Physical and Chemical Changes — REET Science Notes
This page covers the 'Physical and Chemical Changes' topic from REET Level 2 Science, aligned with the Class 7 NCERT syllabus — how to identify both types of changes, their examples, and the key differences between them. In the exam, questions often give an example and ask you to identify the type of change, so every point here is explained with examples.
- 0 questions in REET 2024
- Class 6–8 level
- 7 min read
How this topic is asked in REET
No direct question from the 'Physical and Chemical Changes' topic was recorded in the REET 2024 paper, so the question pattern here is based on the language of the Class 7 NCERT syllabus, not on any verified previous question. Typically the question gives an everyday example and asks whether the change is physical or chemical, so it's essential to understand the difference between the two clearly.
What is a physical change
A change in which the physical properties of a substance (such as its size, shape, or state) change, but no new substance is formed and the chemical composition of the substance stays the same, is called a physical change. It is usually temporary and, in most cases, easily reversible. In a physical change the mass of the substance also usually remains unchanged — only its outward form or state changes.
Examples: wax melting, ice turning into water, tearing or folding paper, dissolving sugar or salt in water, stretching a rubber band, bending an iron rod. In all of these, the chemical nature of the substance stays the same; only the physical state or shape changes.
A few other common examples: water boiling to become steam, churning milk to separate the cream, placing an iron nail near a magnet to make it temporarily magnetic, and breaking glass — in all of these, the mass and chemical composition of the substance remain unchanged, only the physical state, shape, or properties change. It is worth noting that not all physical changes are reversible — breaking glass or cutting wood, for instance, are physical changes (no new substance forms in either case), but it is not possible to restore them to their original form. So reversibility is not a compulsory condition for a physical change, just a common tendency.
What is a chemical change
A change in which one or more new substances form, whose properties are entirely different from the original substance, is called a chemical change. It is usually permanent, and generally it cannot be easily reversed.
Examples: iron rusting, paper or wood burning, milk turning sour (curdling), digestion of food, magnesium ribbon burning, firecrackers exploding. In all of these, a new substance forms with properties completely different from the original substance, and it is generally not possible to get the original substance back.
Main grounds for identifying physical and chemical changes
| Basis | Physical change | Chemical change |
|---|---|---|
| New substance | Does not form | Forms |
| Reversibility | Usually reversible | Usually irreversible |
| Clear signs | Generally no special sign | Signs such as gas being released, colour change, temperature change, or a precipitate forming |
| Example | Ice melting, tearing paper | Rusting, combustion (burning) |
Signs that help identify a chemical change
- Change in colour
- For example, iron developing brown-coloured rust
- Release of a gas
- For example, carbon dioxide gas forming when vinegar is added to baking soda
- Change in temperature
- For example, heat being released in combustion (exothermic), or heat being absorbed in some reactions (endothermic)
- Formation of a precipitate
- An insoluble solid substance forming when two solutions are mixed
- Change in smell
- For example, food going bad, which changes its smell
- Absorption or release of energy
- In an exothermic reaction heat is released (as in combustion); in an endothermic reaction heat is absorbed (as in photosynthesis)
Rusting — an important chemical change
Rusting of iron is a slow chemical change in which iron reacts with oxygen and moisture (water vapour) in the air to form hydrated iron oxide (rust). Rusting weakens the structure of the iron. The main ways to prevent it are applying a coat of paint or oil, galvanising (coating with a layer of zinc), and using an alloy such as stainless steel. Chemically, rust is called hydrated iron(III) oxide (Fe₂O₃·xH₂O). In galvanising, a thin layer of zinc is applied to the surface of the iron, which protects it from direct contact with moisture and oxygen; stainless steel, on the other hand, is an alloy of iron, chromium and nickel, in which the chromium forms a protective oxide layer on the surface that prevents rusting.
It's worth noting that both physical and chemical changes can happen together in some processes — for example, when a candle burns, the melting of the wax is a physical change, while the burning of the wick is a chemical change. Similarly, crystallisation — such as obtaining salt crystals from seawater — is a physical change, in which crystals of the pure substance are obtained.
Classifying everyday changes
- Ice melting / water freezing
- Physical change — reversible in both directions
- Kneading dough and baking roti
- Kneading dough is a physical change, while the change in colour and texture that happens while baking the roti is a chemical change
- Boiling an egg
- Chemical change — the structure of the protein changes permanently, and it cannot be turned back into a raw egg
- Sublimation of camphor
- Physical change — the solid changes directly into a gas, and no new substance is formed
- A firecracker bursting
- Chemical change — new substances form along with light, heat, and gas
- Making butter from curd (churning)
- Physical change — the fat particles separate out, and no new substance is formed
- Making paneer from milk (curdling milk)
- Chemical change — adding acid causes the milk protein (casein) to set permanently, and it cannot be turned back into milk
How to teach this to students
Classroom activity
Light a candle and show it to students, then ask them which part is a physical change (the wax melting) and which is a chemical change (the wick burning) — this makes both concepts clear in a single activity. Have students watch the melted wax cool down so they also grasp reversibility.
Teaching aids
Safely demonstrating the reaction between baking soda and vinegar (gas bubbles forming) in class is a practical way to show the signs of a chemical change. Alongside this, you can also have students dissolve salt in water and recover the crystals back through evaporation.
