Phase changes involve substances transforming between solid, liquid, and gas... Show more
AP Chemistry15 views·Updated May 29, 2026·2 pagesUnderstanding Phase Changes in AP Chemistry 6.5
1
of 2
Energy of Phase Changes and Phase Diagrams
When substances change phase, energy is either absorbed or released while the temperature remains constant. Endothermic phase changes (solid→liquid and liquid→gas) absorb energy, while exothermic phase changes (liquid→solid and gas→liquid) release energy. The energy absorbed in one direction equals the energy released in the reverse direction.
Phase diagrams show how substances exist at different temperatures and pressures. The triple point represents where a substance can exist simultaneously in all three phases. The critical point marks the highest combination of temperature and pressure at which a substance can exist between gas and liquid states.
Special phase transitions include sublimation (solid directly to gas) and deposition (gas directly to solid). These transitions bypass the liquid phase entirely and can be seen in everyday examples like dry ice.
Did you know? While most substances expand when melting, water is unique - it actually expands when freezing, which is why ice floats!
2
of 2
Heating/Cooling Curves and State Changes
Heating curves show what happens as energy is added to a substance. During a phase change, the temperature stays flat while energy goes into breaking molecular bonds rather than increasing kinetic energy. This creates the characteristic "plateau" sections in heating curves.
When you add heat energy (endothermic process), substances move from solid→liquid→gas. The energy required for these transitions is called the heat of fusion (melting) and heat of vaporization. Removing heat energy (exothermic process) reverses the direction, moving from gas→liquid→solid.
A substance in the supercritical fluid region exists beyond the critical point where distinct liquid and gas phases no longer exist. This state has properties of both phases and is useful in industrial applications.
Remember this! During phase changes on a heating/cooling curve, temperature remains constant even though energy is being added or removed. All that energy goes into changing the arrangement of molecules, not increasing their speed.
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI companion is specifically built for the needs of students. Based on the millions of content pieces we have on the platform we can provide truly meaningful and relevant answers to students. But its not only about answers, the companion is even more about guiding students through their daily learning challenges, with personalised study plans, quizzes or content pieces in the chat and 100% personalisation based on the students skills and developments.
Where can I download the Knowunity app?
You can download the app in the Google Play Store and in the 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.
Similar Content
Most popular content in AP Chemistry
9Most popular content
9Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5App Store
4.7/5Google 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 SiOS 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 KlichAndroid 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.
AnnaiOS user
AP Chemistry15 views·Updated May 29, 2026·2 pagesUnderstanding Phase Changes in AP Chemistry 6.5
Phase changes involve substances transforming between solid, liquid, and gas states. These transformations follow predictable patterns that can be mapped on phase diagrams and visualized through heating and cooling curves.
1
of 2Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
Energy of Phase Changes and Phase Diagrams
When substances change phase, energy is either absorbed or released while the temperature remains constant. Endothermic phase changes (solid→liquid and liquid→gas) absorb energy, while exothermic phase changes (liquid→solid and gas→liquid) release energy. The energy absorbed in one direction equals the energy released in the reverse direction.
Phase diagrams show how substances exist at different temperatures and pressures. The triple point represents where a substance can exist simultaneously in all three phases. The critical point marks the highest combination of temperature and pressure at which a substance can exist between gas and liquid states.
Special phase transitions include sublimation (solid directly to gas) and deposition (gas directly to solid). These transitions bypass the liquid phase entirely and can be seen in everyday examples like dry ice.
Did you know? While most substances expand when melting, water is unique - it actually expands when freezing, which is why ice floats!
2
of 2Sign up to see the content. It's free!
- Access to all documents
- Improve your grades
- Join milions of students
Heating/Cooling Curves and State Changes
Heating curves show what happens as energy is added to a substance. During a phase change, the temperature stays flat while energy goes into breaking molecular bonds rather than increasing kinetic energy. This creates the characteristic "plateau" sections in heating curves.
When you add heat energy (endothermic process), substances move from solid→liquid→gas. The energy required for these transitions is called the heat of fusion (melting) and heat of vaporization. Removing heat energy (exothermic process) reverses the direction, moving from gas→liquid→solid.
A substance in the supercritical fluid region exists beyond the critical point where distinct liquid and gas phases no longer exist. This state has properties of both phases and is useful in industrial applications.
Remember this! During phase changes on a heating/cooling curve, temperature remains constant even though energy is being added or removed. All that energy goes into changing the arrangement of molecules, not increasing their speed.
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI companion is specifically built for the needs of students. Based on the millions of content pieces we have on the platform we can provide truly meaningful and relevant answers to students. But its not only about answers, the companion is even more about guiding students through their daily learning challenges, with personalised study plans, quizzes or content pieces in the chat and 100% personalisation based on the students skills and developments.
Where can I download the Knowunity app?
You can download the app in the Google Play Store and in the 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.
Similar Content
Most popular content in AP Chemistry
9Most popular content
9Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5App Store
4.7/5Google 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 SiOS 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 KlichAndroid 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.
AnnaiOS user