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Megan
11/12/2025
Chemistry
WJEC AS-level chemistry Unit 1 topic 1 formulae and equations
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11 Dec 2025
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Master Chemistry Unit 1: Formulae and Equations – WJEC AS
Megan
@megan_0306
1 / 7
Common Compounds and Their Formulae
You'll need to memorise these essential chemical formulae - they appear in nearly every chemistry exam! Water (H₂O), oxygen (O₂), and carbon dioxide (CO₂) are the absolute basics you must know.
The acids are particularly important: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃). These three pop up constantly in reactions and calculations.
Don't forget the common bases and compounds like sodium hydroxide (NaOH), sodium carbonate (Na₂CO₃), and ammonia (NH₃). Getting these right from the start will make everything else much easier.
Quick Tip: Make flashcards for these formulae - you'll thank yourself later when they become second nature!
Understanding Ions
Positive ions (cations) form when atoms lose electrons, whilst negative ions (anions) form when atoms gain electrons. It's that simple! Metals typically become cations, and non-metals become anions.
There's a brilliant shortcut for working out ion charges using the periodic table. Group 1 elements have a +1 charge, Group 2 have +2, Group 6 have -2, and Group 7 have -1 charge.
For example, sodium (Na) from Group 1 becomes Na⁺, whilst chlorine (Cl) from Group 7 becomes Cl⁻. This pattern works because atoms want to achieve a stable electron arrangement.
Memory Trick: Remember that metals "give away" electrons (become positive), whilst non-metals "take" electrons (become negative).
Types of Ions and Conservation of Mass
Simple ions contain just one element (like H⁺ or O²⁻), whilst complex ions contain multiple elements bonded together (like NH₄⁺ or SO₄²⁻). Complex ions act as single units in reactions.
The conservation of mass principle is crucial - the total mass of reactants always equals the total mass of products. In the reaction CH₄ + 2O₂ → CO₂ + 2H₂O, both sides add up to 80g.
State symbols show the physical state of each substance: (s) for solid, (l) for liquid, (g) for gas, and (aq) for dissolved in water. Only include these if the question specifically asks for them.
Exam Tip: Always check your balanced equations by counting atoms on both sides - they must be equal!
Ionic Equations and Reactions
Ionic equations show only the ions that actually react, ignoring the "spectator ions" that don't change. Both sides must have the same overall charge - this is your key check.
In the reaction NaI(aq) + AgNO₃(aq) → AgI(s) + NaNO₃(aq), the ionic equation becomes: I⁻(aq) + Ag⁺(aq) → AgI(s). The Na⁺ and NO₃⁻ are spectators.
Spectator ions remain unchanged throughout the reaction and can be cancelled out from both sides. They're like bystanders watching the real action happen.
Pro Tip: Look for the precipitate (solid formed) - this usually shows you which ions are actually reacting!
Redox Reactions Basics
Redox reactions involve the transfer of electrons between substances. One substance gets oxidised (loses electrons) whilst another gets reduced (gains electrons) - they always happen together.
Oxidation means losing electrons, gaining oxygen, or losing hydrogen. Reduction means gaining electrons, losing oxygen, or gaining hydrogen. The classic memory aid is OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
An oxidising agent causes oxidation in other substances by accepting their electrons, but gets reduced itself. Think of it as the electron "thief" in the reaction.
Memory Aid: Remember OIL RIG - it's the most reliable way to identify what's happening in redox reactions!
Oxidation Numbers and Common Ions
A reducing agent donates electrons to other substances, causing them to be reduced, but gets oxidised itself. It's the electron "donor" in reactions.
Oxidation numbers follow simple rules: elements alone are zero, simple ions equal their charge, and all oxidation numbers in a compound must add up to zero (or the overall charge for complex ions).
Key ions you must memorise include: H⁺ (hydrogen), Ag⁺ (silver), Zn²⁺ (zinc), OH⁻ (hydroxide), NO₃⁻ (nitrate), and SO₄²⁻ (sulfate). These appear constantly in equations.
Quick Check: In H₂O, hydrogen is +1 and oxygen is -2, giving 2(+1) + (-2) = 0. Perfect!
Essential Ion Table
The ammonium ion (NH₄⁺) and carbonate ion (CO₃²⁻) complete your essential ion collection. These complex ions behave as single units in all reactions.
Master these common ions and you'll find writing chemical equations becomes much more straightforward. They're the building blocks for countless reactions you'll encounter.
Final Tip: Practice writing formulae for compounds using these ions - it's the best way to make them stick in your memory!
