This answer key helps students differentiate between physical and chemical properties and changes through clear definitions and practice examples.

The image you've uploaded is a completed worksheet titled "Physical and Chemical Properties and Changes", which includes definitions, examples, and answers to identify whether given properties or changes are physical (P) or chemical (C). Below is a detailed explanation of the concepts and solutions for each section.

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🔹 Definitions Recap

#### ✔ Physical Property
- Can be observed or measured without changing the substance’s identity.
- Examples: color, density, melting point, solubility, odor.

#### ✔ Chemical Property
- Describes how a substance interacts with other substances to form new substances.
- Involves a chemical reaction.
- Examples: flammability, reactivity with acids, ability to rust.

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#### ✔ Physical Change
- A change in form (size, shape, state), but no new substance is formed.
- Reversible in many cases.
- Example: melting ice, cutting paper.

#### ✔ Chemical Change
- Results in the formation of one or more new substances.
- Often involves color change, gas production, temperature change, or odor.
- Example: rusting iron, burning wood.

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## 🧪 Part 1: Identify as Physical (P) or Chemical (C) Property

| # | Property | Answer | Explanation |
|----|--------|--------|-----------|
| 1 | Blue color | P | Observed by sight; no change in composition. |
| 2 | Density | P | Measured without altering the substance. |
| 3 | Flammability (burns) | C | Indicates ability to react with oxygen → combustion. |
| 4 | Solubility (dissolves) | P | Dissolving doesn’t change the substance’s chemical identity (e.g., sugar in water). |
| 5 | Reacts with acid | C | A chemical reaction occurs (e.g., metal + acid → salt + hydrogen). |
| 6 | Supports combustion | C | Involves reacting with fuel (like oxygen); e.g., O₂ supports burning. |
| 7 | Sour taste | P | Sensory observation; though it may indicate acidity, the taste itself is physical. |
| 8 | Melting point | P | Temperature at which solid turns to liquid—no new substance. |
| 9 | Reacts with water | C | Chemical reaction (e.g., sodium + water → NaOH + H₂). |
| 10 | Hardness | P | Can be tested without changing substance. |
| 11 | Boiling point | P | Phase change from liquid to gas; no new substance. |
| 12 | Luster | P | How light reflects off surface; observable without change. |
| 13 | Odor | P | Smell is detected without changing matter. |
| 14 | Reacts with air | C | Oxidation or corrosion (e.g., rusting iron). |

✔ All answers match the correct classification based on whether a new substance forms.

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## 🔁 Part 2: Identify as Physical (P) or Chemical (C) Change

| # | Change | Answer | Explanation |
|----|------|--------|-----------|
| 1 | NaCl dissolves in water | P | Salt breaks into ions but remains Na⁺ and Cl⁻—no new substance. |
| 2 | Ag (Silver) tarnishes | C | Silver reacts with sulfur in air to form silver sulfide (Ag₂S)—black tarnish. |
| 3 | An apple is cut | P | Only size/shape changes; apple pieces are still apples. |
| 4 | Heat changes H₂O to steam | P | Phase change (liquid → gas); same molecules (H₂O). |
| 5 | Baking soda reacts with vinegar | C | Produces CO₂ gas, water, and salt—new substances. |
| 6 | Fe (Iron) rusts | C | Iron oxidizes to form iron oxide (Fe₂O₃)—new compound. |
| 7 | Alcohol evaporates | P | Liquid to vapor—same substance. |
| 8 | Ice melts | P | Solid to liquid—still H₂O. |
| 9 | Milk sours | C | Bacteria convert lactose to lactic acid—new substance formed. |
| 10 | Sugar dissolves in water | P | Sugar molecules disperse but remain unchanged. |
| 11 | Wood rots | C | Decomposition by fungi/microbes—organic matter breaks down chemically. |
| 12 | Pancakes cook | P? Wait! Actually C | Cooking involves Maillard reaction (browning), denaturation of proteins—chemical change. But here marked P, which is incorrect. ✘ Let's fix this. |
| 13 | Grass grows | C | Photosynthesis creates glucose—new substances via chemical reactions. |
| 14 | A tire is inflated | P | Air pressure increases; no chemical change. |
| 15 | Food is digested | C | Enzymes break down food into simpler molecules (e.g., starch → glucose)—chemical change. |
| 16 | Paper towel absorbs water | P | Water is held by capillary action; no chemical reaction. |

⚠️ Correction Note:
Item #12: "Pancakes cook" is labeled P in the key, but this is incorrect.
👉 Correct answer should be C (Chemical Change) because cooking pancakes involves:
- Browning (Maillard reaction),
- Denaturation of proteins,
- Formation of new compounds.

So, #12 should be C, not P.

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✔ Final Summary

#### ✔️ Correct Answers (with correction):

| # | Change | Correct Answer |
|----|-------|----------------|
| 1 | NaCl dissolves | P ✔ |
| 2 | Silver tarnishes | C ✔ |
| 3 | Apple cut | P ✔ |
| 4 | Water to steam | P ✔ |
| 5 | Baking soda + vinegar | C ✔ |
| 6 | Iron rusts | C ✔ |
| 7 | Alcohol evaporates | P ✔ |
| 8 | Ice melts | P ✔ |
| 9 | Milk sours | C ✔ |
| 10 | Sugar dissolves | P ✔ |
| 11 | Wood rots | C ✔ |
| 12 | Pancakes cook | C ✘ (was marked P — error!) |
| 13 | Grass grows | C ✔ |
| 14 | Tire inflated | P ✔ |
| 15 | Food digested | C ✔ |
| 16 | Paper towel absorbs water | P ✔ |

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📌 Conclusion

This worksheet correctly identifies most physical and chemical properties and changes, except for #12 ("Pancakes cook"), which should be classified as a chemical change (C) due to irreversible chemical reactions during cooking.

> ✔ Recommendation: Correct #12 from P → C for accuracy.

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