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
Show all topics
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
Show all topics
270
•
11 Dec 2025
•
Understanding acids might seem intimidating, but it's actually quite straightforward... Show more
The main difference between strong and weak acids is how completely they break apart in water. Think of it like sugar dissolving - some substances dissolve completely whilst others only partially dissolve.
Strong acids completely dissociate (split up) in water. For example, hydrochloric acid (HCl) breaks apart entirely: HCl(aq) → H⁺(aq) + Cl⁻(aq). The forward reaction goes to completion - there's no going back.
Weak acids only partially dissociate in water. Ethanoic acid (CH₃COOH) is a perfect example: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq). Notice the double arrow - this means the reaction is reversible and doesn't go to completion.
Quick Check: Strong acids = complete dissociation, weak acids = partial dissociation. It's that simple!
The pH scale is basically a shortcut for measuring how many H⁺ ions are floating around in your solution. It's much easier than dealing with tiny decimal numbers all the time.
The mathematical expression is: pH = -log. Don't panic about the maths - it's just a way to convert very small numbers into manageable ones. Always give your pH to 2 decimal places, and remember it has no units.
Here's the key relationship to remember: smaller pH = more H⁺ ions = more acidic. A pH change of just one unit means the H⁺ concentration has changed by 10 times - that's a massive difference!
You can also work backwards using: = 10⁻ᵖᴴ. This is handy when you know the pH but need the concentration.
Remember: pH 2 is 10 times more acidic than pH 3, and 100 times more acidic than pH 4!
Calculating the pH of strong acids is straightforward because they completely dissociate. For monobasic acids (acids that donate one H⁺ ion), the concentration of H⁺ ions equals the concentration of the acid.
Let's work through an example: Calculate the pH of 1.56 × 10⁻² mol dm⁻³ HCl. Since HCl completely dissociates, = = 1.56 × 10⁻². Therefore, pH = -log(1.56 × 10⁻²) = 1.81.
Working backwards is just as easy. If HCl has a pH of 3.79, then = 10⁻³·⁷⁹ = 1.62 × 10⁻⁴ mol dm⁻³. Since it's a strong monobasic acid, this is also the concentration of the original acid.
Top Tip: For strong monobasic acids, always equals - this makes calculations much simpler!
When you dilute acids, the pH changes because you're reducing the concentration of H⁺ ions. The key formula is: new concentration = original concentration × (volume of reagent ÷ total volume).
Let's see this in action: 10 cm³ of 0.250 mol dm⁻³ HCl is diluted to 200 cm³ with water. Before dilution, pH = -log(0.25) = 0.60.
After dilution, the new concentration becomes: 0.250 × (10/200) = 0.0125 mol dm⁻³. The new pH = -log(0.0125) = 1.90.
The pH change is 1.90 - 0.60 = 1.3. Notice how diluting the acid made it less acidic (higher pH), which makes perfect sense!
Key Point: Dilution always increases pH (makes solutions less acidic) - you're adding water, not more acid!
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.
App Store
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
Understanding acids might seem intimidating, but it's actually quite straightforward once you grasp the basics. This topic covers how acids behave differently in water and introduces the pH scale - a simple way to measure how acidic a solution is.
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
The main difference between strong and weak acids is how completely they break apart in water. Think of it like sugar dissolving - some substances dissolve completely whilst others only partially dissolve.
Strong acids completely dissociate (split up) in water. For example, hydrochloric acid (HCl) breaks apart entirely: HCl(aq) → H⁺(aq) + Cl⁻(aq). The forward reaction goes to completion - there's no going back.
Weak acids only partially dissociate in water. Ethanoic acid (CH₃COOH) is a perfect example: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq). Notice the double arrow - this means the reaction is reversible and doesn't go to completion.
Quick Check: Strong acids = complete dissociation, weak acids = partial dissociation. It's that simple!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
The pH scale is basically a shortcut for measuring how many H⁺ ions are floating around in your solution. It's much easier than dealing with tiny decimal numbers all the time.
