Every day you see matter changing around you - ice...
Science2 vues·Mis à jour Jun 12, 2026·5 pagesUnderstanding Chemical and Physical Changes
1 / 5
1
of 5
Understanding the Basics
You'll find this topic everywhere in science, so getting these definitions right will make everything else much easier. A physical change happens when something looks different but is still the same substance underneath - like ice melting into water (it's still H₂O). A chemical change creates completely new substances with different properties.
Think of it this way: if you can easily undo the change, it's probably physical. If you can't turn it back to what it was, it's likely chemical.
The key players in chemical changes are reactants (what you start with) and products (what you end up with). Most physical changes are reversible, whilst chemical changes are usually irreversible.
Top Tip: Ask yourself "Is a new substance formed?" - this is the ultimate test for spotting chemical changes.
2
of 5
Spotting the Differences
This comparison table is absolutely essential for your exams - you'll definitely need to know these differences. Physical changes keep the same substance but change its form (like crushing ice or boiling water). Chemical changes create entirely new substances with different properties.
The big giveaway is reversibility. You can easily freeze water back into ice, but you can't turn ash back into wood. Energy changes happen in both types, but chemical reactions often produce much more dramatic energy changes.
Don't worry about the mass though - it's always conserved in both types of changes. The total mass before equals the total mass after, every single time.
Remember: Changes of state (melting, freezing, boiling) are ALWAYS physical changes, no matter how dramatic they look.
3
of 5
Signs of Chemical Reactions
When you're in the lab, these signs will help you spot chemical reactions happening right before your eyes. Look for colour changes (like metal turning rusty), gas production (bubbling or fizzing), or a precipitate forming (when a solid suddenly appears in a clear liquid).
Energy changes are huge clues too. Exothermic reactions give out heat and warm up the container, whilst endothermic reactions absorb heat and cool things down. You might also notice light, sound, or new smells appearing.
Here's the catch though - one sign alone might not be enough. Boiling water produces bubbles, but that's still a physical change. You need to think about whether a genuinely new substance is being created.
Lab Alert: Don't just rely on one sign - look at the whole picture to determine if new substances are actually forming.
4
of 5
Real-World Examples
Let's test your understanding with some everyday scenarios. Dissolving salt in water is physical because no new substances form - you can get your salt back by evaporating the water. The salt and water are still chemically the same.
Frying an egg is definitely chemical. The clear, runny egg white becomes solid and white through heat, and the proteins change permanently. You'll never turn that fried egg back into a raw one!
Rusting happens when iron reacts with oxygen and water to form iron oxide - a completely new substance that's flaky and reddish-brown. Making tea is mostly physical (boiling, dissolving, diffusion), though some tiny chemical processes might occur with the tea compounds.
Exam Strategy: Always explain your reasoning - don't just say "physical" or "chemical", explain why based on whether new substances form.
5
of 5
Key Points for Success
You've got this! The Law of Conservation of Mass applies to everything - mass is never lost or gained, whether it's physical or chemical changes. Remember that dissolving is always physical (the substance is just spread out, not changed), and all changes of state are physical.
For chemical changes, watch for the classic signs: fizzing, colour changes, precipitates, and energy changes. The ultimate question remains: "Is a new substance formed?"
Keep practising with everyday examples around you. Is cooking chemical or physical? What about breaking glass? The more you apply these concepts to real life, the more confident you'll become.
Confidence Boost: Once you master the "new substance" rule, you'll find this topic much easier than it first appears!
Si on te demande...
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Science2 vues·Mis à jour Jun 12, 2026·5 pagesUnderstanding Chemical and Physical Changes
Every day you see matter changing around you - ice melting in your drink, bread toasting, or your bike getting rusty. Understanding whether these are physical or chemical changes is crucial for understanding how the world works and will help...
1
of 5
Inscris-toi pour voir le contenu. C'est gratuit!
- Accès à tous les documents
- Améliore tes notes
- Rejoins des millions d'étudiants
Understanding the Basics
You'll find this topic everywhere in science, so getting these definitions right will make everything else much easier. A physical change happens when something looks different but is still the same substance underneath - like ice melting into water (it's still H₂O). A chemical change creates completely new substances with different properties.
Think of it this way: if you can easily undo the change, it's probably physical. If you can't turn it back to what it was, it's likely chemical.
The key players in chemical changes are reactants (what you start with) and products (what you end up with). Most physical changes are reversible, whilst chemical changes are usually irreversible.
Top Tip: Ask yourself "Is a new substance formed?" - this is the ultimate test for spotting chemical changes.
2
of 5Inscris-toi pour voir le contenu. C'est gratuit!
- Accès à tous les documents
- Améliore tes notes
- Rejoins des millions d'étudiants
Spotting the Differences
This comparison table is absolutely essential for your exams - you'll definitely need to know these differences. Physical changes keep the same substance but change its form (like crushing ice or boiling water). Chemical changes create entirely new substances with different properties.
