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Chemistry
acid & bases, pH curves, & calculating pH of buffer solutions | AQA A-Level Physical Chemistry
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11 Dec 2025
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Understanding Acids, Bases, pH Curves, and Buffer Solutions | AQA A-Level Chemistry Notes
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Brønsted-Lowry Theory and pH Basics
Think of acids and bases as chemical matchmakers - they're all about proton transfer. A Brønsted-Lowry acid donates protons (like HCl or H₂SO₄), whilst a Brønsted-Lowry base accepts them (like NaOH or NH₃).
Water is the ultimate multitasker here. It can act as both an acid and a base depending on what it's reacting with. When water meets ammonia, it donates a proton (acting as an acid). When it meets HCl, it accepts a proton (acting as a base).
The pH scale measures hydrogen ion concentration using the formula pH = -log₁₀. For strong acids, the hydrogen ion concentration equals the acid concentration - so 1M HCl gives pH = 0.
Quick Tip: Remember that pH is just a shorthand way to express tiny hydrogen ion concentrations without writing loads of zeros!
Water's Ionic Product and Weak Acids
Water isn't completely innocent - it does ionise slightly on its own. The ionic product of water (Kw) describes this self-ionisation: Kw = = 1 × 10⁻¹⁴ at 298K.
Weak acids are the shy cousins of strong acids - they only partially dissociate in solution. This is where Ka (acid dissociation constant) comes in handy. A large Ka means the acid ionises loads (stronger), whilst a small Ka means it barely ionises (weaker).
The formula Ka = / helps you calculate concentrations. For convenience, we often use pKa = -log₁₀Ka. The smaller the pKa value, the stronger the weak acid.
Exam Focus: You'll often need to use the relationship pKa = pH at the half-equivalence point - this is a common exam question!
Titrations and pH Curves
Titrations let you find unknown concentrations by carefully adding one solution to another until you reach the equivalence point - where moles of acid equal moles of base.
Different combinations of strong/weak acids and bases give distinctly shaped pH curves. Strong acid + strong base gives a sharp, symmetric S-curve. Weak acid + strong base starts lower and has a less sharp transition.
Indicators change colour at specific pH ranges. Phenolphthalein works brilliantly for weak acid/strong base titrations. Methyl orange (red to yellow) suits strong acid/weak base titrations better.
The half-equivalence point occurs when exactly half the acid has reacted, and here's where pKa = pH - dead useful for calculations.
Lab Success: Concordant results (within 0.1 cm³ of each other) show you've mastered your titration technique!
Buffer Solutions
Buffer solutions are the pH police - they resist changes when you add acid or base. They're absolutely essential in biological systems and industrial processes.
Acid buffers contain a weak acid plus its salt (like methanoic acid and sodium methanoate). When you add acid, the conjugate base mops up the extra H⁺ ions. Add base, and the weak acid donates more H⁺ ions to neutralise it.
Basic buffers work similarly but maintain pH above 7, using a weak base and its salt (like ammonia and ammonium chloride).
For calculations, assume the salt fully dissociates and the weak acid stays mostly unionised. Then use: = Ka × /.
Real-World Connection: Your blood is buffered around pH 7.4 - without this, tiny changes would be fatal!
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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 😍😁😲🤑💗✨🎀😮
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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
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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
Understanding Acids, Bases, pH Curves, and Buffer Solutions | AQA A-Level Chemistry Notes
B
bv
@bv
Acids and bases are everywhere - from the citric acid in your orange juice to the ammonia in cleaning products. Understanding how they behave, especially their strength and how they interact with water, is crucial for A-level chemistry success.
Brønsted-Lowry Theory and pH Basics
Think of acids and bases as chemical matchmakers - they're all about proton transfer. A Brønsted-Lowry acid donates protons (like HCl or H₂SO₄), whilst a Brønsted-Lowry base accepts them (like NaOH or NH₃).
Water is the ultimate multitasker here. It can act as both an acid and a base depending on what it's reacting with. When water meets ammonia, it donates a proton (acting as an acid). When it meets HCl, it accepts a proton (acting as a base).
The pH scale measures hydrogen ion concentration using the formula pH = -log₁₀. For strong acids, the hydrogen ion concentration equals the acid concentration - so 1M HCl gives pH = 0.
Quick Tip: Remember that pH is just a shorthand way to express tiny hydrogen ion concentrations without writing loads of zeros!
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Water's Ionic Product and Weak Acids
Water isn't completely innocent - it does ionise slightly on its own. The ionic product of water (Kw) describes this self-ionisation: Kw = = 1 × 10⁻¹⁴ at 298K.
Weak acids are the shy cousins of strong acids - they only partially dissociate in solution. This is where Ka (acid dissociation constant) comes in handy. A large Ka means the acid ionises loads (stronger), whilst a small Ka means it barely ionises (weaker).
The formula Ka = / helps you calculate concentrations. For convenience, we often use pKa = -log₁₀Ka. The smaller the pKa value, the stronger the weak acid.
Exam Focus: You'll often need to use the relationship pKa = pH at the half-equivalence point - this is a common exam question!
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Titrations and pH Curves
Titrations let you find unknown concentrations by carefully adding one solution to another until you reach the equivalence point - where moles of acid equal moles of base.
Different combinations of strong/weak acids and bases give distinctly shaped pH curves. Strong acid + strong base gives a sharp, symmetric S-curve. Weak acid + strong base starts lower and has a less sharp transition.
Indicators change colour at specific pH ranges. Phenolphthalein works brilliantly for weak acid/strong base titrations. Methyl orange (red to yellow) suits strong acid/weak base titrations better.
The half-equivalence point occurs when exactly half the acid has reacted, and here's where pKa = pH - dead useful for calculations.
Lab Success: Concordant results (within 0.1 cm³ of each other) show you've mastered your titration technique!
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Buffer Solutions
Buffer solutions are the pH police - they resist changes when you add acid or base. They're absolutely essential in biological systems and industrial processes.
Acid buffers contain a weak acid plus its salt (like methanoic acid and sodium methanoate). When you add acid, the conjugate base mops up the extra H⁺ ions. Add base, and the weak acid donates more H⁺ ions to neutralise it.
Basic buffers work similarly but maintain pH above 7, using a weak base and its salt (like ammonia and ammonium chloride).
For calculations, assume the salt fully dissociates and the weak acid stays mostly unionised. Then use: = Ka × /.
Real-World Connection: Your blood is buffered around pH 7.4 - without this, tiny changes would be fatal!
We thought you’d never ask...
What is the Knowunity AI companion?
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Where can I download the Knowunity app?
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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