20 Important Words (Meanings in Hindi)

• Physical Change — भौतिक परिवर्तन: केवल रूप/आकार बदलता है, पदार्थ वही रहता है
• Chemical Change — रासायनिक परिवर्तन: नया पदार्थ बनता है, गुण बदल जाते हैं
• Reversible — उलटने योग्य: जो आसानी से वापस किया जा सके
• Irreversible — अपरिवर्तनीय: जिसे वापस नहीं बदला जा सके
• Rusting — जंग लगना: लोहे पर ऑक्सीजन और नमी के कारण परत बनना
• Galvanisation — गैल्वनाइजेशन: लोहे पर जिंक की परत चढ़ाना
• Crystallisation — स्फटीकीकरण: क्रिस्टल बनाना (जैसे नमक)
• Evaporation — वाष्पीकरण: तरल का गैस में बदलना
• Condensation — संघनन: वाष्प का तरल में बदलना
• Combustion — दहन: जलने की प्रक्रिया
• Oxidation — ऑक्सीकरण: ऑक्सीजन से प्रतिक्रिया
• Precipitate — अवक्षेप: द्रव से ठोस पदार्थ का गिरना
• Corrosion — क्षरण: धातु का खराब होना
• Expansion — प्रसार: पदार्थ का फैलना
• Contraction — संकुचन: पदार्थ का सिकुड़ना
• Melting — गलना: ठोस से तरल में बदलना
• Freezing — जमना: तरल से ठोस बनना
• Photosynthesis — प्रकाश संश्लेषण: पौधों में भोजन बनाने की प्रक्रिया
• Respiration — श्वसन: ऑक्सीजन लेकर ऊर्जा बनाना
• Displacement Reaction — विस्थापन अभिक्रिया: एक धातु दूसरी को हटाकर यौगिक बनाए

Important Notes

• Changes are broadly of two types: Physical and Chemical.
• Physical change: No new substance is formed. Only physical properties (shape, size, state) change. Reversible. Example: melting of ice.
• Chemical change: A new substance is formed with different properties. Irreversible. Example: burning of paper, rusting of iron.
• Rusting requires both oxygen and moisture (H\(_2\)O). Prevention: galvanisation, painting, oiling.
• Crystallisation is a method to purify solids like salt or sugar.
• Combustion is a chemical change as it releases new products and energy.
• Equations:
  \[ \text{Iron} + \text{Oxygen} + \text{Water} \;\rightarrow\; \text{Hydrated Iron Oxide (Rust)} \]
  \[ \text{Magnesium} + \text{Oxygen} \;\rightarrow\; \text{Magnesium Oxide} \]

20 One-Word Answer Questions

1. Burning of candle is what type of change?
   Chemical change (though melting is physical).
2. Process of sugar dissolving in water?
   Physical change.
3. Name a method of preventing rusting.
   Galvanisation.
4. Formation of crystals from solution is?
   Crystallisation.
5. Rust is chemically called?
   Hydrated iron oxide.
6. Burning of magnesium ribbon gives?
   Magnesium oxide (white ash).
7. Cutting of paper is?
   Physical change.
8. Formation of curd from milk?
   Chemical change.
9. Reaction between copper sulphate and iron nail?
   Displacement reaction.
10. Gas needed for rusting?
    Oxygen.
11. Gas released in burning of paper?
    Carbon dioxide.
12. Process of cooling vapour into liquid?
    Condensation.
13. Heating of sugar gives?
    Carbon and water vapour.
14. Rusting is what type of change?
    Chemical change.
15. Stretching of rubber band?
    Physical change.
16. Explosion of crackers?
    Chemical change.
17. Mixing sand in water?
    Physical change.
18. Boiling of water?
    Physical change.
19. Burning of coal produces?
    Carbon dioxide, ash, heat.
20. Change of water into ice?
    Physical change.

