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Updated Mar 26, 2026
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GCSE Chemistry Grade 9: Comprehensive Acids and Alkalis Mindmap/Notes
potaeto
@username_3579
Investigating Neutralisation
Neutralisation occurs when acids and bases react to form salt and water. When you gradually add an alkali to an acid, the pH changes in a specific pattern - slowly at first, then rapidly, then slowly again.
A titration is a precise way to measure this neutralisation process. You use a burette (a tall glass tube with markings) to add alkali to acid and record the volume needed for neutralisation. This helps determine the exact amount needed to create a pure salt.
To make a pure dry salt, first perform a titration to find the exact volume of acid needed, then add that precise amount, and finally evaporate the water from the solution.
Top Tip: During a titration, the slower you add the reagent, the more accurately you can measure the pH at the neutralisation point!
Acid Reactions with Metals and Carbonates
When acids react with metals, they produce salt and hydrogen gas. You'll observe bubbles, the metal dissolving, and the solution warming up. The general equation is: Metal + Acid → Salt + Hydrogen
With carbonates, the reaction is: Carbonate + Acid → Salt + Water + Carbon Dioxide
These reactions can be written as ionic equations that show changes to ions. For example, when magnesium reacts with hydrochloric acid: Mg + 2HCl → MgCl₂ + H₂
Bases, Alkalis and Balancing Equations
A base neutralises acids to form salt and water. An alkali is simply a base that dissolves in water. Common alkalis include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)₂).
When balancing equations for these reactions, remember that the numbers of atoms must be equal on both sides. For example: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
Not all bases dissolve in water - insoluble bases like copper oxide can still neutralise acids, but they're not considered alkalis.
Acids, Alkalis and Indicators
The pH scale measures how acidic or alkaline a substance is. Acids have pH below 7, neutral substances have pH of 7, and alkalis have pH above 7. As the pH decreases by 1 unit, the concentration of H⁺ ions increases tenfold!
Different indicators change colour at different pH values:
- Litmus (red in acid, blue in alkali)
- Methyl orange (red in acid, orange in alkali)
- Phenolphthalein (colourless in acid, pink in alkali)
- Universal indicator (shows a range of colours from red to purple)
Acids produce excess hydrogen ions (H⁺) in water, while alkalis produce excess hydroxide ions (OH⁻).
Acids in Detail
The terms concentrated and dilute refer to the amount of acid dissolved in solution, while strong and weak refer to how completely the acid dissociates into ions.
Strong acids like hydrochloric (HCl), nitric (HNO₃), and sulfuric (H₂SO₄) dissociate completely in water, releasing many H⁺ ions. Weak acids like ethanoic acid (CH₃COOH) only partially dissociate.
Concentration is calculated as: amount dissolved (g) ÷ volume of solution (dm³)
Bases and Salts
When an acid reacts with a base, it forms a salt and water. The type of salt produced depends on the acid and metal used:
- Hydrochloric acid forms chlorides
- Sulfuric acid forms sulfates
- Nitric acid forms nitrates
To prepare a soluble salt, warm some acid and add small amounts of the metal oxide until no more dissolves. Then filter to remove excess oxide and evaporate the water to obtain crystals.
Solubility and Precipitation
Not all salts dissolve in water. Most nitrates and sodium compounds are soluble, while most carbonates and hydroxides are insoluble.
A precipitate is an insoluble solid formed when two solutions mix, creating a cloudy appearance. This reaction is useful for preparing insoluble salts through precipitation reactions.
Practical Example: Preparing Copper Sulfate
To prepare copper sulfate crystals:
- Warm dilute sulfuric acid
- Add excess copper oxide (the solution turns blue)
- Filter to remove unreacted copper oxide
- Evaporate some water until crystallisation begins
- Leave to cool and form blue diamond-shaped crystals
Remember: Adding excess base ensures all the acid is neutralised, which is crucial for obtaining pure salt crystals!
