Physical and Chemical Changes – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Explain why melting of ice is a physical change. Use features to support your answer.
Answer:
- Melting of ice is a physical change because the substance remains water (H2O).
- Only the state changes from solid to liquid.
- No new substance is formed during melting.
- The change is reversible. We can freeze water to get ice again.
- The basic properties of water remain the same.
- Energy is involved, but there is no chemical reaction.
Q2. Boiling water forms steam. Explain how this shows the characteristics of a physical change.
Answer:
- Boiling water changes liquid water to gas (steam).
- The substance stays water in both forms.
- No new substance is formed in boiling.
- The change is reversible by condensation. Steam cools back to water.
- It is a change of state, which is a physical change.
- There may be heat involved, but there is no chemical change.
Q3. A student dissolves sugar in water. Explain why it is a physical change and how the sugar can be recovered.
Answer:
- Dissolving sugar in water is a physical change.
- The sugar is still sugar, only spread out in water.
- No new substance is made in this process.
- The change is reversible. We can get sugar back.
- By evaporating the water, sugar is left behind.
- So, both dissolving and evaporation are physical processes.
Q4. Rusting of iron shows clear signs of a chemical change. Explain with reasons.
Answer:
- In rusting, new substances like iron oxide are formed.
- There is a clear color change from shiny iron to reddish-brown rust.
- The process is usually irreversible under normal conditions.
- Energy changes are involved as the iron reacts with oxygen and moisture.
- The properties change. Rust is flaky and weak, unlike iron.
- Thus, rusting is a chemical change.
Q5. Compare physical and chemical changes using examples and key features.
Answer:
- In a physical change, no new substance is formed (e.g., ice melting, water boiling).
- In a chemical change, new substances are formed (e.g., wood burning, iron rusting).
- Physical changes are often reversible. Chemical changes are often irreversible.
- Physical changes show little or no energy change. Chemical changes often show heat or light.
- Physical changes involve state or form. Chemical changes involve composition.
- So, the core test is “new substance formed” vs “no new substance formed.”
High Complexity (Analysis & Scenario-Based)
Q6. During an experiment, water boils and steam touches a cool lid, forming droplets. Classify each step and justify.
Answer:
- Boiling water to form steam is a physical change.
- The substance remains water, only the state changes to gas.
- When steam touches the cool lid, it condenses back to water.
- Condensation is also a physical change and is reversible.
- In both steps, no new substance is formed.
- These steps show state changes with no chemical reaction.
Q7. A wooden log is burnt in a bonfire. Analyze the observations and conclude the type of change.
Answer:
- On burning, wood turns into ash, smoke, and gases.
- New substances are formed, so it is a chemical change.
- There is a strong energy change. We see heat and light.
- The original wood cannot be recovered, so it is irreversible.
- There is also a color and smell change during burning.
- Thus, burning wood is a clear chemical change.
Q8. Apple slices turn brown when left in air. Use clues to decide the type of change and explain.
Answer:
- The color change to brown is a key sign of a chemical change.
- New substances are formed when apple reacts with oxygen in air.
- The change is usually irreversible in normal conditions.
- There is no simple way to get the fresh apple surface back.
- The properties of the surface change from fresh to brown.
- So, browning of apples is a chemical change.
Q9. In a lab, a student notices bubbles forming in a beaker and the beaker becomes warm. What kind of change is this? Justify your answer.
Answer:
- The formation of bubbles suggests gas is produced.
- The beaker becoming warm shows an energy change.
- Both clues point to a chemical change.
- In a chemical change, new substances are formed.
- It is often irreversible and shows visible signs like gas and heat.
- Thus, the observations match a chemical reaction.
Q10. A student dissolves sugar in water, heats the solution to dryness, and then condenses the vapor. Classify each step with reasons.
Answer:
- Dissolving sugar in water is a physical change. No new substance forms.
- Heating the solution evaporates water. Evaporation is a physical change.
- The sugar remains behind, showing the change is reversible.
- The vapor is still water in the gas state.
- Condensing the vapor back to liquid water is also a physical change.
- All steps involve state or form changes, with no chemical reaction.