Science2 閲覧数·更新日 Jun 12, 2026·5 ページ

Understanding Chemical and Physical Changes

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Every day you see matter changing around you - ice...

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# Chemical vs. Physical
Changes

An introduction to physical and chemical changes

Everything in the world is made of matter, and matter can

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.

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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.

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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.

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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.

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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!

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Science2 閲覧数·更新日 Jun 12, 2026·5 ページ

Understanding Chemical and Physical Changes

Googleソースに追加

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...

of 5

Understanding the Basics

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.

Top Tip: Ask yourself "Is a new substance formed?" - this is the ultimate test for spotting chemical changes.

of 5

Spotting the Differences

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.

of 5

Signs of Chemical Reactions

Lab Alert: Don't just rely on one sign - look at the whole picture to determine if new substances are actually forming.

of 5

Real-World Examples

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!

Exam Strategy: Always explain your reasoning - don't just say "physical" or "chemical", explain why based on whether new substances form.

of 5

Key Points for Success

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!