is neutral (like pure water), and pH greater than 7 is alkaline.
Quick Tip: The more H⁺ ions present, the lower the pH becomes - so stronger acids have lower pH values!
Indicators and Acid Strength
Indicators are your best friends for identifying acids and alkalis in practicals. Universal indicator gives you an approximate pH through colour changes, whilst electronic pH probes give exact measurements for precise work.
Understanding strong acids is essential for your exams. Sulphuric, nitric, and hydrochloric acids are all strong because they're completely ionised - every single molecule splits into ions when dissolved in water.
The chemical equations show this perfectly: HCl → H⁺ + Cl⁻ means every HCl molecule breaks apart completely. This complete ionisation makes these acids particularly reactive and dangerous.
Remember: Strong doesn't mean concentrated - these are different concepts that often confuse students!
Weak Acids and Concentration
Weak acids like ethanoic acid and citric acid are only partially ionised - just a small percentage of their molecules split into ions. This makes them less reactive than strong acids, even at the same concentration.
Don't confuse strength with concentration! Concentration tells you how much acid is dissolved in the water. You can have a dilute strong acid (few molecules, all ionised) or a concentrated weak acid (lots of molecules, few ionised).
When acids react with metals or metal compounds, they form salts. Think of it as the hydrogen in the acid getting replaced by a metal - like HNO₃ reacting with sodium to make NaNO₃.
Exam Tip: The stronger the acid, the lower the pH - this relationship appears frequently in exam questions!
Salt Formation and Acid Reactions
Each acid creates specific types of salts: hydrochloric acid makes metal chlorides, sulphuric acid produces metal sulfates, and nitric acid forms metal nitrates. Learning these patterns saves you loads of time in exams.
Acid reactions follow predictable patterns that you absolutely must memorise. When acids react with metals, you get salt plus hydrogen gas. With metal hydroxides or oxides, you get salt plus water.
The most important reaction is with metal carbonates, giving you salt, water, and carbon dioxide. This fizzing reaction is easy to spot in practicals and often appears in exam questions.
Key Pattern: All acid reactions produce salts, but the second product depends on what the acid reacts with!
Neutralisation and Crystallisation
Bases neutralise acids to form water in neutralisation reactions. Some metal hydroxides dissolve in water to form alkaline solutions (these are alkalis), whilst others don't dissolve but still neutralise acids.
The required practical for crystallisation helps you make solid salts from insoluble bases. You'll heat dilute acid, add base until it's in excess, filter off unreacted base, then evaporate the solution carefully.
Redox happens during metal-acid reactions - the metal loses electrons (gets oxidised) whilst hydrogen gains electrons (gets reduced). This electron transfer explains why hydrogen gas bubbles form.
Practical Tip: Use a water bath for evaporation rather than direct heating to avoid salt crystals spitting out!
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Anna
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Best app on earth! no words because it’s too good
Thomas R
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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
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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
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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
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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
GCSE Chemistry AQA: Acid Reactions Notes
Mason Haynes
@masonhaynes_nyun
Understanding acids and alkalis is crucial for GCSE Chemistry, as these substances are everywhere - from the acid in your stomach to the alkaline cleaning products under your sink. You'll need to master concepts like pH, indicators, and salt formation... Show more
Understanding Acids and Alkalis
Ever wondered why lemon juice tastes sour or why soap feels slippery? It's all about acids and alkalis! Acids are compounds that release H⁺ ions when dissolved in water, whilst alkalis release OH⁻ ions.
The three main acids you'll encounter are sulphuric acid (H₂SO₄), nitric acid (HNO₃), and hydrochloric acid (HCl). These pop up constantly in reactions, so memorise their formulas now.
The pH scale runs from 1 to 14 and tells you how acidic or alkaline something is. Solutions with pH less than 7 are acidic, pH 7 is neutral (like pure water), and pH greater than 7 is alkaline.
Quick Tip: The more H⁺ ions present, the lower the pH becomes - so stronger acids have lower pH values!
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Indicators and Acid Strength
Indicators are your best friends for identifying acids and alkalis in practicals. Universal indicator gives you an approximate pH through colour changes, whilst electronic pH probes give exact measurements for precise work.
Understanding strong acids is essential for your exams. Sulphuric, nitric, and hydrochloric acids are all strong because they're completely ionised - every single molecule splits into ions when dissolved in water.
The chemical equations show this perfectly: HCl → H⁺ + Cl⁻ means every HCl molecule breaks apart completely. This complete ionisation makes these acids particularly reactive and dangerous.
Remember: Strong doesn't mean concentrated - these are different concepts that often confuse students!
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Weak Acids and Concentration
Weak acids like ethanoic acid and citric acid are only partially ionised - just a small percentage of their molecules split into ions. This makes them less reactive than strong acids, even at the same concentration.
Don't confuse strength with concentration! Concentration tells you how much acid is dissolved in the water. You can have a dilute strong acid (few molecules, all ionised) or a concentrated weak acid (lots of molecules, few ionised).
When acids react with metals or metal compounds, they form salts. Think of it as the hydrogen in the acid getting replaced by a metal - like HNO₃ reacting with sodium to make NaNO₃.
Exam Tip: The stronger the acid, the lower the pH - this relationship appears frequently in exam questions!
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By signing up you accept Terms of Service and Privacy Policy
Salt Formation and Acid Reactions
Each acid creates specific types of salts: hydrochloric acid makes metal chlorides, sulphuric acid produces metal sulfates, and nitric acid forms metal nitrates. Learning these patterns saves you loads of time in exams.
Acid reactions follow predictable patterns that you absolutely must memorise. When acids react with metals, you get salt plus hydrogen gas. With metal hydroxides or oxides, you get salt plus water.
The most important reaction is with metal carbonates, giving you salt, water, and carbon dioxide. This fizzing reaction is easy to spot in practicals and often appears in exam questions.
Key Pattern: All acid reactions produce salts, but the second product depends on what the acid reacts with!
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Neutralisation and Crystallisation
Bases neutralise acids to form water in neutralisation reactions. Some metal hydroxides dissolve in water to form alkaline solutions (these are alkalis), whilst others don't dissolve but still neutralise acids.
The required practical for crystallisation helps you make solid salts from insoluble bases. You'll heat dilute acid, add base until it's in excess, filter off unreacted base, then evaporate the solution carefully.
Redox happens during metal-acid reactions - the metal loses electrons (gets oxidised) whilst hydrogen gains electrons (gets reduced). This electron transfer explains why hydrogen gas bubbles form.
Practical Tip: Use a water bath for evaporation rather than direct heating to avoid salt crystals spitting out!
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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4.9/5
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4.8/5
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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