Class 7 Chemistry Worksheet on Chapter 6 Physical and Chemical ... - Free Printable

Problem Analysis:

The image contains a series of questions related to chemistry, focusing on concepts such as physical and chemical changes, rusting, galvanization, crystallization, and the properties of metals. Below is a detailed solution for each question.

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Q9. State true or false. Condensation of steam is not a chemical change.

Solution:
- Condensation is the process where a gas (steam) changes into a liquid (water). This is a physical change because no new substances are formed; only the state of matter changes.
- Therefore, the statement "Condensation of steam is not a chemical change" is True.

Answer: True

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Q10. Match the following.

| Column A | Column B |
|------------------------------|-----------------------------------|
| Physical Change | Blue |
| Chemical Change | Green |
| Burning of magnesium | Physical properties are changed |
| Iron Sulphate | Magnesium Oxide |
| Copper Sulphate | New substances are produced |

Solution:
1. Physical Change: This refers to a change in the physical properties of a substance without forming new substances.
- Match: Physical properties are changed.

2. Chemical Change: This refers to a change where new substances are formed with different chemical properties.
- Match: New substances are produced.

3. Burning of magnesium: When magnesium burns in air, it reacts with oxygen to form magnesium oxide. This is a chemical change.
- Match: Magnesium Oxide.

4. Iron Sulphate: Iron sulphate is a compound that appears green in color.
- Match: Green.

5. Copper Sulphate: Copper sulphate is a compound that appears blue in color.
- Match: Blue.

Answer:
- Physical Change → Physical properties are changed
- Chemical Change → New substances are produced
- Burning of magnesium → Magnesium Oxide
- Iron Sulphate → Green
- Copper Sulphate → Blue

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Q11. Define crystallisation.

Solution:
Crystallisation is the process by which a solid forms from a solution, melt, or vapor. It involves the arrangement of molecules or ions into a highly ordered crystalline structure. Crystallisation is often used to purify substances by separating them from impurities.

Answer: Crystallisation is the process of forming a solid crystal from a solution, melt, or vapor, resulting in a highly ordered crystalline structure.

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Q12. Define galvanisation.

Solution:
Galvanisation is the process of coating a metal (usually iron or steel) with a layer of zinc to protect it from corrosion (rusting). This is typically done by immersing the metal in a bath of molten zinc or through electrolytic deposition. The zinc layer acts as a protective barrier, preventing oxygen and moisture from reaching the underlying metal.

Answer: Galvanisation is the process of coating a metal (usually iron or steel) with a layer of zinc to prevent corrosion.

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Q13. Why do stainless steel utensils not rust?

Solution:
Stainless steel does not rust because it contains a significant amount of chromium (typically around 10–30%). Chromium reacts with oxygen in the presence of water to form a thin, passive layer of chromium oxide on the surface of the steel. This layer is highly resistant to further oxidation and protects the underlying metal from rusting. Additionally, other elements like nickel and molybdenum enhance the corrosion resistance of stainless steel.

Answer: Stainless steel does not rust because it contains chromium, which forms a protective layer of chromium oxide on its surface, preventing further oxidation.

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Q14. Name the product formed when a piece of magnesium is burnt in the air.

Solution:
When magnesium burns in the air, it reacts with oxygen to form magnesium oxide (\( \text{MgO} \)). The reaction is as follows:
\[
\text{Mg} + \text{O}_2 \rightarrow \text{MgO}
\]

Answer: Magnesium oxide (\( \text{MgO} \))

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Q15. What is rust?

Solution:
Rust is the reddish-brown substance that forms on the surface of iron or steel when it is exposed to moisture and oxygen over time. Rust is primarily composed of hydrated iron(III) oxide (\( \text{Fe}_2\text{O}_3 \cdot n\text{H}_2\text{O} \)) and is a result of the corrosion of iron. The process of rust formation involves the oxidation of iron in the presence of water and oxygen.

Answer: Rust is the reddish-brown substance formed on iron or steel due to corrosion, primarily consisting of hydrated iron(III) oxide (\( \text{Fe}_2\text{O}_3 \cdot n\text{H}_2\text{O} \)).

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Q16. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of gas. What kind of change is it? Explain.

Solution:
- Baking soda (\( \text{NaHCO}_3 \)) reacts with the citric acid in lemon juice to produce carbon dioxide gas (\( \text{CO}_2 \)), water (\( \text{H}_2\text{O} \)), and sodium citrate. The reaction is as follows:
\[
\text{NaHCO}_3 + \text{C}_6\text{H}_8\text{O}_7 \rightarrow \text{CO}_2 + \text{H}_2\text{O} + \text{Na}_3\text{C}_6\text{H}_5\text{O}_7
\]
- Since new substances (\( \text{CO}_2 \), \( \text{H}_2\text{O} \), and sodium citrate) are formed, this is a chemical change.

Answer: This is a chemical change because new substances (carbon dioxide, water, and sodium citrate) are formed during the reaction.

