Physical vs Chemical Changes Key - Studocu - Free Printable
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Step-by-step solution for: Physical vs Chemical Changes Key - Studocu
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Step-by-step solution for: Physical vs Chemical Changes Key - Studocu
The image you've uploaded is a completed worksheet titled "Physical vs. Chemical Changes" from a chemistry educational resource. The task involves classifying various processes as either physical changes or chemical changes, based on the definitions provided:
- Physical change: The original substance still exists, only its form has changed.
- Chemical change: A new substance is formed; energy changes usually accompany these.
Below is a detailed explanation of each answer, verifying whether it's correct and explaining why:
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- Explanation: Dissolving NaOH in water is a physical change because no new substances are formed (Na⁺ and OH⁻ ions exist already). The process is reversible by evaporation.
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- Explanation: This is a neutralization reaction: HCl + KOH → KCl + H₂O. New substances (salt and water) are formed — a clear chemical change.
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- Explanation: Cutting the sodium only changes its size and shape; the substance remains sodium. No new substances are formed.
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- Explanation: Phase change from liquid to gas (evaporation). Water molecules remain H₂O — just in a different state.
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- Explanation: Decomposition reaction: 2KClO₃ → 2KCl + 3O₂. New substances (oxygen gas and KCl) are formed — chemical change.
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- Explanation: Rusting is oxidation: Fe + O₂ + H₂O → Fe₂O₃·xH₂O (rust). A new compound forms — chemical change.
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- Explanation: Reaction: 2Na + 2H₂O → 2NaOH + H₂↑. Hydrogen gas and sodium hydroxide are new products — definite chemical change.
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- Explanation: Liquid turns into vapor — phase change without altering molecular composition.
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- Explanation: Solid water becomes liquid water — same substance, different state.
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- Explanation: Lactic acid bacteria convert lactose to lactic acid, changing the chemical composition — souring is irreversible and involves new substances.
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- Explanation: Sugar molecules disperse in water but do not chemically react. Can be recovered by evaporation — physical change.
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- Explanation: Decomposition by fungi and bacteria breaks down cellulose into simpler compounds — new substances form.
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- Explanation: Browning (Maillard reaction), browning of proteins and sugars — irreversible chemical changes occur.
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- Explanation: Photosynthesis occurs: CO₂ + H₂O → glucose + O₂. New substances are synthesized — biological chemical change.
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- Explanation: Air is compressed into the tire — no chemical reactions. Gases are simply added; the air remains unchanged.
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- Explanation: Enzymes break down food molecules (e.g., proteins → amino acids, carbs → sugars) — chemical transformations.
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- Explanation: Capillary action draws water into the fibers — no chemical reaction. Water can be removed by drying — physical change.
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All answers on the worksheet are correctly classified.
This worksheet demonstrates an excellent understanding of the distinction between physical and chemical changes. Key indicators used include:
- Formation of new substances
- Irreversibility
- Energy changes (like heat or light)
- Color change, gas evolution, precipitate formation
These are hallmarks of chemical changes, while phase changes, dissolving, cutting, and compression are typically physical.
Let me know if you'd like this turned into a study guide or flashcards!
- Physical change: The original substance still exists, only its form has changed.
- Chemical change: A new substance is formed; energy changes usually accompany these.
Below is a detailed explanation of each answer, verifying whether it's correct and explaining why:
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1. Sodium hydroxide dissolves in water. → *Physical* ✔
- Explanation: Dissolving NaOH in water is a physical change because no new substances are formed (Na⁺ and OH⁻ ions exist already). The process is reversible by evaporation.
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2. Hydrochloric acid reacts with potassium hydroxide to produce a salt, water, and heat. → *Chemical* ✔
- Explanation: This is a neutralization reaction: HCl + KOH → KCl + H₂O. New substances (salt and water) are formed — a clear chemical change.
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3. A pellet of sodium is sliced in two. → *Physical* ✔
- Explanation: Cutting the sodium only changes its size and shape; the substance remains sodium. No new substances are formed.
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4. Water is heated and changed to steam. → *Physical* ✔
- Explanation: Phase change from liquid to gas (evaporation). Water molecules remain H₂O — just in a different state.
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5. Potassium chlorate decomposes to potassium chloride and oxygen gas. → *Chemical* ✔
- Explanation: Decomposition reaction: 2KClO₃ → 2KCl + 3O₂. New substances (oxygen gas and KCl) are formed — chemical change.
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6. Iron rusts. → *Chemical* ✔
- Explanation: Rusting is oxidation: Fe + O₂ + H₂O → Fe₂O₃·xH₂O (rust). A new compound forms — chemical change.
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7. When placed in H₂O, a sodium pellet catches on fire as hydrogen gas is liberated and sodium hydroxide forms. → *Chemical* ✔
- Explanation: Reaction: 2Na + 2H₂O → 2NaOH + H₂↑. Hydrogen gas and sodium hydroxide are new products — definite chemical change.
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8. Evaporation → *Physical* ✔
- Explanation: Liquid turns into vapor — phase change without altering molecular composition.
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9. Ice melting → *Physical* ✔
- Explanation: Solid water becomes liquid water — same substance, different state.
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10. Milk sours. → *Chemical* ✔
- Explanation: Lactic acid bacteria convert lactose to lactic acid, changing the chemical composition — souring is irreversible and involves new substances.
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11. Sugar dissolves in water. → *Physical* ✔
- Explanation: Sugar molecules disperse in water but do not chemically react. Can be recovered by evaporation — physical change.
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12. Wood rotting → *Chemical* ✔
- Explanation: Decomposition by fungi and bacteria breaks down cellulose into simpler compounds — new substances form.
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13. Pancakes cooking on a griddle → *Chemical* ✔
- Explanation: Browning (Maillard reaction), browning of proteins and sugars — irreversible chemical changes occur.
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14. Grass growing in a lawn → *Chemical* ✔
- Explanation: Photosynthesis occurs: CO₂ + H₂O → glucose + O₂. New substances are synthesized — biological chemical change.
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15. A tire is inflated with air. → *Physical* ✔
- Explanation: Air is compressed into the tire — no chemical reactions. Gases are simply added; the air remains unchanged.
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16. Food is digested in the stomach. → *Chemical* ✔
- Explanation: Enzymes break down food molecules (e.g., proteins → amino acids, carbs → sugars) — chemical transformations.
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17. Water is absorbed by a paper towel. → *Physical* ✔
- Explanation: Capillary action draws water into the fibers — no chemical reaction. Water can be removed by drying — physical change.
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✔ Final Assessment:
All answers on the worksheet are correctly classified.
This worksheet demonstrates an excellent understanding of the distinction between physical and chemical changes. Key indicators used include:
- Formation of new substances
- Irreversibility
- Energy changes (like heat or light)
- Color change, gas evolution, precipitate formation
These are hallmarks of chemical changes, while phase changes, dissolving, cutting, and compression are typically physical.
Let me know if you'd like this turned into a study guide or flashcards!
Parent Tip: Review the logic above to help your child master the concept of physical vs chemical change worksheet answers.
