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2 Dec 2025
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Chemical Analysis: Key Concepts in Combined Science
Charlotte
@charlotte26
1 / 4
Pure Substances vs Mixtures
Ever wondered why chocolate melts at a specific temperature but your mixed sweets don't? That's because pure substances contain only one element or compound, whilst mixtures are two or more different substances that aren't chemically bonded together.
The easiest way to tell them apart is through melting points. Pure substances have a sharp melting point - they melt at one exact temperature. Mixtures melt over a range of temperatures because each component melts at different points.
Formulations are special mixtures where each ingredient is carefully measured for a specific purpose. Think about medicines, paints, cleaning products, fuel, and even your favourite foods - they're all formulations designed to work perfectly for their job.
Quick Check: Next time you're cooking, notice how butter (pure fat) melts sharply whilst margarine (a formulation) melts more gradually!
Paper Chromatography
Paper chromatography is like giving substances a race up a piece of paper - and it's brilliant for separating things like food colourings, inks, and dyes to see what they're really made of.
Here's the method: draw a pencil line , place your ink dots on the line, then dip the paper so the line sits just above the water. As the paper absorbs the solvent, your substances race upwards at different speeds.
The mobile phase is your solvent doing the moving, whilst the stationary phase is the paper staying put. If your substance doesn't move, it's insoluble. One colour means it's pure, multiple colours reveal it's a mixture.
Lab Tip: Always use a pencil for your baseline - ink would interfere with your results and ruin the whole experiment!
Reading Chromatograms and Rf Values
Your chromatogram tells you loads about what you're testing. Pure substances show up as one spot, whilst impure substances create multiple spots - it's like a fingerprint for chemicals.
Rf values are your secret weapon for identifying unknown substances. Calculate them using: distance travelled by substance ÷ distance travelled by solvent. These values stay consistent for the same substance in the same solvent, so you can compare with reference tables.
Think of Rf values like a chemical ID card - they help you identify mystery substances by matching them against known chemicals.
Exam Tip: Always measure from the centre of spots to the pencil line and solvent front for accurate Rf calculations!
Gas Tests You Need to Know
These four gas tests are absolute exam favourites, and each has a memorable result that makes identification dead easy.
Oxygen - "relight my fire!" A glowing splint relights because oxygen supports combustion. Hydrogen gives the classic "squeaky pop" when you hold a lighted splint near the test tube opening.
Carbon dioxide makes limewater go "cloudy for carbon dioxide" - it turns milky white when you bubble the gas through calcium hydroxide solution. Chlorine bleaches damp litmus paper white because it's an acidic gas used for bleaching.
Memory Hack: These rhyming memory aids aren't just silly - they'll save you marks in your exams when you're under pressure!
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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
Chemical Analysis: Key Concepts in Combined Science
Charlotte
@charlotte26
Chemical analysis is all about figuring out what's actually in the substances around you - from the ink in your pen to the air you breathe. You'll learn how to identify pure substances, separate mixtures, and test for specific gases... Show more
Pure Substances vs Mixtures
Ever wondered why chocolate melts at a specific temperature but your mixed sweets don't? That's because pure substances contain only one element or compound, whilst mixtures are two or more different substances that aren't chemically bonded together.
The easiest way to tell them apart is through melting points. Pure substances have a sharp melting point - they melt at one exact temperature. Mixtures melt over a range of temperatures because each component melts at different points.
Formulations are special mixtures where each ingredient is carefully measured for a specific purpose. Think about medicines, paints, cleaning products, fuel, and even your favourite foods - they're all formulations designed to work perfectly for their job.
Quick Check: Next time you're cooking, notice how butter (pure fat) melts sharply whilst margarine (a formulation) melts more gradually!
Sign up to see the contentIt's free!
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Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Paper Chromatography
Paper chromatography is like giving substances a race up a piece of paper - and it's brilliant for separating things like food colourings, inks, and dyes to see what they're really made of.
Here's the method: draw a pencil line , place your ink dots on the line, then dip the paper so the line sits just above the water. As the paper absorbs the solvent, your substances race upwards at different speeds.
The mobile phase is your solvent doing the moving, whilst the stationary phase is the paper staying put. If your substance doesn't move, it's insoluble. One colour means it's pure, multiple colours reveal it's a mixture.
Lab Tip: Always use a pencil for your baseline - ink would interfere with your results and ruin the whole experiment!
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Reading Chromatograms and Rf Values
Your chromatogram tells you loads about what you're testing. Pure substances show up as one spot, whilst impure substances create multiple spots - it's like a fingerprint for chemicals.
Rf values are your secret weapon for identifying unknown substances. Calculate them using: distance travelled by substance ÷ distance travelled by solvent. These values stay consistent for the same substance in the same solvent, so you can compare with reference tables.
Think of Rf values like a chemical ID card - they help you identify mystery substances by matching them against known chemicals.
Exam Tip: Always measure from the centre of spots to the pencil line and solvent front for accurate Rf calculations!
Sign up to see the contentIt's free!
Access to all documents
Improve your grades
Join milions of students
By signing up you accept Terms of Service and Privacy Policy
Gas Tests You Need to Know
These four gas tests are absolute exam favourites, and each has a memorable result that makes identification dead easy.
Oxygen - "relight my fire!" A glowing splint relights because oxygen supports combustion. Hydrogen gives the classic "squeaky pop" when you hold a lighted splint near the test tube opening.
Carbon dioxide makes limewater go "cloudy for carbon dioxide" - it turns milky white when you bubble the gas through calcium hydroxide solution. Chlorine bleaches damp litmus paper white because it's an acidic gas used for bleaching.
Memory Hack: These rhyming memory aids aren't just silly - they'll save you marks in your exams when you're under pressure!
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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Most popular content: Chromatography
Most popular content in Chemistry
Most popular content
Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.9/5
App Store
4.8/5
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 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