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

Very Short Answer Type Questions

Q1: What is a physical change? Provide an example.
Ans: A physical change is a change in which a substance undergoes changes only in its physical properties (e.g., shape, size, color, state), and no new substance is formed. Example: melting of butter.

Q2: What is a chemical change? Provide an example.
Ans: A chemical change is a change in which two substances react chemically to form a new substance with different chemical properties. Example: the burning of magnesium to form magnesium oxide.

Q3: Is a physical change temporary or permanent?
Ans: A physical change is temporary.

Q4: Is a chemical change permanent or temporary?
Ans: A chemical change is permanent.

Q5: What forms of energy are typically emitted or absorbed during a chemical change?
Ans: Forms of energy such as heat, light, or electricity may be emitted or absorbed during a chemical change.

Q6: Give an example of a chemical change.
Ans: The reaction between copper sulfate and iron, leading to the formation of iron sulfate and copper, is an example of a chemical change.

Q7: What is rust, and how does it form?
Ans: Rust is a brownish material that forms on the surface of iron when it comes in contact with oxygen and water. It is a chemical change known as rusting.

Q8: Why is it important to dry wet iron objects in the kitchen?
Ans: Wet iron objects can rust in the kitchen if not properly dried because rust forms when iron reacts with water and oxygen.

Q9: How can you prevent the rusting of iron objects?
Ans: Rusting of iron objects can be prevented by methods such as oiling, greasing, painting, or coating with other metals like zinc.

Q10: What is crystallization, and how does it help obtain pure crystals?
Ans: Crystallization is a process that involves cooling a hot, concentrated solution to create large, pure crystals. It helps obtain pure crystals from an impure solid substance.

Short Answer Type Questions

Q11: Explain the properties of a physical change.
Ans: Properties of a physical change include being temporary, no new substance formation, no emission or absorption of forms of energy, and being generally reversible.

Q12: Describe the properties of a chemical change.
Ans: Properties of a chemical change include being permanent, forming a new substance, the potential emission or absorption of forms of energy, and being generally irreversible.

Q13: Provide an example of a chemical change and explain the new substances formed.
Ans: In the reaction between copper sulfate and iron, copper sulfate solution (blue) reacts with iron to form iron sulfate solution (green) and copper (brown deposit). New substances formed are iron sulfate and copper.

Q14: Why does rusting occur, and what factors are essential for rusting to take place?
Ans: Rusting occurs when iron comes in contact with oxygen and water, forming iron oxide (rust). Rusting requires both oxygen and water or water vapor. Higher moisture content in the air increases the speed of rusting.

Q15: How can you prevent the rusting of iron objects, and what methods are commonly used?
Ans: Rusting of iron objects can be prevented by methods such as oiling, greasing, painting, coating with other metals (galvanization), or converting iron into stainless steel.

Q16: What is crystallization, and how is it different from the process of evaporation?
Ans: Crystallization is a process of obtaining large, pure crystals from a solution by cooling it. It is different from evaporation, which simply involves the removal of the solvent to obtain solid solute.

Q17: Explain the steps involved in the crystallization of copper sulfate.
Ans: Steps involved in the crystallization of copper sulfate include preparing a concentrated solution of copper sulfate, cooling the solution without disturbing it, and observing the formation of copper sulfate crystals.

Q18: Why is crystallization used to obtain pure crystals from an impure solid substance?
Ans: Crystallization is used to obtain pure crystals because it allows the controlled formation of large, well-defined crystals, separating them from impurities present in the solution.

Q19: How can you test for the presence of carbon dioxide, and what is the result of this test?
Ans: The presence of carbon dioxide can be tested by passing the gas through freshly prepared lime water. If carbon dioxide is present, lime water turns milky due to the formation of calcium carbonate.

Q20: Explain how vinegar and baking soda react and the result of this reaction.
Ans: When vinegar (acetic acid) is mixed with baking soda (sodium hydrogen carbonate), they react to produce carbon dioxide gas. This can be confirmed by passing the gas through lime water, which turns milky due to the formation of calcium carbonate. The chemical reaction is: Carbon dioxide (CO₂) + Lime water [Ca(OH)₂] → Calcium Carbonate (CaCO₃) + Water (H₂O).