Class 7 Chemistry Worksheet on Chapter 6 Physical and Chemical ... - Free Printable
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Step-by-step solution for: Class 7 Chemistry Worksheet on Chapter 6 Physical and Chemical ...
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Step-by-step solution for: Class 7 Chemistry Worksheet on Chapter 6 Physical and Chemical ...
Here is the complete solution to all the questions from Q9 to Q20, based on standard science curriculum (typically Class 7 or 8 level):
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**Q9. State true or false.
Condensation of steam is not a chemical change.**
✔ Answer: True
Explanation:
Condensation of steam (water vapor turning into liquid water) is a physical change because only the state of matter changes — no new substance is formed. The chemical composition (H₂O) remains the same.
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Q10. Match the following.
| Column A | Column B |
|-----------------------|----------------------------|
| Physical Change | → Physical properties are changed |
| Chemical Change | → New substances are produced |
| Burning of magnesium | → Magnesium Oxide |
| Iron Sulphate | → Green |
| Copper Sulphate | → Blue |
✔ Correct Matching:
- Physical Change → Physical properties are changed
- Chemical Change → New substances are produced
- Burning of magnesium → Magnesium Oxide
- Iron Sulphate → Green
- Copper Sulphate → Blue
Explanation:
- Physical change involves change in shape, size, state — no new substance.
- Chemical change produces new substances with different properties.
- Burning magnesium produces white powder — magnesium oxide.
- Iron sulphate crystals are green.
- Copper sulphate crystals are blue.
---
Q11. Define crystallisation.
✔ Answer:
Crystallisation is the process of forming solid crystals from a homogeneous solution by cooling or evaporating the solvent. It is used to purify solids.
Example: Salt crystals formed when seawater evaporates.
---
Q12. Define galvanisation.
✔ Answer:
Galvanisation is the process of coating iron or steel with a layer of zinc to prevent rusting.
Note: The image says “aluminium metal” — that’s incorrect. Galvanisation uses zinc, not aluminium. Zinc forms a protective layer and also acts sacrificially if scratched.
---
Q13. Why do stainless steel utensils not rust?
✔ Answer:
Stainless steel is an alloy of iron, chromium, and nickel. Chromium forms a thin, invisible, protective oxide layer on the surface which prevents further oxidation (rusting).
---
Q14. Name the product formed when a piece of magnesium is burnt in the air.
✔ Answer:
Magnesium oxide (MgO)
Chemical equation:
2Mg + O₂ → 2MgO
It burns with a dazzling white flame and forms a white powdery ash.
---
Q15. What is rust?
✔ Answer:
Rust is a reddish-brown flaky substance formed when iron reacts with oxygen and moisture in the air. Its chemical name is hydrated ferric oxide, represented as Fe₂O₃·nH₂O.
---
Q16. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of gas. What kind of change is it? Explain.
✔ Answer:
This is a chemical change.
Explanation:
Baking soda (sodium bicarbonate, NaHCO₃) reacts with citric acid in lemon juice to produce carbon dioxide gas (CO₂), water, and salt. The formation of bubbles (gas) indicates a new substance is formed — which is a sign of chemical change.
Reaction:
NaHCO₃ + H⁺ (from citric acid) → CO₂↑ + H₂O + salt
---
Q17. How does painting an iron grill prevent it from rusting?
✔ Answer:
Painting creates a barrier between the iron surface and the atmosphere (oxygen and moisture). Since rusting requires both oxygen and water, preventing their contact with iron stops rusting.
---
Q18. What is a chemical change? State any four characteristics of chemical change.
✔ Answer:
A chemical change is a change in which one or more substances are transformed into new substances with different chemical compositions and properties.
Four characteristics:
1. Formation of new substances.
2. Irreversible (usually).
3. Change in color, odor, or temperature.
4. Evolution of gas or formation of precipitate.
---
Q19. What is rusting? How can it be prevented?
✔ Answer:
Rusting is the corrosion of iron and steel when they react with oxygen and moisture in the air to form hydrated ferric oxide (rust).
Prevention methods:
1. Painting or greasing to create a barrier.
2. Galvanisation (coating with zinc).
3. Using alloys like stainless steel.
4. Keeping iron objects dry and away from moisture.
---
Q20. Differentiate between a chemical and physical change.
✔ Answer:
| Feature | Physical Change | Chemical Change |
|-----------------------|------------------------------------------|-------------------------------------------|
| New substance formed? | No | Yes |
| Reversibility | Usually reversible | Usually irreversible |
| Composition | Same molecules before and after | Molecules rearranged; new compounds formed |
| Energy change | Little or no energy change | Often involves heat/light/gas evolution |
| Examples | Melting ice, dissolving sugar | Burning paper, rusting iron |
---
✔ Summary Note for Q12 (Galvanisation):
The statement in the image — *“Iron objects are galvanised with a layer of aluminium metal.”* — is FALSE.