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Q17. How does painting an iron grill prevent it from rusting?

Solution:
Painting an iron grill prevents rusting by acting as a barrier between the iron and the environment (moisture and oxygen). The paint coat seals the surface of the iron, preventing water and oxygen from coming into contact with the metal. Without these reactants, the chemical reaction leading to rust formation cannot occur.

Answer: Painting an iron grill prevents rusting by creating a barrier that blocks moisture and oxygen from reaching the iron surface, thus inhibiting the rusting process.

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Q18. What is a chemical change? State any four characteristics of chemical change.

Solution:
A chemical change is a process in which one or more substances are transformed into new substances with different chemical properties. Four key characteristics of chemical changes are:

1. Formation of new substances: New compounds or elements are produced with different chemical properties.
2. Change in energy: Chemical changes often involve the release or absorption of energy, such as heat, light, or sound.
3. Irreversibility: Chemical changes are usually irreversible, meaning the original substances cannot be easily recovered.
4. Change in color, odor, or state: Chemical changes may result in observable changes such as a change in color, odor, or state of matter.

Answer: A chemical change is a process in which new substances are formed. Characteristics include:
1. Formation of new substances
2. Change in energy
3. Irreversibility
4. Change in color, odor, or state

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Q19. What is rusting? How can it be prevented?

Solution:
- Rusting: Rusting is the corrosion of iron or steel, which occurs when iron reacts with oxygen and moisture to form hydrated iron(III) oxide (\( \text{Fe}_2\text{O}_3 \cdot n\text{H}_2\text{O} \)). It is a chemical process that weakens the metal over time.
- Prevention of rusting: Rusting can be prevented using the following methods:
1. Coating: Applying a protective layer such as paint, oil, or galvanisation (zinc coating) to prevent exposure to moisture and oxygen.
2. Alloying: Using alloys like stainless steel, which contain chromium and resist corrosion.
3. Dry Environment: Keeping the metal in a dry environment to prevent moisture from reacting with the iron.
4. Anodizing: Electrochemically altering the surface of the metal to create a protective oxide layer.

Answer: Rusting is the corrosion of iron or steel due to the reaction with oxygen and moisture. It can be prevented by coating, alloying, maintaining a dry environment, or anodizing.

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Q20. Differentiate between a chemical and physical change.

Solution:
| Aspect | Chemical Change | Physical Change |
|------------------------------|-----------------------------------|-----------------------------------|
| Definition | Involves the formation of new substances with different chemical properties. | Involves a change in the physical properties of a substance without forming new substances. |
| Examples | Burning wood, rusting iron, cooking food. | Melting ice, boiling water, tearing paper. |
| Reversibility | Usually irreversible. | Reversible. |
| Energy Change | Often involves the release or absorption of energy. | May involve a change in energy but is generally less significant. |
| New Substances Formed | Yes. | No. |

Answer:
- Chemical Change: Involves the formation of new substances with different chemical properties. Examples include burning wood and rusting iron.
- Physical Change: Involves a change in physical properties without forming new substances. Examples include melting ice and boiling water.

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Final Answers:

1. Q9: True
2. Q10:
- Physical Change → Physical properties are changed
- Chemical Change → New substances are produced
- Burning of magnesium → Magnesium Oxide
- Iron Sulphate → Green
- Copper Sulphate → Blue
3. Q11: Crystallisation is the process of forming a solid crystal from a solution, melt, or vapor, resulting in a highly ordered crystalline structure.
4. Q12: Galvanisation is the process of coating a metal (usually iron or steel) with a layer of zinc to prevent corrosion.
5. Q13: Stainless steel does not rust because it contains chromium, which forms a protective layer of chromium oxide on its surface, preventing further oxidation.
6. Q14: Magnesium oxide (\( \text{MgO} \))
7. Q15: Rust is the reddish-brown substance formed on iron or steel due to corrosion, primarily consisting of hydrated iron(III) oxide (\( \text{Fe}_2\text{O}_3 \cdot n\text{H}_2\text{O} \)).
8. Q16: This is a chemical change because new substances (carbon dioxide, water, and sodium citrate) are formed during the reaction.
9. Q17: Painting an iron grill prevents rusting by creating a barrier that blocks moisture and oxygen from reaching the iron surface, thus inhibiting the rusting process.
10. Q18: A chemical change is a process in which new substances are formed. Characteristics include:
1. Formation of new substances
2. Change in energy
3. Irreversibility
4. Change in color, odor, or state
11. Q19: Rusting is the corrosion of iron or steel due to the reaction with oxygen and moisture. It can be prevented by coating, alloying, maintaining a dry environment, or anodizing.
12. Q20:
- Chemical Change: Involves the formation of new substances with different chemical properties. Examples include burning wood and rusting iron.
- Physical Change: Involves a change in physical properties without forming new substances. Examples include melting ice and boiling water.

Boxed Final Answer:
\[
\boxed{\text{See detailed solutions above}}
\]