The big giveaway is reversibility. You can easily freeze water back into ice, but you can't turn ash back into wood. Energy changes happen in both types, but chemical reactions often produce much more dramatic energy changes.
Don't worry about the mass though - it's always conserved in both types of changes. The total mass before equals the total mass after, every single time.
Remember: Changes of state (melting, freezing, boiling) are ALWAYS physical changes, no matter how dramatic they look.
3
of 5Inscris-toi pour voir le contenu. C'est gratuit!
- Accès à tous les documents
- Améliore tes notes
- Rejoins des millions d'étudiants
Signs of Chemical Reactions
When you're in the lab, these signs will help you spot chemical reactions happening right before your eyes. Look for colour changes (like metal turning rusty), gas production (bubbling or fizzing), or a precipitate forming (when a solid suddenly appears in a clear liquid).
Energy changes are huge clues too. Exothermic reactions give out heat and warm up the container, whilst endothermic reactions absorb heat and cool things down. You might also notice light, sound, or new smells appearing.
Here's the catch though - one sign alone might not be enough. Boiling water produces bubbles, but that's still a physical change. You need to think about whether a genuinely new substance is being created.
Lab Alert: Don't just rely on one sign - look at the whole picture to determine if new substances are actually forming.
4
of 5Inscris-toi pour voir le contenu. C'est gratuit!
- Accès à tous les documents
- Améliore tes notes
- Rejoins des millions d'étudiants
Real-World Examples
Let's test your understanding with some everyday scenarios. Dissolving salt in water is physical because no new substances form - you can get your salt back by evaporating the water. The salt and water are still chemically the same.
Frying an egg is definitely chemical. The clear, runny egg white becomes solid and white through heat, and the proteins change permanently. You'll never turn that fried egg back into a raw one!
Rusting happens when iron reacts with oxygen and water to form iron oxide - a completely new substance that's flaky and reddish-brown. Making tea is mostly physical (boiling, dissolving, diffusion), though some tiny chemical processes might occur with the tea compounds.
Exam Strategy: Always explain your reasoning - don't just say "physical" or "chemical", explain why based on whether new substances form.
5
of 5Inscris-toi pour voir le contenu. C'est gratuit!
- Accès à tous les documents
- Améliore tes notes
- Rejoins des millions d'étudiants
Key Points for Success
You've got this! The Law of Conservation of Mass applies to everything - mass is never lost or gained, whether it's physical or chemical changes. Remember that dissolving is always physical (the substance is just spread out, not changed), and all changes of state are physical.
For chemical changes, watch for the classic signs: fizzing, colour changes, precipitates, and energy changes. The ultimate question remains: "Is a new substance formed?"
Keep practising with everyday examples around you. Is cooking chemical or physical? What about breaking glass? The more you apply these concepts to real life, the more confident you'll become.
Confidence Boost: Once you master the "new substance" rule, you'll find this topic much easier than it first appears!
Si on te demande...
Qu'est-ce que le compagnon IA de Knowunity ?
Notre compagnon IA est spécialement conçu pour répondre aux besoins des étudiants. Sur la base des millions d'éléments de contenu que nous avons sur la plateforme, nous pouvons fournir des réponses vraiment significatives et pertinentes aux étudiants. Mais il ne s'agit pas seulement de réponses, le compagnon a encore plus pour but de guider les élèves dans leurs défis d'apprentissage quotidiens, avec des plans d'étude personnalisés, des quiz ou des éléments de contenu dans le chat et une personnalisation à 100% basée sur les compétences et les développements de l'étudiant.
Où puis-je télécharger l'appli Knowunity ?
Tu peux télécharger l'application dans Google Play Store et dans l'App Store d'Apple.
L'application est-elle vraiment gratuite ?
Oui, tu as un accès entièrement gratuit à tous les contenus de l'appli, tu peux chatter ou suivre les créateurs à tout moment. De plus, nous proposons Knowunity Premium, qui te permet de réviser sans limites!
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4.6/5App Store
4.7/5Google Play
L'application est très facile d'utilisation et bien conçue. Jusqu'à présent, j'ai trouvé tout ce que je cherchais et j'ai pu apprendre beaucoup de choses grâce aux présentations ! Je vais certainement utiliser l'application pour un travail en classe ! Et comme source d'inspiration personnelle, elle est bien sûr aussi très utile.
Stefan Sutilisateur iOS
Cette application est vraiment super. Il y a tellement de fiches de révision et d'aide, [...]. Par exemple, la matière qui me pose problème est le français et l'appli a un choix d'aide très large. Grâce à cette application, je me suis améliorée en français. Je la recommanderais à tout le monde.
Samantha Klichutilisatrice Android
Waouh, je suis vraiment abasourdi. J'ai essayé l'application parce que je l'avais déjà vue plusieurs fois dans la publicité et j'ai été absolument choquée. Cette appli est L'AIDE dont on rêve pour l'école et surtout, elle propose tellement de choses, comme des rédactions et des fiches qui m'ont personnellement TRÈS bien aidé.
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