Class 7 Science: Physical & Chemical Changes + Acids and Bases

All questions and answers in simple words

1. Physical and Chemical Changes

Now tell

Q1. Give two examples of fast change.
Ans: Burning paper, bursting crackers.

Q2. What kind of change is the eruption of volcano?
Ans: Fast and chemical change.

Q3. What kind of change is the cloud burst?
Ans: Fast and physical change.

Q4. What kind of change is the balloon bursting?
Ans: Fast and physical change.

Exercise 1: Tick the correct option

Q1(i). In physical changes...
Ans: (d) Only physical properties change ✓
No new thing is made. Only shape, size or state changes.

Q1(ii). Which of the following changes is reversible?
Ans: (d) melting of ghee ✓
Ghee can become solid again when cooled.

Q1(iii). Which of the following is a chemical change?
Ans: (d) burning of paper ✓
Paper turns into ash and smoke. New things form.

Q2. Write an event in which both physical and chemical changes take place.
Ans: Burning of candle – Wax melts (physical). Wick burns and makes heat, light, CO₂ (chemical).

Q3. Classify into physical and chemical changes:
Ans:
Physical: melting wax, separating husk from paddy, melting of ice
Chemical: cooking of rice, making curd of milk, digestion of food, rust, photosynthesis, making bread from flour, formation of leaves

Q. How are copper sulphate crystals formed? Explain with example.
Ans: Dissolve copper sulphate in hot water. Cool it slowly. Blue crystals form.

Q. Why an iron cupboard is painted?
Ans: To stop rusting. Paint blocks air and water from touching iron.

2. Corrosion, Rust & Heat Changes

Q6. Explain why there is more corrosion in iron articles in marine areas than in desert areas.
Ans: Sea air has salt + water. Salt makes iron rust faster. Deserts are dry, so less rust.

Q7. Take the difference between physical and chemical changes with examples.
Ans:
Physical: No new substance forms. Easy to reverse. Ex: ice to water.
Chemical: New substance forms. Cannot reverse easily. Ex: milk to curd.

Q8. Give an example of:
(i) fast change → Burning matchstick
(ii) endothermic change → Melting ice – takes heat from outside
(iii) exothermic change → Burning wood – gives out heat

Q9. What is rust? What are the disadvantages? What measures can be taken to avoid this?
Ans:
Rust: Red-brown layer on iron from air + water.
Problems: Makes iron weak, holes form, things get destroyed.
Avoid by: Painting, oiling, greasing, galvanizing, or keeping iron dry.

Q10. Steel spoon should not be put in pickle box. Why?
Ans: Pickle has acid. Acid reacts with steel. It spoils the pickle and can make the spoon rust.

3. Acids, Bases and Indicators

Now tell

Q1. Can I taste all the ingredients to find out the taste?
Ans: No. Some things can be harmful or poisonous. Never taste unknown things.

Q2. Sunil soaked the turmeric shirt and applied soap, the shirt turned red. Why?
Ans: Turmeric is a natural indicator. Soap is basic. Turmeric turns red in base.

Q3. Acid + indicator = ?
Ans: Colour change. Ex: Blue litmus turns red.

Q4. Base + indicator = ?
Ans: Colour change. Ex: Red litmus turns blue.

Q5. Anita noticed fish in the lake in front of the factory suddenly started dying. Why?
Ans: Factory released acidic/basic waste in lake. It polluted water and killed fish.

Exercise 1: MCQs

Q1(i). Red and blue litmus papers dipped in vinegar. What will happen?
Ans: (b) Blue litmus will turn red ✓ Vinegar is acid

Q1(ii). When you wash turmeric stains with soap, its colour becomes?
Ans: (c) red ✓ Soap is basic

Q1(iii). If red litmus remains red and blue litmus is blue, then solution is?
Ans: (c) neutral ✓

Q1(iv). On mixing acid and base in right quantity?
Ans: (c) The quality of both is destroyed ✓ This is neutralization

Q1(v). If soil is acidic, what will farmer put in soil?
Ans: (c) Slaked lime or quick lime ✓ Lime is basic

Q2. What is the difference between acid and base?
Ans:
Acid: Sour taste, turns blue litmus red. Ex: lemon juice.
Base: Bitter taste, soapy feel, turns red litmus blue. Ex: soap.

Q3. Litmus solution is obtained from which plant?
Ans: Lichen plant.

Q4. Write the names of three indicators.
Ans: Litmus, turmeric, china rose (hibiscus).

Q5. What do sodium hydroxide and hydrochloric acid make each other neutral? Write.
Ans: They make salt + water.
NaOH + HCl → NaCl + H₂O

4. Neutralization & True/False

Q6. What happens when:
(i) A person suffering from acidity takes antacids:
Ans: Antacid is basic. It neutralizes extra acid in stomach and gives relief.
(ii) Soda is put on area bitten by ants:
Ans: Ant injects acid. Soda (sodium hydrogen carbonate) is basic. It neutralizes acid and reduces pain.
(iii) In factory, waste water is neutralized before being discharged:
Ans: To stop water pollution. Acidic/basic waste can kill fish and plants.
(iv) The farmer puts lime in the soil of his fields:
Ans: To remove extra acidity from soil. Lime is basic. Crops grow better in neutral soil.

Q7. Is distilled water acidic/basic/neutral? How do you confirm this?
Ans: Neutral. Test with litmus paper – no colour change. Red litmus stays red, blue stays blue.

Q8. If the following statement is true, write (✓) or false (✗):
(i) Nitric acid turns red litmus blue. → ✗ False Acid turns blue litmus red
(ii) Sodium hydroxide turns blue litmus red. → ✗ False Base turns red litmus blue
(iii) NaOH and HCl neutralize each other to form salts and water. → ✓ True
(iv) Indicator shows different colours in acidic and basic solutions. → ✓ True
(v) Dental caries is caused by alkali. → ✗ False Caused by acid made by bacteria

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