✔ Correction: Galvanisation uses zinc, not aluminium. Aluminium coating is called "aluminising", which is different.
---
Let me know if you want these answers formatted for printing or study notes!
---
**Q9. State true or false.
Condensation of steam is not a chemical change.**
✔ Answer: True
Explanation:
Condensation of steam (water vapor turning into liquid water) is a physical change because only the state of matter changes — no new substance is formed. The chemical composition (H₂O) remains the same.
---
Q10. Match the following.
| Column A | Column B |
|-----------------------|----------------------------|
| Physical Change | → Physical properties are changed |
| Chemical Change | → New substances are produced |
| Burning of magnesium | → Magnesium Oxide |
| Iron Sulphate | → Green |
| Copper Sulphate | → Blue |
✔ Correct Matching:
- Physical Change → Physical properties are changed
- Chemical Change → New substances are produced
- Burning of magnesium → Magnesium Oxide
- Iron Sulphate → Green
- Copper Sulphate → Blue
Explanation:
- Physical change involves change in shape, size, state — no new substance.
- Chemical change produces new substances with different properties.
- Burning magnesium produces white powder — magnesium oxide.
- Iron sulphate crystals are green.
- Copper sulphate crystals are blue.
---
Q11. Define crystallisation.
✔ Answer:
Crystallisation is the process of forming solid crystals from a homogeneous solution by cooling or evaporating the solvent. It is used to purify solids.
Example: Salt crystals formed when seawater evaporates.
---
Q12. Define galvanisation.
✔ Answer:
Galvanisation is the process of coating iron or steel with a layer of zinc to prevent rusting.
Note: The image says “aluminium metal” — that’s incorrect. Galvanisation uses zinc, not aluminium. Zinc forms a protective layer and also acts sacrificially if scratched.
---
Q13. Why do stainless steel utensils not rust?
✔ Answer:
Stainless steel is an alloy of iron, chromium, and nickel. Chromium forms a thin, invisible, protective oxide layer on the surface which prevents further oxidation (rusting).
---
Q14. Name the product formed when a piece of magnesium is burnt in the air.
✔ Answer:
Magnesium oxide (MgO)
Chemical equation:
2Mg + O₂ → 2MgO
It burns with a dazzling white flame and forms a white powdery ash.
---
Q15. What is rust?
✔ Answer:
Rust is a reddish-brown flaky substance formed when iron reacts with oxygen and moisture in the air. Its chemical name is hydrated ferric oxide, represented as Fe₂O₃·nH₂O.
---
Q16. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of gas. What kind of change is it? Explain.
✔ Answer:
This is a chemical change.
Explanation:
Baking soda (sodium bicarbonate, NaHCO₃) reacts with citric acid in lemon juice to produce carbon dioxide gas (CO₂), water, and salt. The formation of bubbles (gas) indicates a new substance is formed — which is a sign of chemical change.
Reaction:
NaHCO₃ + H⁺ (from citric acid) → CO₂↑ + H₂O + salt
---
Q17. How does painting an iron grill prevent it from rusting?
✔ Answer:
Painting creates a barrier between the iron surface and the atmosphere (oxygen and moisture). Since rusting requires both oxygen and water, preventing their contact with iron stops rusting.
---
Q18. What is a chemical change? State any four characteristics of chemical change.
✔ Answer:
A chemical change is a change in which one or more substances are transformed into new substances with different chemical compositions and properties.
Four characteristics:
1. Formation of new substances.
2. Irreversible (usually).
3. Change in color, odor, or temperature.
4. Evolution of gas or formation of precipitate.
---
Q19. What is rusting? How can it be prevented?
✔ Answer:
Rusting is the corrosion of iron and steel when they react with oxygen and moisture in the air to form hydrated ferric oxide (rust).
Prevention methods:
1. Painting or greasing to create a barrier.
2. Galvanisation (coating with zinc).
3. Using alloys like stainless steel.
4. Keeping iron objects dry and away from moisture.
---
Q20. Differentiate between a chemical and physical change.
✔ Answer:
| Feature | Physical Change | Chemical Change |
|-----------------------|------------------------------------------|-------------------------------------------|
| New substance formed? | No | Yes |
| Reversibility | Usually reversible | Usually irreversible |
| Composition | Same molecules before and after | Molecules rearranged; new compounds formed |
| Energy change | Little or no energy change | Often involves heat/light/gas evolution |
| Examples | Melting ice, dissolving sugar | Burning paper, rusting iron |
---
✔ Summary Note for Q12 (Galvanisation):
The statement in the image — *“Iron objects are galvanised with a layer of aluminium metal.”* — is FALSE.
✔ Correction: Galvanisation uses zinc, not aluminium. Aluminium coating is called "aluminising", which is different.
---
Let me know if you want these answers formatted for printing or study notes!
Parent Tip: Review the logic above to help your child master the concept of physical chemical changes worksheet.
