7th Class Changes Around Us: Physical and Chemical
Understanding Physical and Chemical Changes
1. Which two statements best describe a physical change?
(i) Only the shape or state may alter.
(ii) A brand-new substance appears.
(iii) Properties stay the same.
(iv) Chemical bonds break forever.
Answer: Option (a). In a physical change, matter keeps its identity; only shape or state alters, giving no new substance.
2. Which of these changes can be reversed easily?
Answer: Option (c). Sugar can be recovered by evaporating water, so the change is reversible, unlike stitching, burning or baking.
3. Why is dissolving salt in water called a reversible physical change?
Answer: Heating the salty water lets water escape as vapour, leaving unchanged salt crystals; therefore dissolving salt is a reversible physical change.
4. Explain why curdling of milk is considered a chemical change.
Answer: Curdling forms curd and butter milk because bacteria create lactic acid and curd which cannot be reversed into milk again; new substances form, so change is chemical.
5. Which option shows a chemical change?
Answer: Option (b). Burning paper produces ash, smoke and gases with entirely new properties; this irreversible formation proves a chemical change occurred.
6. What is meant by a chemical change? Give one everyday example.
Answer: A chemical change makes one or more new substances with different properties. Example: Rusting of iron, cooking of food where new substances are formed.
7. Which process below mainly involves physical weathering of rocks?
Answer: Option (c). Strong winds carrying sand grind rock surfaces into smaller chips without altering minerals; therefore wind abrasion is simple physical weathering.
8. Why does burning a candle show both physical and chemical changes?
Answer: Wax melting is a physical change. Simultaneously, burning wax vapour forms carbon dioxide and soot, showing chemical change too.
9. Is rusting of iron a physical or chemical change? Give a reason.
Answer: Rusting is a slow chemical change because iron reacts with oxygen and moisture, forming brown iron-oxide that cannot turn back into shiny iron.
10. Explain why melting of ice is a reversible physical change.
Answer: This is because ice can be changed into water on heating and water again can be changed into ice on cooling, so no new product is formed. Hence, it is a reversible physical process.
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