The mathematical expression is: pH = -log. Don't panic about the maths - it's just a way to convert very small numbers into manageable ones. Always give your pH to 2 decimal places, and remember it has no units.
Here's the key relationship to remember: smaller pH = more H⁺ ions = more acidic. A pH change of just one unit means the H⁺ concentration has changed by 10 times - that's a massive difference!
You can also work backwards using: = 10⁻ᵖᴴ. This is handy when you know the pH but need the concentration.
Remember: pH 2 is 10 times more acidic than pH 3, and 100 times more acidic than pH 4!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Calculating the pH of strong acids is straightforward because they completely dissociate. For monobasic acids (acids that donate one H⁺ ion), the concentration of H⁺ ions equals the concentration of the acid.
Let's work through an example: Calculate the pH of 1.56 × 10⁻² mol dm⁻³ HCl. Since HCl completely dissociates, = = 1.56 × 10⁻². Therefore, pH = -log(1.56 × 10⁻²) = 1.81.
Working backwards is just as easy. If HCl has a pH of 3.79, then = 10⁻³·⁷⁹ = 1.62 × 10⁻⁴ mol dm⁻³. Since it's a strong monobasic acid, this is also the concentration of the original acid.
Top Tip: For strong monobasic acids, always equals - this makes calculations much simpler!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
When you dilute acids, the pH changes because you're reducing the concentration of H⁺ ions. The key formula is: new concentration = original concentration × (volume of reagent ÷ total volume).
Let's see this in action: 10 cm³ of 0.250 mol dm⁻³ HCl is diluted to 200 cm³ with water. Before dilution, pH = -log(0.25) = 0.60.
After dilution, the new concentration becomes: 0.250 × (10/200) = 0.0125 mol dm⁻³. The new pH = -log(0.0125) = 1.90.
The pH change is 1.90 - 0.60 = 1.3. Notice how diluting the acid made it less acidic (higher pH), which makes perfect sense!
Key Point: Dilution always increases pH (makes solutions less acidic) - you're adding water, not more acid!
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.
6
Smart Tools NEW
Transform this note into: ✓ 50+ Practice Questions ✓ Interactive Flashcards ✓ Full Mock Exam ✓ Essay Outlines
Explore the fundamentals of neutralization reactions between acids and bases, including pH scale, indicators, and the formation of salts and water. This summary covers key concepts essential for chemistry exams and revision.
ChemistryExplore the essential techniques of acid-base titrations, including key definitions, step-by-step methods, and calculations for determining unknown concentrations and molar masses. This summary also covers measurement uncertainties and how to effectively record results. Ideal for chemistry students preparing for practical assessments.
ChemistryExplore a comprehensive set of exam questions focused on weak acids and bases, including topics such as acid-base titrations, buffer solutions, and the acid dissociation constant (Ka). This resource is ideal for students preparing for chemistry exams, providing practice on key concepts like pH calculations, neutralization reactions, and the behavior of carboxylic acids.
ChemistryExplore the fundamentals of acid-base chemistry, including conjugate acid-base pairs, the acid dissociation constant (Ka), and the relationship between pKa and pH. This summary covers key calculations for weak acids and strong bases, essential for A-Level Chemistry students. Understand how to determine pH and the significance of Kw in aqueous solutions.
ChemistryExplore the fundamentals of Brønsted-Lowry acid-base theory, pH calculations, and buffer solutions. This comprehensive guide covers key concepts such as acid-base equilibria, the ionic product of water (Kw), dissociation constants (Ka, pKa), and practical applications in acid-base titrations. Ideal for AQA A-Level Physical Chemistry students.
ChemistryExplore key concepts in acids, bases, and buffers through detailed practice questions and model answers. This resource covers acid-base titrations, pKa values, conjugate acid-base pairs, and the Henderson-Hasselbalch equation, providing essential calculations and explanations for mastering the topic.
ChemistryApp Store
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
Sociology