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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
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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
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
Master Chemistry Unit 1: Formulae and Equations – WJEC AS
Megan
@megan_0306
Chemical formulae and equations are the language of chemistry - they tell us exactly what substances are present and how they react with each other. Understanding ions, balanced equations, and redox reactions will help you tackle any chemistry problem with... Show more
Common Compounds and Their Formulae
You'll need to memorise these essential chemical formulae - they appear in nearly every chemistry exam! Water (H₂O), oxygen (O₂), and carbon dioxide (CO₂) are the absolute basics you must know.
The acids are particularly important: hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃). These three pop up constantly in reactions and calculations.
Don't forget the common bases and compounds like sodium hydroxide (NaOH), sodium carbonate (Na₂CO₃), and ammonia (NH₃). Getting these right from the start will make everything else much easier.
Quick Tip: Make flashcards for these formulae - you'll thank yourself later when they become second nature!
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Understanding Ions
Positive ions (cations) form when atoms lose electrons, whilst negative ions (anions) form when atoms gain electrons. It's that simple! Metals typically become cations, and non-metals become anions.
There's a brilliant shortcut for working out ion charges using the periodic table. Group 1 elements have a +1 charge, Group 2 have +2, Group 6 have -2, and Group 7 have -1 charge.
For example, sodium (Na) from Group 1 becomes Na⁺, whilst chlorine (Cl) from Group 7 becomes Cl⁻. This pattern works because atoms want to achieve a stable electron arrangement.
Memory Trick: Remember that metals "give away" electrons (become positive), whilst non-metals "take" electrons (become negative).
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Types of Ions and Conservation of Mass
Simple ions contain just one element (like H⁺ or O²⁻), whilst complex ions contain multiple elements bonded together (like NH₄⁺ or SO₄²⁻). Complex ions act as single units in reactions.
The conservation of mass principle is crucial - the total mass of reactants always equals the total mass of products. In the reaction CH₄ + 2O₂ → CO₂ + 2H₂O, both sides add up to 80g.
State symbols show the physical state of each substance: (s) for solid, (l) for liquid, (g) for gas, and (aq) for dissolved in water. Only include these if the question specifically asks for them.
Exam Tip: Always check your balanced equations by counting atoms on both sides - they must be equal!
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Ionic Equations and Reactions
Ionic equations show only the ions that actually react, ignoring the "spectator ions" that don't change. Both sides must have the same overall charge - this is your key check.
In the reaction NaI(aq) + AgNO₃(aq) → AgI(s) + NaNO₃(aq), the ionic equation becomes: I⁻(aq) + Ag⁺(aq) → AgI(s). The Na⁺ and NO₃⁻ are spectators.
Spectator ions remain unchanged throughout the reaction and can be cancelled out from both sides. They're like bystanders watching the real action happen.
Pro Tip: Look for the precipitate (solid formed) - this usually shows you which ions are actually reacting!
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By signing up you accept Terms of Service and Privacy Policy
Redox Reactions Basics
Redox reactions involve the transfer of electrons between substances. One substance gets oxidised (loses electrons) whilst another gets reduced (gains electrons) - they always happen together.
Oxidation means losing electrons, gaining oxygen, or losing hydrogen. Reduction means gaining electrons, losing oxygen, or gaining hydrogen. The classic memory aid is OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
An oxidising agent causes oxidation in other substances by accepting their electrons, but gets reduced itself. Think of it as the electron "thief" in the reaction.
Memory Aid: Remember OIL RIG - it's the most reliable way to identify what's happening in redox reactions!
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Oxidation Numbers and Common Ions
A reducing agent donates electrons to other substances, causing them to be reduced, but gets oxidised itself. It's the electron "donor" in reactions.
Oxidation numbers follow simple rules: elements alone are zero, simple ions equal their charge, and all oxidation numbers in a compound must add up to zero (or the overall charge for complex ions).
Key ions you must memorise include: H⁺ (hydrogen), Ag⁺ (silver), Zn²⁺ (zinc), OH⁻ (hydroxide), NO₃⁻ (nitrate), and SO₄²⁻ (sulfate). These appear constantly in equations.
Quick Check: In H₂O, hydrogen is +1 and oxygen is -2, giving 2(+1) + (-2) = 0. Perfect!
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Essential Ion Table
The ammonium ion (NH₄⁺) and carbonate ion (CO₃²⁻) complete your essential ion collection. These complex ions behave as single units in all reactions.
Master these common ions and you'll find writing chemical equations becomes much more straightforward. They're the building blocks for countless reactions you'll encounter.
Final Tip: Practice writing formulae for compounds using these ions - it's the best way to make them stick in your memory!
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?
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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