TEACHGYAN • CLASS 7 SCIENCE • CURIOSITY
Chapter 5 — Changes Around Us: Physical and Chemical
Reversible Changes • New Substances • Reactions • Everyday Evidence
🌟 Chapter at a Glance
Change is everywhere: ice melts, paper is torn, iron rusts and food cooks. A key scientific question is whether a new substance has formed and whether the change can be reversed.
| Feature | Physical change | Chemical change |
|---|---|---|
| New substance | Usually no | Yes |
| Reversibility | Often easier to reverse | Often difficult to reverse |
| Example | Melting ice | Rusting iron |
🧊 1. Physical Changes
In a physical change, the identity of the substance is generally retained even though its state, shape or size changes. Melting, freezing, cutting and dissolving are common examples. Not every physical change is easily reversible, so reversibility alone is not a perfect test.
🔥 2. Chemical Changes
A chemical change produces one or more new substances with different properties. Burning, rusting and many cooking processes involve chemical changes.
🔎 3. Clues of Chemical Change
| Observation | What it may suggest |
|---|---|
| Gas produced | A chemical reaction may be occurring |
| New colour | May indicate a new substance |
| Temperature/light change | May indicate energy change |
| Precipitate/solid formed | May indicate a new substance |
These are clues, not isolated proof in every situation. The full context matters.
⚖️ 4. Reversible and Irreversible Changes
Some changes can be reversed easily, such as freezing water. Others are difficult to reverse, such as cooking an egg. However, “irreversible” does not automatically mean “chemical”; tearing paper is difficult to reverse but remains a physical change.
🧪 5. Investigating Changes
Observe before and after. Record colour, temperature, state, gas formation, smell only when safe and instructed, and whether a new material appears. Never deliberately smell or taste unknown reaction products.
🧠 Worked Examples
- Ice → water: physical change because the substance remains water.
- Iron → rust: chemical change because rust has different properties from iron.
- Paper cut into pieces: physical change because its composition remains the same.
- Milk turning into curd: chemical change involving formation of new substances.
🎯 Concept Check
- What is the main difference between physical and chemical change?
- Why is melting ice physical?
- Why is rusting chemical?
- Why is reversibility alone not enough to classify every change?
🧩 Interactive MCQs
- Melting ice is mainly a: A physical change B chemical change C combustion D neutralisation
- Rusting of iron is a: A chemical change B physical change C change of shape only D freezing process
- Formation of a new substance is strong evidence of: A chemical change B only physical change C measurement error D filtration
- Cutting paper is generally: A physical B chemical C combustion D neutralisation
Answers: 1-A, 2-A, 3-A, 4-A
📌 Case-Based Learning — The Rusty Bicycle
A bicycle left outdoors develops a reddish-brown coating on exposed iron parts.
- What type of change is occurring?
- What conditions encourage the change?
- Suggest two prevention methods.
- Why is painting useful?
🔥 HOTS / Competition Corner
- Classify dissolving sugar in water and explain your reasoning.
- A student says every irreversible change is chemical. Give a counterexample.
- Design a safe investigation comparing a physical and chemical change.
- Why can colour change be a clue but not automatically proof of a chemical reaction?
📝 Worksheet
| Level | Task |
|---|---|
| 1 | Classify common changes. |
| 2 | Identify evidence of chemical change. |
| 3 | Distinguish reversibility from chemical identity. |
| 4 | Analyse real-life situations. |
| 5 | Design an observation-based investigation. |
🏆 Mastery Checklist
- ☐ I can distinguish physical and chemical changes.
- ☐ I can use new-substance formation as evidence.
- ☐ I understand reversible and irreversible changes.
- ☐ I can identify clues of chemical change safely.
🔁 One-Minute Revision
Physical change: identity generally remains the same. Chemical change: new substance(s) form. Use multiple observations and evidence rather than relying on one clue alone.