20 Very Short Answer Type (1–2 lines)

1. Define physical change with one example.
   Change where no new substance forms. Example: melting ice.
2. Define chemical change with one example.
   Change where new substance forms. Example: rusting iron.
3. What is galvanisation?
   Coating iron with zinc to prevent rusting.
4. Name one irreversible physical change.
   Breaking of glass.
5. Why is burning of wood chemical?
   New substances (ash, gases) form, irreversible.
6. What is corrosion?
   Slow eating away of metals due to environment.
7. Why is rusting harmful?
   Weakens iron objects, reduces strength.
8. What are crystals?
   Pure solids with definite shape, formed by crystallisation.
9. Give one example of displacement reaction.
   Iron + CuSO\(_4\) → FeSO\(_4\) + Copper.
10. Is dissolving salt reversible?
    Yes, by evaporation.
11. What is expansion?
    Increase in size/volume with heat.
12. What is contraction?
    Decrease in size/volume on cooling.
13. Name the gas used in combustion.
    Oxygen.
14. Why is rusting slow in dry air?
    Moisture is absent, both oxygen and water are needed.
15. What happens when Mg ribbon burns?
    Forms white powder of magnesium oxide.
16. Why is evaporation a physical change?
    No new substance forms, reversible.
17. What is common in chemical changes?
    Formation of new products, energy change.
18. State one use of crystallisation.
    Purification of salts.
19. Is freezing of water reversible?
    Yes, ice melts back to water.
20. Why does curd formation represent chemical change?
    New product with different taste/properties forms.

20 Short Answer Type Questions (2–3 lines)

1. Differentiate between physical and chemical change.
   Physical: no new substance, reversible (e.g., ice → water). Chemical: new substance, irreversible (e.g., rusting).
2. Explain rusting of iron.
   Iron reacts with oxygen and moisture forming hydrated iron oxide (rust), a reddish-brown flaky substance.
3. How can rusting be prevented?
   By galvanisation, painting, oiling, or alloying iron with other metals.
4. Why is burning of candle both physical and chemical?
   Melting of wax is physical; burning of wax with oxygen is chemical producing CO\(_2\) and water.
5. What is crystallisation? Give one use.
   Process of obtaining pure crystals from solution; used to purify salts like copper sulphate.
6. Why is cooking food a chemical change?
   New products with different taste and properties form; irreversible.
7. Explain the displacement reaction with iron and copper sulphate.
   Fe + CuSO\(_4\) → FeSO\(_4\) + Cu; iron displaces copper due to higher reactivity.
8. What are reversible and irreversible changes? Examples.
   Reversible: ice melting. Irreversible: burning paper.
9. Why are physical changes important?
   They change state or form without altering composition, useful in shaping and processing materials.
10. Give two examples of corrosion other than rusting.
    Tarnishing of silver, green coating on copper.
11. Why does iron rust faster near sea?
    Because salt in moist air increases conductivity and speeds corrosion.
12. Is breaking a glass a physical or chemical change? Why?
    Physical change, no new substance formed, only shape changes.
13. Why does magnesium burn with dazzling light?
    It reacts with oxygen releasing large amount of heat and light, forming MgO.
14. How can you show that dissolving salt in water is reversible?
    By evaporating water, salt is obtained back.
15. Give one difference between expansion and contraction.
    Expansion: on heating, particles move apart. Contraction: on cooling, particles come closer.
16. Why does rusting not occur on plastic?
    Rusting is chemical change of metals with oxygen/moisture. Plastics are non-metallic.
17. Why is digestion a chemical change?
    Food is broken down into new simpler substances with different properties.
18. Why is respiration a chemical change?
    Glucose reacts with oxygen producing CO\(_2\), water, and energy; new substances form.
19. Why are chemical changes usually permanent?
    New substances form with different properties, cannot be reversed easily.
20. Give two examples each of physical and chemical changes.
    Physical: melting ice, tearing paper. Chemical: burning wood, rusting iron.

NCERT Exercise Answers – Chapter 7

1. Classify changes below as physical or chemical: (a) melting of ice, (b) burning of candle, (c) rusting of iron, (d) mixing salt in water, (e) boiling of water.
   (a) Physical, (b) Both (melting = physical, burning = chemical), (c) Chemical, (d) Physical, (e) Physical.
2. Iron pillar near Qutub Minar has not rusted. Why?
   Special composition of iron and dry climate prevent rusting.
3. Galvanisation is method to prevent rusting. Define.
   It is coating iron with zinc layer to prevent exposure to moisture/air.
4. Name two methods to prevent rusting.
   Painting, oiling, galvanisation.
5. Explain crystallisation with an activity.
   Prepare saturated solution of salt, leave it to cool → crystals form. This is crystallisation used to purify solids.
6. Classify into physical/chemical: (a) tearing paper, (b) cooking rice, (c) bending metal, (d) digesting food.
   (a) Physical, (b) Chemical, (c) Physical, (d) Chemical.

Important Equations Reminder

\[ 4Fe + 3O_2 + 6H_2O \;\rightarrow\; 4Fe(OH)_3 \;\;\; \text{(Rust)} \]

\[ 2Mg + O_2 \;\rightarrow\; 2MgO \]