Common misconception
Students often assume that a change of state (like water boiling) is always a chemical change. Make it clear that in a change of state (solid-liquid-gas), no new substance forms, so it is a physical change. Similarly, some students think that any change involving a colour shift must be chemical, which is not always true.
Assessment idea
Give students 10 everyday examples and ask them to classify each as physical or chemical, along with a reason for every answer — this assesses a deeper understanding of the concept.
A trick to remember this
Remember the signs for identifying a chemical change — change in smell, release of a gas, change in colour, formation of a precipitate — if any one of these signs is present, the change is chemical. If the chemical nature of the substance stays the same and only its shape or state changes, the change is physical.
Practice questions
Question 1
Which of the following is a physical change?
- Aलोहे में जंग लगना
- Bदूध का खट्टा होना
- Cबर्फ का पिघलना
- Dकागज का जलना
Explanation
Ice melting into water is only a change of state — no new substance forms, so it is a physical change.
Question 2
Which of the following is a chemical change?
- Aमैग्नीशियम रिबन जलाना
- Bपानी में नमक घोलना
- Cरबर बैंड खींचना
- Dकागज मोड़ना
Explanation
When a magnesium ribbon burns, a new substance, magnesium oxide, forms, whose properties are different from the original magnesium — this is a chemical change.
Question 3
The main characteristic of a chemical change is
- Aनए पदार्थ का बनना
- Bआकार में परिवर्तन
- Cअवस्था में परिवर्तन
- Dरंग वही रहना
Explanation
In a chemical change, a new substance forms with properties different from the original substance, and that is its defining feature.
Question 4
Rusting requires
- Aकेवल ऑक्सीजन
- Bकेवल नमी
- Cकेवल ऊष्मा
- Dऑक्सीजन व नमी दोनों
Explanation
For rust (hydrated iron oxide) to form, iron needs both oxygen from the air and moisture.
Question 5
Which of the following is a reversible change?
- Aकागज जलाना
- Bमोम का पिघलना
- Cदूध का खट्टा होना
- Dपटाखे का फटना
Explanation
Wax melts into a liquid and can turn solid again on cooling, so it is a reversible physical change.
Question 6
In the process of burning a candle,
- Aकेवल भौतिक परिवर्तन होता है
- Bभौतिक व रासायनिक दोनों परिवर्तन होते हैं
- Cकेवल रासायनिक परिवर्तन होता है
- Dकोई परिवर्तन नहीं होता
Explanation
The melting of wax is a physical change, while the burning of the wick (combustion) is a chemical change, so both kinds of change happen together when a candle burns.
Question 7
Which of the following is NOT a way to prevent rusting?
- Aपेंट करना
- Bगैल्वेनाइजेशन
- Cतेल की परत चढ़ाना
- Dपानी में डुबोकर रखना
Explanation
Keeping iron soaked in water would expose it to even more moisture, which would make it rust faster — this is not a way to prevent rusting.
Question 8
What type of change is crystallization, and what is it used for?
- Aभौतिक; शुद्ध क्रिस्टल प्राप्त करने के लिए
- Bरासायनिक; नया यौगिक बनाने के लिए
- Cरासायनिक; गैस बनाने के लिए
- Dभौतिक; रंग बदलने के लिए
Explanation
Crystallisation is a physical change in which pure crystals of a substance are obtained from a solution, such as crystals of salt or copper sulphate.
Question 9
When teaching physical and chemical changes in Class 7, the most effective activity is
- Aकेवल परिभाषा रटवाना
- Bकेवल किताब से उदाहरण पढ़वाना
- Cमोमबत्ती जलाकर दोनों प्रकार के परिवर्तन एक साथ दिखाना
- Dपरीक्षा में पूछे गए प्रश्न याद करवाना
Explanation
Burning a candle in class clearly shows both physical (melting) and chemical (burning) change within a single example.
Question 10
If a student says, 'Boiling of water is a chemical change because it turns water into steam,' the teacher should explain that
- Aयह सही है
- Bयह गलत है क्योंकि तापमान नहीं बदला
- Cयह आंशिक रूप से सही है
- Dयह गलत है क्योंकि कोई नया पदार्थ नहीं बना, केवल अवस्था बदली है
Explanation
Boiling water is only a change of state (liquid to gas) — the chemical composition of water (H₂O) stays the same, no new substance forms, so it is a physical change.
Frequently asked questions
What is the main difference between a physical and a chemical change?+
In a physical change no new substance forms and it is usually reversible, while in a chemical change a new substance forms and it is usually permanent.
Is burning a candle a physical or a chemical change?+
Burning a candle involves both changes — the melting of the wax is a physical change, while the burning of the wick (combustion) is a chemical change. This example is often asked in the exam because it shows both types of change happening together.
What kind of change is rusting?+
Rusting is a chemical change, in which iron reacts with oxygen and moisture to form hydrated iron oxide. This reaction is slow, so rusting takes time, but the change is permanent.
What kind of change is crystallisation?+
Crystallisation is a physical change, used to obtain crystals of a pure substance from a solution, such as crystals of salt or copper sulphate.
What are the main signs of a chemical change?+
A change in colour, release of a gas, change in temperature, formation of a precipitate, and a change in smell — these are the main signs of a chemical change. However, no single sign is conclusive proof on its own, so it's important to check whether a new substance has actually formed.
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Change Log
- 17 September 2026 — English page published — mirrors the fact-checked Hindi page, independently bilingual-accuracy checked