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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
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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
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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
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THE QUIZES AND FLASHCARDS ARE SO USEFUL AND I LOVE Knowunity AI. 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 Knowunity AI. 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 Grade 9: Comprehensive Acids and Alkalis Mindmap/Notes
potaeto
@username_3579
Acids and bases are fundamental in chemistry, affecting everything from your kitchen to industrial processes. This guide will help you understand neutralisation, pH, and the essential reactions between acids and bases - concepts that feature frequently in exams and practical... Show more
Investigating Neutralisation
Neutralisation occurs when acids and bases react to form salt and water. When you gradually add an alkali to an acid, the pH changes in a specific pattern - slowly at first, then rapidly, then slowly again.
A titration is a precise way to measure this neutralisation process. You use a burette (a tall glass tube with markings) to add alkali to acid and record the volume needed for neutralisation. This helps determine the exact amount needed to create a pure salt.
To make a pure dry salt, first perform a titration to find the exact volume of acid needed, then add that precise amount, and finally evaporate the water from the solution.
Top Tip: During a titration, the slower you add the reagent, the more accurately you can measure the pH at the neutralisation point!
Acid Reactions with Metals and Carbonates
When acids react with metals, they produce salt and hydrogen gas. You'll observe bubbles, the metal dissolving, and the solution warming up. The general equation is: Metal + Acid → Salt + Hydrogen
With carbonates, the reaction is: Carbonate + Acid → Salt + Water + Carbon Dioxide
These reactions can be written as ionic equations that show changes to ions. For example, when magnesium reacts with hydrochloric acid: Mg + 2HCl → MgCl₂ + H₂
Bases, Alkalis and Balancing Equations
A base neutralises acids to form salt and water. An alkali is simply a base that dissolves in water. Common alkalis include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)₂).
When balancing equations for these reactions, remember that the numbers of atoms must be equal on both sides. For example: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O
Not all bases dissolve in water - insoluble bases like copper oxide can still neutralise acids, but they're not considered alkalis.
Acids, Alkalis and Indicators
The pH scale measures how acidic or alkaline a substance is. Acids have pH below 7, neutral substances have pH of 7, and alkalis have pH above 7. As the pH decreases by 1 unit, the concentration of H⁺ ions increases tenfold!
Different indicators change colour at different pH values:
- Litmus (red in acid, blue in alkali)
- Methyl orange (red in acid, orange in alkali)
- Phenolphthalein (colourless in acid, pink in alkali)
- Universal indicator (shows a range of colours from red to purple)
Acids produce excess hydrogen ions (H⁺) in water, while alkalis produce excess hydroxide ions (OH⁻).
Acids in Detail
The terms concentrated and dilute refer to the amount of acid dissolved in solution, while strong and weak refer to how completely the acid dissociates into ions.
Strong acids like hydrochloric (HCl), nitric (HNO₃), and sulfuric (H₂SO₄) dissociate completely in water, releasing many H⁺ ions. Weak acids like ethanoic acid (CH₃COOH) only partially dissociate.
Concentration is calculated as: amount dissolved (g) ÷ volume of solution (dm³)
Bases and Salts
When an acid reacts with a base, it forms a salt and water. The type of salt produced depends on the acid and metal used:
- Hydrochloric acid forms chlorides
- Sulfuric acid forms sulfates
- Nitric acid forms nitrates
To prepare a soluble salt, warm some acid and add small amounts of the metal oxide until no more dissolves. Then filter to remove excess oxide and evaporate the water to obtain crystals.
Solubility and Precipitation
Not all salts dissolve in water. Most nitrates and sodium compounds are soluble, while most carbonates and hydroxides are insoluble.
A precipitate is an insoluble solid formed when two solutions mix, creating a cloudy appearance. This reaction is useful for preparing insoluble salts through precipitation reactions.
Practical Example: Preparing Copper Sulfate
To prepare copper sulfate crystals:
- Warm dilute sulfuric acid
- Add excess copper oxide (the solution turns blue)
- Filter to remove unreacted copper oxide
- Evaporate some water until crystallisation begins
- Leave to cool and form blue diamond-shaped crystals
Remember: Adding excess base ensures all the acid is neutralised, which is crucial for obtaining pure salt crystals!
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?
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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4.6/5
App Store
4.7/5
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 Knowunity AI. 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 Knowunity AI. 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