Class 7 Science Chapter 5 Question Answers - Physical and Chemical Changes

Q1. How does painting an iron grill prevent it from rusting?
Ans: Painting an iron grill prevents the contact of iron with air and moisture, so there is no interaction. Thus, painting an iron grill prevents it from rusting.

Q2. What is a physical change?
Ans: A physical change occurs when the physical properties of the substance change, but no new substances are formed.

Q3. What is a chemical change?
Ans: A chemical change occurs when two substances interact to produce one or more new substances with different properties.

Q4. Mention some examples of chemical changes.
Ans: Rusting of iron, fermentation of food, photosynthesis, burning of a candle, and digestion of food are some examples of chemical changes.

Q5. Write the chemical equation for rusting of iron.
Ans: The chemical equation for rusting iron is Iron + Oxygen + Water → Rust (Iron Oxide).
4 Fe + 3 O₂ + x.H₂O → 2 Fe₂O₃.

Q6. What is rust?
Ans: The brownish deposit on an iron material left open for some time is called rust.

Q7. What are the molecular formula and chemical name of rust?
Ans: The molecular formula and chemical name of rust are Fe2O3 and ferric oxide, respectively.

Q8. How would you show that curdling of milk is a chemical change?
Ans: Curdling of milk is a chemical change because a new substance, i.e., lactic acid, is formed. The curd has a different taste than the milk. Also, once the curd is formed, we cannot restore milk from it.

Q9. Why is fermented food warmer than ordinary food?
Ans: Fermented food is warmer than everyday food because a chemical reaction occurs during fermentation leading to the elimination of carbon dioxide and heat.

Q10. Explain why burning wood and cutting it into small pieces are considered two different types of changes.
Ans: The burning of wood is a chemical change as after burning it, wood changes into ash, and we cannot recover it from ash. In contrast, cutting wood into small pieces is a physical change as it involves a difference in the size of the wood.

Q11. Name the two conditions that are essential for rusting.
Ans: The two conditions necessary for rusting are:

• Oxygen
• Water or water vapor

Q12. When baking soda is mixed with lemon juice, bubbles are formed with the evolution of gas. What kind of change is it? Explain.
Ans: When baking soda is mixed with lemon juice, carbon dioxide (CO2) gas is released in the form of bubbles. The chemical composition of the substance changes, and a new product is formed after mixing. Thus, it is a chemical change.

Q13. Explain why rusting of iron objects is faster in coastal areas than in deserts.
Ans: Rusting of iron objects occurs if iron comes in contact with moist air. It is faster in coastal areas than in deserts because air contains a high percentage of moisture in coastal areas. In contrast, the air is dry and hot in the deserts. Therefore, rusting is more prominent in coastal provinces than in deserts.

Q14. Why is crystallisation regarded as a physical change?
Ans: Crystallisation is a physical change as it does not result in the formation of a new substance. Moreover, the change is reversible because crystals formed in the process can be reversed back to the non-crystalline form by dissolving it in water.

Q15. Why do stainless steel utensils not rust?
Ans: Stainless steel is an alloy with a minimum chromium content of 10.5%. The chromium reacts with the oxygen in the air and forms a protective layer that makes stainless steel highly resistant to corrosion and rust.